Search

The enzyme serine hydroxymethyltransferase (SHMT) plays a key role in folate metabolism and is conserved in all kingdoms of life. SHMT is a pyridoxal 5'-phosphate (PLP) - dependent enzyme that catalyzes the conversion of L-serine and (6S)-tetrahydrofolate to glycine and 5,10-methylene tetrahydrofolate. Crystal structures of multiple members of the SHMT family have shown that the enzyme has a single conserved cis proline, which is located near the active site. Here, we have characterized a Pro to Ser amino acid variant (P285S) that affects this conserved cis proline in soybean SHMT8. P285S was identified as one of a set of mutations that affect the resistance of soybean to the agricultural pathogen soybean cyst nematode. We find that replacement of Pro285 by serine eliminates PLP-mediated catalytic activity of SHMT8, reduces folate binding, decreases enzyme stability, and affects the dimer-tetramer ratio of the enzyme in solution. Crystal structures at 1.9-2.2 Å resolution reveal a local reordering of the polypeptide chain that extends an α-helix and shifts a turn region into the active site. This results in a dramatically perturbed PLP-binding pose, where the ring of the cofactor is flipped by ∼180° with concomitant loss of conserved enzyme-PLP interactions. A nearby region of the polypeptide becomes disordered, evidenced by missing electron density for ∼10 residues. These structural perturbations are consistent with the loss of enzyme activity and folate binding and underscore the important role of the Pro285 cis-peptide in SHMT structure and function., (© 2024 The Author(s). Published by Portland Press Limited on behalf of the Biochemical Society.)

Tetrahydrofolate and its derivatives participate in one-carbon transfer reactions in all organisms. The cellular form of tetrahydrofolate (THF) is modified by multiple glutamate residues and polyglutamylation plays a key role in organellar and cellular folate homeostasis. In addition, polyglutamylation of THF is known to increase the binding affinity to enzymes in the folate cycle, many of which can utilize polyglutamylated THF as a substrate. Here, we use X-ray crystallography to provide a high-resolution view of interactions between the enzyme serine hydroxymethyltransferase (SHMT), which provides one carbon precursors for the folate cycle, and a polyglutamylated form of THF. Our 1.7 Å crystal structure of soybean SHMT8 in complex with diglutamylated 5-formyl-THF reveals, for the first time, a structural rearrangement of a loop at the entrance to the folate binding site accompanied by the formation of novel specific interactions between the enzyme and the diglutamyl tail of the ligand. Biochemical assays show that additional glutamate moieties on the folate ligand increase both enzyme stability and binding affinity. Together these studies provide new information on SHMT structure and function and inform the design of anti-folate agents., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Owuocha, Mitchum and Beamer.)

Two amino acid variants in soybean serine hydroxymethyltransferase 8 (SHMT8) are associated with resistance to the soybean cyst nematode (SCN), a devastating agricultural pathogen with worldwide economic impacts on soybean production. SHMT8 is a cytoplasmic enzyme that catalyzes the pyridoxal 5-phosphate-dependent conversion of serine and tetrahydrofolate (THF) to glycine and 5,10-methylenetetrahydrofolate. A previous study of the P130R/N358Y double variant of SHMT8, identified in the SCN-resistant soybean cultivar (cv.) Forrest, showed profound impairment of folate binding affinity and reduced THF-dependent enzyme activity, relative to the highly active SHMT8 in cv. Essex, which is susceptible to SCN. Given the importance of SCN-resistance in soybean agriculture, we report here the biochemical and structural characterization of the P130R and N358Y single variants to elucidate their individual effects on soybean SHMT8. We find that both single variants have reduced THF-dependent catalytic activity relative to Essex SHMT8 (10- to 50-fold decrease in k cat /K m ) but are significantly more active than the P130R/N368Y double variant. The kinetic data also show that the single variants lack THF-substrate inhibition as found in Essex SHMT8, an observation with implications for regulation of the folate cycle. Five crystal structures of the P130R and N358Y variants in complex with various ligands (resolutions from 1.49 to 2.30 Å) reveal distinct structural impacts of the mutations and provide new insights into allosterism. Our results support the notion that the P130R/N358Y double variant in Forrest SHMT8 produces unique and unexpected effects on the enzyme, which cannot be easily predicted from the behavior of the individual variants., (© 2023 Federation of European Biochemical Societies.)

Abnormal polyol metabolism is predominantly associated with diabetes, where excess glucose is converted to sorbitol by aldose reductase (AR). Recently, abnormal polyol metabolism has been implicated in phosphomannomutase 2 congenital disorder of glycosylation (PMM2-CDG) and an AR inhibitor, epalrestat, proposed as a potential therapy. Considering that the PMM2 enzyme is not directly involved in polyol metabolism, the increased polyol production and epalrestat's therapeutic mechanism in PMM2-CDG remained elusive. PMM2-CDG, caused by PMM2 deficiency, presents with depleted GDP-mannose and abnormal glycosylation. Here, we show that, apart from glycosylation abnormalities, PMM2 deficiency affects intracellular glucose flux, resulting in polyol increase. Targeting AR with epalrestat decreases polyols and increases GDP-mannose both in patient-derived fibroblasts and in pmm2 mutant zebrafish. Using tracer studies, we demonstrate that AR inhibition diverts glucose flux away from polyol production toward the synthesis of sugar nucleotides, and ultimately glycosylation. Finally, PMM2-CDG individuals treated with epalrestat show a clinical and biochemical improvement., Competing Interests: Declaration of interests Mayo Clinic and E.M. have a financial interest related to this research. This research has been reviewed by the Mayo Clinic Conflict of Interest Review Board and is being conducted in compliance with Mayo Clinic Conflict of Interest policies. E.M. has the following patents planned, issued, or pending: application title, “Methods and Materials for Treating Glycosylation Disorders”; application # 16/973,210; filing date, 12/08/2020; Mayo Case # 2018-132., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

Phosphoglucomutase 1 (PGM1) plays a central role in glucose homeostasis in human cells. Missense variants of this enzyme cause an inborn error of metabolism, which is categorized as a congenital disorder of glycosylation. Here, two disease-related variants of PGM1, T337M and G391V, which are both located in domain 3 of the four-domain protein, were characterized via X-ray crystallography and biochemical assays. The studies show multiple impacts resulting from these dysfunctional variants, including both short- and long-range structural perturbations. In the T337M variant these are limited to a small shift in an active-site loop, consistent with reduced enzyme activity. In contrast, the G391V variant produces a cascade of structural perturbations, including displacement of both the catalytic phosphoserine and metal-binding loops. This work reinforces several themes that were found in prior studies of dysfunctional PGM1 variants, including increased structural flexibility and the outsized impacts of mutations affecting interdomain interfaces. The molecular mechanisms of PGM1 variants have implications for newly described inherited disorders of related enzymes.

We describe the first reported case of transient distal ureteric obstruction attributed to post-surgical oedema in a patient with a solitary kidney. This occurred following combined pelvic floor repair and sacrospinous fixation for recurrent pelvic organ prolapse and manifested clinically as anuria, radiological hydroureter and acute kidney injury in the postoperative period. The transient nature of this obstruction, which was managed by a temporary percutaneous nephrostomy, indicates that it was caused by ureteric compression secondary to soft tissue oedema following surgery. We highlight the importance of this potential complication in females with a history of nephrectomy, unilateral renal tract anomalies or severely diminished renal reserve., Competing Interests: Competing interests: None declared., (© BMJ Publishing Group Limited 2021. No commercial re-use. See rights and permissions. Published by BMJ.)

Once experimentally prohibitive, structural studies of individual missense variants in proteins are increasingly feasible, and can provide a new level of insight into human genetic disease. One example of this is the recently identified inborn error of metabolism known as phosphoglucomutase-1 (PGM1) deficiency. Just as different variants of a protein can produce different patient phenotypes, they may also produce distinct biochemical phenotypes, affecting properties such as catalytic activity, protein stability, or 3D structure/dynamics. Experimental studies of missense variants, and particularly structural characterization, can reveal details of the underlying biochemical pathomechanisms of missense variants. Here, we review four examples of enzyme dysfunction observed in disease-related variants of PGM1. These studies are based on 11 crystal structures of wild-type (WT) and mutant enzymes, and multiple biochemical assays. Lessons learned include the value of comparing mutant and WT structures, synergy between structural and biochemical studies, and the rich understanding of molecular pathomechanism provided by experimental characterization relative to the use of predictive algorithms. We further note functional insights into the WT enzyme that can be gained from the study of pathogenic variants., Competing Interests: Declaration of competing interest The author declares no conflict of interest., (Copyright © 2020. Published by Elsevier B.V.)

The transcription factor, nuclear factor erythroid 2-related factor 2 (Nrf2), plays a major role in regulating the antioxidant defense system through the Kelch-like ECH-associated protein 1-Nrf2-antioxidant response element (Keap1-Nrf2-ARE) pathway. Small-molecule inhibitors targeting Keap1-Nrf2 protein-protein interaction (PPI) decrease the rate of Nrf2 degradation by the 26S proteasome and thus increase the intracellular level of Nrf2, which translocates into the nucleus, leading to upregulated expression of cytoprotective and antioxidant enzymes. Such inhibitors can be developed into potential preventive and therapeutic agents of diseases caused by oxidative damage. To more effectively identify promising Nrf2 activators through the inhibition of Keap1-Nrf2 PPI, a homogeneous time-resolved fluorescence resonance energy transfer (TR-FRET) assay was developed in this work by indirectly labeling the Keap1 Kelch domain protein with Tb-anti-His antibody as the donor and using, as the acceptor, fluorescein isothiocyanate (FITC)-labeled 9mer Nrf2 peptide amide, the same fluorescent probe that was used in an earlier fluorescence polarization (FP) assay. Assay conditions, including concentrations of the various components, buffer type, and incubation time, were optimized in the TR-FRET competition assay with known small-molecule inhibitors of Keap1-Nrf2 PPI. Under the optimized conditions, the Keap1-Nrf2 TR-FRET assay exhibited great sensitivity with a high dynamic range and considerable stability for as long as 5 h. The Z' factor was determined to be 0.82, suggesting that the assay is suitable for high-throughput screening and lead optimization of inhibitors of Keap1-Nrf2 PPI. Furthermore, the TR-FRET assay is capable of differentiating potent inhibitors of Keap1-Nrf2 PPI down to the subnanomolar inhibition constant ( K i ) range.

Missense variants of human phosphoglucomutase 1 (PGM1) cause the inherited metabolic disease known as PGM1 deficiency. This condition is categorised as both a glycogen storage disease and a congenital disorder of glycosylation. Approximately 20 missense variants of PGM1 are linked to PGM1 deficiency, and biochemical studies have suggested that they fall into two general categories: those affecting the active site and catalytic efficiency, and those that appear to impair protein folding and/or stability. In this study, we characterise a novel variant of Arg422, a residue distal from the active site of PGM1 and the site of a previously identified disease-related variant (Arg422Trp). In prior studies, the R422W variant was found to produce insoluble protein in a recombinant expression system, precluding further in vitro characterisation. Here we investigate an alternative variant of this residue, Arg422Gln, which is amenable to experimental characterisation presumably due to its more conservative physicochemical substitution. Biochemical, crystallographic, and computational studies of R422Q establish that this variant causes only minor changes in catalytic efficiency and 3D structure, but is nonetheless dramatically reduced in stability. Unexpectedly, binding of a substrate analog is found to further destabilise the protein, in contrast to its stabilising effect on wild-type PGM1 and several other missense variants. This work establishes Arg422 as a lynchpin residue for the stability of PGM1 and supports the impairment of protein stability as a pathomechanism for variants that cause PGM1 deficiency. SYNOPSIS: Biochemical and structural studies of a missense variant far from the active site of human PGM1 identify a residue with a key role in enzyme stability., (© 2020 SSIEM.)

Management of the agricultural pathogen soybean cyst nematode (SCN) relies on the use of SCN-resistant soybean cultivars, a strategy that has been failing in recent years. An underutilized source of resistance in the soybean genotype Peking is linked to two polymorphisms in serine hydroxy-methyltransferase 8 (SHMT8). SHMT is a pyridoxal 5'-phosphate-dependent enzyme that converts l-serine and (6 S )-tetrahydrofolate to glycine and 5,10-methylenetetrahydrofolate. Here, we determined five crystal structures of the 1884-residue SHMT8 tetramers from the SCN-susceptible cultivar (cv.) Essex and the SCN-resistant cv. Forrest (whose resistance is derived from the SHMT8 polymorphisms in Peking); the crystal structures were determined in complex with various ligands at 1.4-2.35 Å resolutions. We find that the two Forrest-specific polymorphic substitutions (P130R and N358Y) impact the mobility of a loop near the entrance of the (6 S )-tetrahydrofolate-binding site. Ligand-binding and kinetic studies indicate severely reduced affinity for folate and dramatically impaired enzyme activity in Forrest SHMT8. These findings imply widespread effects on folate metabolism in soybean cv. Forrest that have implications for combating the widespread increase in virulent SCN., (© 2020 Korasick et al.)

α-Phosphomannomutase/phosphoglucomutase (αPMM/PGM) from P. aeruginosa is involved in bacterial cell wall assembly and is implicated in P. aeruginosa virulence, yet few studies have addressed αPMM/PGM inhibition from this important Gram-negative bacterial human pathogen. Four structurally different α-d-glucopyranose 1-phosphate (αG1P) derivatives including 1- C -fluoromethylated analogues ( 1 - 3 ), 1,2-cyclic phosph(on)ate analogues ( 4 - 6 ), isosteric methylene phosphono analogues ( 7 and 8 ), and 6-fluoro-αG1P ( 9 ), were synthesized and assessed as potential time-dependent or reversible αPMM/PGM inhibitors. The resulting kinetic data were consistent with the crystallographic structures of the highly homologous Xanthomonas citri αPGM with inhibitors 3 and 7 - 9 binding to the enzyme active site (1.65-1.9 Å). These structural and kinetic insights will enhance the design of future αPMM/PGM inhibitors.

Glucokinase phosphorylated a series of C-1 fluorinated α-d-gluco-heptuloses. These phosphorylated products were discovered to be inhibitors of α-phosphomannomutase/phosphoglucomutase (αPMM/PGM) and β-phosphoglucomutase (βPGM). Inhibition potency with both mutases inversely correlated to the degree of fluorination. Structural analysis with αPMM demonstrated the inhibitor binding to the active site, with the phosphate in the phosphate binding site and the anomeric hydroxyl directed to the catalytic site., Competing Interests: The authors declare no competing financial interest.

Enzymes are known to adopt various conformations at different points along their catalytic cycles. Here, we present a comprehensive analysis of 15 isomorphous, high resolution crystal structures of the enzyme phosphoglucomutase from the bacterium Xanthomonas citri . The protein was captured in distinct states critical to function, including enzyme-substrate, enzyme-product, and enzyme-intermediate complexes. Key residues in ligand recognition and regions undergoing conformational change are identified and correlated with the various steps of the catalytic reaction. In addition, we use principal component analysis to examine various subsets of these structures with two goals: (1) identifying sites of conformational heterogeneity through a comparison of room temperature and cryogenic structures of the apo-enzyme and (2) a priori clustering of the enzyme-ligand complexes into functionally related groups, showing sensitivity of this method to structural features difficult to detect by traditional methods. This study captures, in a single system, the structural basis of diverse substrate recognition, the subtle impact of covalent modification, and the role of ligand-induced conformational change in this representative enzyme of the α-D-phosphohexomutase superfamily.

Human phosphoglucomutase 1 (PGM1) plays a central role in cellular glucose homeostasis, catalyzing the conversion of glucose 1-phosphate and glucose 6-phosphate. Recently, missense variants of this enzyme were identified as causing an inborn error of metabolism, PGM1 deficiency, with features of a glycogen storage disease and a congenital disorder of glycosylation. Previous studies of selected PGM1 variants have revealed various mechanisms for enzyme dysfunction, including regions of structural disorder and side-chain rearrangements within the active site. Here, we examine variants within a substrate-binding loop in domain 4 (D4) of PGM1 that cause extreme impairment of activity. Biochemical, structural, and computational studies demonstrate multiple detrimental impacts resulting from these variants, including loss of conserved ligand-binding interactions and reduced mobility of the D4 loop, due to perturbation of its conformational ensemble. These potentially synergistic effects make this conserved ligand-binding loop a hotspot for disease-related variants in PGM1 and related enzymes., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Enzymes in the α-d-phosphohexomutase (PHM) superfamily catalyze a multistep reaction, entailing two successive phosphoryl transfers. Key to this reaction is a conserved phosphoserine in the active site, which serves alternately as a phosphoryl donor and acceptor during the catalytic cycle. In addition to its role in the enzyme mechanism, the phosphorylation state of the catalytic phosphoserine has recently been found to have widespread effects on the structural flexibility of enzymes in this superfamily. These effects must be carefully accounted for when assessing other perturbations to these enzymes, such as mutations or ligand binding. In this chapter, we focus on methods for assessing and modulating the phosphorylation state of the catalytic serine, as well as straightforward ways to probe the impacts of this modification on protein structure/flexibility. This knowledge is essential for producing homogeneous and stable samples of these proteins for biophysical studies. The methods described herein should be widely applicable to enzymes across the PHM superfamily and may also be useful in characterizing the effects of posttranslational modifications on other proteins., (© 2018 Elsevier Inc. All rights reserved.)

Phosphoglucosamine mutase (PNGM) is an evolutionarily conserved bacterial enzyme in the peptidoglycan biosynthetic pathway, catalyzing the reversible conversion between glucosamine 1- and 6-phosphate. Previous structural studies of PNGM from the pathogen Bacillus anthracis revealed its dimeric assembly and highlighted the rotational mobility of its C-terminal domain. Recent studies of two other enzymes in the same superfamily have demonstrated the long-range effects on the conformational flexibility associated with phosphorylation of the conserved, active site phosphoserine involved in phosphoryl transfer. Building on this work, we use a combination of experimental and computational studies to show that the active, phosphorylated version of B. anthracis PNGM has decreased flexibility relative to its inactive, dephosphorylated state. Limited proteolysis reveals an enhanced and accelerated cleavage of the dephosphorylated enzyme. 15 N transverse relaxation-optimized NMR spectra corroborate a conformational adjustment with broadening and shifts of peaks relative to the phospho-enzyme. Electrostatic calculations indicate that residues in the mobile, C-terminal domain are linked to the phosphoserine by lines of attraction that are absent in the dephosphorylated enzyme. Phosphorylation-dependent changes in protein flexibility appear linked with the conformational change and enzyme mechanism in PNGM, establishing this as a conserved theme in multiple subgroups of the diverse α-d-phosphohexomutase superfamily., Competing Interests: The authors declare no competing financial interest.

The key metabolic enzyme phosphoglucomutase 1 (PGM1) controls glucose homeostasis in most human cells. Four proteins related to PGM1, known as PGM2, PGM2L1, PGM3 and PGM5, and referred to herein as paralogs, are encoded in the human genome. Although all members of the same enzyme superfamily, these proteins have distinct substrate preferences and different functional roles. The recent association of PGM1 and PGM3 with inherited enzyme deficiencies prompts us to revisit sequence-structure and other relationships among the PGM1 paralogs, which are understudied despite their importance in human biology. Using currently available sequence, structure, and expression data, we investigated evolutionary relationships, tissue-specific expression profiles, and the amino acid preferences of key active site motifs. Phylogenetic analyses indicate both ancient and more recent divergence between the different enzyme sub-groups comprising the human paralogs. Tissue-specific protein and RNA expression profiles show widely varying patterns for each paralog, providing insight into function and disease pathology. Multiple sequence alignments confirm high conservation of key active site regions, but also reveal differences related to substrate specificity. In addition, we find that sequence variants of PGM2, PGM2L1, and PGM5 verified in the human population affect residues associated with disease-related mutants in PGM1 or PGM3. This suggests that inherited diseases related to dysfunction of these paralogs will likely occur in humans.

Enzymes sample multiple conformations during their catalytic cycles. Chemical shifts from Nuclear Magnetic Resonance (NMR) are hypersensitive to conformational changes and ensembles in solution. Phosphomannomutase/phosphoglucomutase (PMM/PGM) is a ubiquitous four-domain enzyme that catalyzes phosphoryl transfer across phosphohexose substrates. We compared states the enzyme visits during its catalytic cycle. Collective responses of Pseudomonas PMM/PGM to phosphosugar substrates and inhibitor were assessed using NMR-detected titrations. Affinities were estimated from binding isotherms obtained by principal component analysis (PCA). Relationships among phosphosugar-enzyme associations emerge from PCA comparisons of the titrations. COordiNated Chemical Shifts bEhavior (CONCISE) analysis provides novel discrimination of three ligand-bound states of PMM/PGM harboring a mutation that suppresses activity. Enzyme phosphorylation and phosphosugar binding appear to drive the open dephosphorylated enzyme to the free phosphorylated state, and on toward ligand-closed states. Domain 4 appears central to collective responses to substrate and inhibitor binding. Hydrogen exchange reveals that binding of a substrate analogue enhances folding stability of the domains to a uniform level, establishing a globally unified structure. CONCISE and PCA of NMR spectra have discovered novel states of a well-studied enzyme and appear ready to discriminate other enzyme and ligand binding states.

The bifunctional flavoenzyme proline utilization A (PutA) catalyzes the two-step oxidation of proline to glutamate using separate proline dehydrogenase (PRODH) and l-glutamate-γ-semialdehyde dehydrogenase active sites. Because PutAs catalyze sequential reactions, they are good systems for studying how metabolic enzymes communicate via substrate channeling. Although mechanistically similar, PutAs vary widely in domain architecture, oligomeric state, and quaternary structure, and these variations represent different structural solutions to the problem of sequestering a reactive metabolite. Here, we studied PutA from Corynebacterium freiburgense (CfPutA), which belongs to the uncharacterized 3B class of PutAs. A 2.7 Å resolution crystal structure showed the canonical arrangement of PRODH, l-glutamate-γ-semialdehyde dehydrogenase, and C-terminal domains, including an extended interdomain tunnel associated with substrate channeling. The structure unexpectedly revealed a novel open conformation of the PRODH active site, which is interpreted to represent the non-activated conformation, an elusive form of PutA that exhibits suboptimal channeling. Nevertheless, CfPutA exhibited normal substrate-channeling activity, indicating that it isomerizes into the active state under assay conditions. Sedimentation-velocity experiments provided insight into the isomerization process, showing that CfPutA dimerizes in the presence of a proline analog and NAD + These results are consistent with the morpheein model of enzyme hysteresis, in which substrate binding induces conformational changes that promote assembly of a high-activity oligomer. Finally, we used domain deletion analysis to investigate the function of the C-terminal domain. Although this domain contains neither catalytic residues nor substrate sites, its removal impaired both catalytic activities, suggesting that it may be essential for active-site integrity., (© 2017 by The American Society for Biochemistry and Molecular Biology, Inc.)

The enzyme phosphoglucomutase 1 (PGM1) plays a central role in glucose homeostasis. Clinical studies have identified mutations in human PGM1 as the cause of PGM1 deficiency, an inherited metabolic disease. One residue, Asp263, has two known variants associated with disease: D263G and D263Y. Biochemical studies have shown that these mutants are soluble and well folded, but have significant catalytic impairment. To better understand this catalytic defect, we determined crystal structures of these two missense variants, both of which reveal a similar and indirect structural change due to the loss of a conserved salt bridge between Asp263 and Arg293. The arginine reorients into the active site, making interactions with residues responsible for substrate binding. Biochemical studies also show that the catalytic phosphoserine of the missense variants is more stable to hydrolysis relative to wild-type enzyme. The structural perturbation resulting from mutation of this single amino acid reveals the molecular mechanism underlying PGM1 deficiency in these missense variants., Database: Structural data are available in the PDB under the accession numbers 5JN5 and 5TR2., (© 2017 Federation of European Biochemical Societies.)

Enzymes in the α-d-phosphohexomutases superfamily catalyze the reversible conversion of phosphosugars, such as glucose 1-phosphate and glucose 6-phosphate. These reactions are fundamental to primary metabolism across the kingdoms of life and are required for a myriad of cellular processes, ranging from exopolysaccharide production to protein glycosylation. The subject of extensive mechanistic characterization during the latter half of the 20th century, these enzymes have recently benefitted from biophysical characterization, including X-ray crystallography, NMR, and hydrogen-deuterium exchange studies. This work has provided new insights into the unique catalytic mechanism of the superfamily, shed light on the molecular determinants of ligand recognition, and revealed the evolutionary conservation of conformational flexibility. Novel associations with inherited metabolic disease and the pathogenesis of bacterial infections have emerged, spurring renewed interest in the long-appreciated functional roles of these enzymes., (© 2017 Elsevier Inc. All rights reserved.)

Most enzymes in the α-D-phosphohexomutase superfamily catalyze the reversible conversion of 1- to 6-phosphosugars. They play important roles in carbohydrate and sugar nucleotide metabolism, and participate in the biosynthesis of polysaccharides, glycolipids, and other exoproducts. Mutations in genes encoding these enzymes are associated with inherited metabolic diseases in humans, including glycogen storage disease and congenital disorders of glycosylation. Enzymes in the superfamily share a highly conserved active site serine that participates in the multi-step phosphoryl transfer reaction. Here we provide data on the effects of various phosphosugar ligands on the phosphorylation of this serine, as monitored by electrospray ionization mass spectrometry (ESI-MS) data on the intact proteins. We also show data on the longevity of the phospho-enzyme under various solution conditions in one member of the superfamily from Pseudomonas aeruginosa , and present inhibition data for several ligands. These data should be useful for the production of homogeneous samples of phosphorylated or unphosphorylated proteins, which are essential for biophysical characterization of these enzymes.

This paper describes the introduction of synchrotron-based macromolecular crystallography (MX) into an undergraduate laboratory class. An introductory 2 week experimental module on MX, consisting of four laboratory sessions and two classroom lectures, was incorporated into a senior-level biochemistry class focused on a survey of biochemical techniques, including the experimental characterization of proteins. Students purified recombinant protein samples, set up crystallization plates and flash-cooled crystals for shipping to a synchrotron. Students then collected X-ray diffraction data sets from their crystals via the remote interface of the Molecular Biology Consortium beamline (4.2.2) at the Advanced Light Source in Berkeley, CA, USA. Processed diffraction data sets were transferred back to the laboratory and used in conjunction with partial protein models provided to the students for refinement and model building. The laboratory component was supplemented by up to 2 h of lectures by faculty with expertise in MX. This module can be easily adapted for implementation into other similar undergraduate classes, assuming the availability of local crystallographic expertise and access to remote data collection at a synchrotron source.

Objective: To define phenotypic groups and identify predictors of disease severity in patients with phosphoglucomutase-1 deficiency (PGM1-CDG)., Study Design: We evaluated 27 patients with PGM1-CDG who were divided into 3 phenotypic groups, and group assignment was validated by a scoring system, the Tulane PGM1-CDG Rating Scale (TPCRS). This scale evaluates measurable clinical features of PGM1-CDG. We examined the relationship between genotype, enzyme activity, and TPCRS score by using regression analysis. Associations between the most common clinical features and disease severity were evaluated by principal component analysis., Results: We found a statistically significant stratification of the TPCRS scores among the phenotypic groups (P < .001). Regression analysis showed that there is no significant correlation between genotype, enzyme activity, and TPCRS score. Principal component analysis identified 5 variables that contributed to 54% variance in the cohort and are predictive of disease severity: congenital malformation, cardiac involvement, endocrine deficiency, myopathy, and growth., Conclusions: We established a scoring algorithm to reliably evaluate disease severity in patients with PGM1-CDG on the basis of their clinical history and presentation. We also identified 5 clinical features that are predictors of disease severity; 2 of these features can be evaluated by physical examination, without the need for specific diagnostic testing and thus allow for rapid assessment and initiation of therapy., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Human phosphoglucomutase 1 (PGM1) plays a central role in cellular glucose homeostasis, mediating the switch between glycolysis and gluconeogenesis through the conversion of glucose 1-phosphate and glucose 6-phosphate. Recent clinical studies have identified mutations in this enzyme as the cause of PGM1 deficiency, an inborn error of metabolism classified as both a glycogen storage disease and a congenital disorder of glycosylation. Reported here are the first crystal structures of two disease-related missense variants of PGM1, along with the structure of the wild-type enzyme. Two independent glycine-to-arginine substitutions (G121R and G291R), both affecting key active site loops of PGM1, are found to induce regions of structural disorder, as evidenced by a nearly complete loss of electron density for as many as 23 aa. The disordered regions are not contiguous in sequence to the site of mutation, and even cross domain boundaries. Other structural rearrangements include changes in the conformations of loops and side chains, some of which occur nearly 20 Å away from the site of mutation. The induced structural disorder is correlated with increased sensitivity to proteolysis and lower-resolution diffraction, particularly for the G291R variant. Examination of the multi-domain effects of these G➔R mutations establishes a correlation between interdomain interfaces of the enzyme and missense variants of PGM1 associated with disease. These crystal structures provide the first insights into the structural basis of enzyme dysfunction in PGM1 deficiency and highlight a growing role for biophysical characterization of proteins in the field of precision medicine., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Recent studies have identified phosphoglucomutase 1 (PGM1) deficiency as an inherited metabolic disorder in humans. PGM1 deficiency is classified as both a muscle glycogenosis (type XIV) and a congenital disorder of glycosylation of types I and II. Affected patients show multiple disease phenotypes, reflecting the central role of the enzyme in glucose homeostasis, where it catalyzes the interconversion of glucose 1-phosphate and glucose 6-phosphate. The influence of PGM1 deficiency on protein glycosylation patterns is also widespread, affecting both biosynthesis and processing of glycans and their precursors. To date, 21 different mutations involved in PGM1 deficiency have been identified, including 13 missense mutations resulting in single amino acid changes. Growing clinical interest in PGM1 deficiency prompts a review of the molecular context of these mutations in the three-dimensional structure of the protein. Here the known crystal structure of PGM from rabbit (97 % sequence identity to human) is used to analyze the mutations associated with disease and find that many map to regions with clear significance to enzyme function. In particular, amino acids in and around the active site cleft are frequently involved, including regions responsible for catalysis, binding of the metal ion required for activity, and interactions with the phosphosugar substrate. Several of the known mutations, however, are distant from the active site and appear to manifest their effects indirectly. An understanding of how the different mutations that cause PGM1 deficiency affect enzyme structure and function is foundational to providing clinical prognosis and the development of effective treatment strategies.

Phosphorylation can modulate the activities of enzymes. The phosphoryl donor in the catalytic cleft of α-D-phosphohexomutases is transiently dephosphorylated while the reaction intermediate completes a 180° reorientation within the cleft. The phosphorylated form of 52 kDa bacterial phosphomannomutase/phosphoglucomutase is less accessible to dye or protease, more stable to chemical denaturation, and widely stabilized against NMR-detected hydrogen exchange across the core of domain 3 to juxtaposed domain 4 (each by ≥ 1.3 kcal/mol) and parts of domains 1 and 2. However, phosphorylation accelerates hydrogen exchange in specific regions of domains 1 and 2, including a metal-binding residue in the active site. Electrostatic field lines reveal attraction across the catalytic cleft between phosphorylated Ser-108 and domain 4, but repulsion when Ser-108 is dephosphorylated. Molecular dynamics (MD) simulated the dephosphorylated form to be expanded due to enhanced rotational freedom of domain 4. The contacts and fluctuations of the MD trajectories enabled correct simulation of more than 80% of sites that undergo either protection or deprotection from hydrogen exchange due to phosphorylation. Electrostatic attraction in the phosphorylated enzyme accounts for 1) domain 4 drawing closer to domains 1 and 3; 2) decreased accessibility; and 3) increased stability within these domains. The electrostriction due to phosphorylation may help capture substrate, whereas the opening of the cleft upon transient dephosphorylation allows rotation of the intermediate. The long-range effects of phosphorylation on hydrogen exchange parallel reports on protein kinases, suggesting a conceptual link among these multidomain, phosphoryl transfer enzymes., (Copyright © 2015 Biophysical Society. Published by Elsevier Inc. All rights reserved.)

Recent studies have identified phosphoglucomutase 1 (PGM1) deficiency as an inherited metabolic disorder in humans. Affected patients show multiple disease phenotypes, including dilated cardiomyopathy, exercise intolerance, and hepatopathy, reflecting the central role of the enzyme in glucose metabolism. We present here the first in vitro biochemical characterization of 13 missense mutations involved in PGM1 deficiency. The biochemical phenotypes of the PGM1 mutants cluster into two groups: those with compromised catalysis and those with possible folding defects. Relative to the recombinant wild-type enzyme, certain missense mutants show greatly decreased expression of soluble protein and/or increased aggregation. In contrast, other missense variants are well behaved in solution, but show dramatic reductions in enzyme activity, with kcat/Km often <1.5% of wild-type. Modest changes in protein conformation and flexibility are also apparent in some of the catalytically impaired variants. In the case of the G291R mutant, severely compromised activity is linked to the inability of a key active site serine to be phosphorylated, a prerequisite for catalysis. Our results complement previous in vivo studies, which suggest that both protein misfolding and catalytic impairment may play a role in PGM1 deficiency., (© 2014 by The American Society for Biochemistry and Molecular Biology, Inc.)

A domain needed for the catalytic efficiency of an enzyme model of simple processivity and domain-domain interactions has been characterized by NMR. This domain 4 from phosphomannomutase/phosphoglucomutase (PMM/PGM) closes upon glucose phosphate and mannose phosphate ligands in the active site, and can modestly reconstitute activity of enzyme truncated to domains 1-3. This enzyme supports biosynthesis of the saccharide-derived virulence factors (rhamnolipids, lipopolysaccharides, and alginate) of the opportunistic bacterial pathogen Pseudomonas aeruginosa. (1)H, (13)C, and (15)N NMR chemical shift assignments of domain 4 of PMM/PGM suggest preservation and independence of its structure when separated from domains 1-3. The face of domain 4 that packs with domain 3 is perturbed in NMR spectra without disrupting this fold. The perturbed residues overlap both the most highly coevolved positions in the interface and residues lining a cavity at the domain interface.

The enzyme phosphomannomutase/phosphoglucomutase (PMM/PGM) from Pseudomonas aeruginosa catalyzes an intramolecular phosphoryl transfer across its phosphosugar substrates, which are precursors in the synthesis of exoproducts involved in bacterial virulence. Previous structural studies of PMM/PGM have established a key role for conformational change in its multistep reaction, which requires a dramatic 180° reorientation of the intermediate within the active site. Here hydrogen-deuterium exchange by mass spectrometry and small angle x-ray scattering were used to probe the conformational flexibility of different forms of PMM/PGM in solution, including its active, phosphorylated state and the unphosphorylated state that occurs transiently during the catalytic cycle. In addition, the effects of ligand binding were assessed through use of a substrate analog. We found that both phosphorylation and binding of ligand produce significant effects on deuterium incorporation. Phosphorylation of the conserved catalytic serine has broad effects on residues in multiple domains and is supported by small angle x-ray scattering data showing that the unphosphorylated enzyme is less compact in solution. The effects of ligand binding are generally manifested near the active site cleft and at a domain interface that is a site of conformational change. These results suggest that dephosphorylation of the enzyme may play two critical functional roles: a direct role in the chemical step of phosphoryl transfer and secondly through propagation of structural flexibility. We propose a model whereby increased enzyme flexibility facilitates the reorientation of the reaction intermediate, coupling changes in structural dynamics with the unique catalytic mechanism of this enzyme.

Unlabelled: Enzymes in the α-d-phosphohexomutase superfamily catalyze the conversion of 1-phosphosugars to their 6-phospho counterparts. Their phosphoryl transfer reaction has long been proposed to require general acid-base catalysts, but candidate residues for these key roles have not been identified. In this study, we show through mutagenesis and kinetic studies that a histidine (His329) in the active site is critical for enzyme activity in a well-studied member of the superfamily, phosphomannomutase/phosphoglucomutase from Pseudomonas aeruginosa. Crystallographic characterization of an H329A mutant protein showed no significant changes from the wild-type enzyme, excluding structural disruption as the source of its compromised activity. Mutation of the structurally analogous lysine residue in a related protein, phosphoglucomutase from Salmonella typhimurium, also results in significant catalytic impairment. Analyses of protein-ligand complexes of the P. aeruginosa enzyme show that His329 is appropriately positioned to abstract a proton from the O1/O6 hydroxyl of the phosphosugar substrates, and thus may serve as the general base in the reaction. Histidine is strongly conserved at this position in many proteins in the superfamily, and lysine is also often conserved at a structurally corresponding position, particularly in the phosphoglucomutase enzyme sub-group. These studies shed light on the mechanism of this important enzyme superfamily, and may facilitate the design of mechanism-based inhibitors., Database: Structural data have been deposited in the Protein Data Bank with accession number 4IL8., (© 2013 The Authors Journal compilation © 2013 FEBS.)

A high-throughput screen (HTS) of the MLPCN library using a homogenous fluorescence polarization assay identified a small molecule as a first-in-class direct inhibitor of Keap1-Nrf2 protein-protein interaction. The HTS hit has three chiral centers; a combination of flash and chiral chromatographic separation demonstrated that Keap1-binding activity resides predominantly in one stereoisomer (SRS)-5 designated as ML334 (LH601A), which is at least 100× more potent than the other stereoisomers. The stereochemistry of the four cis isomers was assigned using X-ray crystallography and confirmed using stereospecific synthesis. (SRS)-5 is functionally active in both an ARE gene reporter assay and an Nrf2 nuclear translocation assay. The stereospecific nature of binding between (SRS)-5 and Keap1 as well as the preliminary but tractable structure-activity relationships support its use as a lead for our ongoing optimization., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

The α-d-phosphohexomutase superfamily comprises enzymes involved in carbohydrate metabolism that are found in all kingdoms of life. Recent biophysical studies have shown for the first time that several of these enzymes exist as dimers in solution, prompting an examination of the oligomeric state of all proteins of known structure in the superfamily (11 different proteins; 31 crystal structures) via computational and experimental analyses. We find that these proteins range in quaternary structure from monomers to tetramers, with 6 of the 11 known structures being likely oligomers. The oligomeric state of these proteins not only is associated in some cases with enzyme subgroup (i.e., substrate specificity) but also appears to depend on domain of life, with the two archaeal proteins existing as higher-order oligomers. Within the oligomers, three distinct interfaces are observed, one of which is found in both archaeal and bacterial proteins. Normal mode analysis shows that the topological arrangement of the oligomers permits domain 4 of each protomer to move independently as required for catalysis. Our analysis suggests that the advantages associated with protein flexibility in this enzyme family are of sufficient importance to be maintained during the evolution of multiple independent oligomers. This study is one of the first showing that global motions may be conserved not only within protein families but also across members of a superfamily with varying oligomeric structures., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Activation of the antioxidant response element (ARE) upregulates enzymes involved in detoxification of electrophiles and reactive oxygen species. The induction of ARE genes is regulated by the interaction between redox sensor protein Keap1 and the transcription factor Nrf2. Fluorescently labeled Nrf2 peptides containing the ETGE motif were synthesized and optimized as tracers in the development of a fluorescence polarization (FP) assay to identify small-molecule inhibitors of the Keap1-Nrf2 interaction. The tracers were optimized to increase the dynamic range of the assay and their binding affinities to the Keap1 Kelch domain. The binding affinities of Nrf2 peptide inhibitors obtained in our FP assay using FITC-9mer Nrf2 peptide amide as the probe were in good agreement with those obtained previously by a surface plasmon resonance assay. The FP assay exhibits considerable tolerance toward DMSO and produced a Z' factor greater than 0.6 in a 384-well format. Further optimization of the probe led to cyanine-labeled 9mer Nrf2 peptide amide, which can be used along with the FITC-9mer Nrf2 peptide amide in a high-throughput screening assay to discover small-molecule inhibitors of Keap1-Nrf2 interaction.

Phosphomannomutase/phosphoglucomutase contributes to the infectivity of Pseudomonas aeruginosa, retains and reorients its intermediate by 180°, and rotates domain 4 to close the deep catalytic cleft. Nuclear magnetic resonance (NMR) spectra of the backbone of wild-type and S108C-inactivated enzymes were assigned to at least 90%. (13)C secondary chemical shifts report excellent agreement of solution and crystallographic structure over the 14 α-helices, C-capping motifs, and 20 of the 22 β-strands. Major and minor NMR peaks implicate substates affecting 28% of assigned residues. These can be attributed to the phosphorylation state and possibly to conformational interconversions. The S108C substitution of the phosphoryl donor and acceptor slowed transformation of the glucose 1-phosphate substrate by impairing k(cat). Addition of the glucose 1,6-bisphosphate intermediate accelerated this reaction by 2-3 orders of magnitude, somewhat bypassing the defect and apparently relieving substrate inhibition. The S108C mutation perturbs the NMR spectra and electron density map around the catalytic cleft while preserving the secondary structure in solution. Diminished peak heights and faster (15)N relaxation suggest line broadening and millisecond fluctuations within four loops that can contact phosphosugars. (15)N NMR relaxation and peak heights suggest that domain 4 reorients slightly faster in solution than domains 1-3, and with a different principal axis of diffusion. This adds to the crystallographic evidence of domain 4 rotations in the enzyme, which were previously suggested to couple to reorientation of the intermediate, substrate binding, and product release.

Coevolution analyses identify residues that co-vary with each other during evolution, revealing sequence relationships unobservable from traditional multiple sequence alignments. Here we describe a coevolutionary analysis of phosphomannomutase/phosphoglucomutase (PMM/PGM), a widespread and diverse enzyme family involved in carbohydrate biosynthesis. Mutual information and graph theory were utilized to identify a network of highly connected residues with high significance. An examination of the most tightly connected regions of the coevolutionary network reveals that most of the involved residues are localized near an interdomain interface of this enzyme, known to be the site of a functionally important conformational change. The roles of four interface residues found in this network were examined via site-directed mutagenesis and kinetic characterization. For three of these residues, mutation to alanine reduces enzyme specificity to ~10% or less of wild-type, while the other has ~45% activity of wild-type enzyme. An additional mutant of an interface residue that is not densely connected in the coevolutionary network was also characterized, and shows no change in activity relative to wild-type enzyme. The results of these studies are interpreted in the context of structural and functional data on PMM/PGM. Together, they demonstrate that a network of coevolving residues links the highly conserved active site with the interdomain conformational change necessary for the multi-step catalytic reaction. This work adds to our understanding of the functional roles of coevolving residue networks, and has implications for the definition of catalytically important residues.

Phosphoglucosamine mutase (PNGM) is a bacterial enzyme that participates in the peptidoglycan biosynthetic pathway. Recent crystal structures of PNGM from two bacterial pathogens, Bacillus anthracis and Francisella tularensis, have revealed key structural features of this enzyme for the first time. Here, we follow up on several novel findings from the crystallographic studies, including the observation of a structurally conserved interface between polypeptide chains and conformational variability of the C-terminal domain. Small-angle X-ray scattering of B. anthracis PNGM shows that this protein is a dimer in solution. Comparisons of the four independent polypeptide chains from the two structures reveals conserved residues and structural changes involved in the conformational variability, as well as a significant rotation of the C-terminal domain, of nearly 60°, between the most divergent conformers. Furthermore, the fluctuation dynamics of PNGM are examined via normal mode analyses. The most mobile region of the protein is its C-terminal domain, consistent with observations from the crystal structures. Large regions of correlated, collective motions are identified exclusively for the dimeric state of the protein, comprising both contiguous and noncontiguous structural domains. The motions observed in the lowest frequency normal mode of the dimer result in dynamically coupled opening and closing of the two active sites. The global motions identified in this study support the importance of the conformational change of PNGM in function, and suggest that the dimeric state of this protein may confer advantages consistent with its evolutionary conservation., (© 2011 The Authors Journal compilation © 2011 FEBS.)

Phosphoglucosamine mutase (PNGM) is an evolutionarily conserved bacterial enzyme that participates in the cytoplasmic steps of peptidoglycan biosynthesis. As peptidoglycan is essential for bacterial survival and is absent in humans, enzymes in this pathway have been the focus of intensive inhibitor design efforts. Many aspects of the structural biology of the peptidoglycan pathway have been elucidated, with the exception of the PNGM structure. We present here the crystal structure of PNGM from the human pathogen and bioterrorism agent Bacillus anthracis. The structure reveals key residues in the large active site cleft of the enzyme which likely have roles in catalysis and specificity. A large conformational change of the C-terminal domain of PNGM is observed when comparing two independent molecules in the crystal, shedding light on both the apo- and ligand-bound conformers of the enzyme. Crystal packing analyses and dynamic light scattering studies suggest that the enzyme is a dimer in solution. Multiple sequence alignments show that residues in the dimer interface are conserved, suggesting that many PNGM enzymes adopt this oligomeric state. This work lays the foundation for the development of inhibitors for PNGM enzymes from human pathogens.

The crystal structure of the enzyme phosphoglucomutase from Salmonella typhimurium (StPGM) is reported at 1.7 A resolution. This is the first high-resolution structural characterization of a bacterial protein from this large enzyme family, which has a central role in metabolism and is also important to bacterial virulence and infectivity. A comparison of the active site of StPGM with that of other phosphoglucomutases reveals conserved residues that are likely involved in catalysis and ligand binding for the entire enzyme family. An alternate crystal form of StPGM and normal mode analysis give insights into conformational changes of the C-terminal domain that occur upon ligand binding. A novel observation from the StPGM structure is an apparent dimer in the asymmetric unit of the crystal, mediated largely through contacts in an N-terminal helix. Analytical ultracentrifugation and small-angle X-ray scattering confirm that StPGM forms a dimer in solution. Multiple sequence alignments and phylogenetic studies show that a distinct subset of bacterial PGMs share the signature dimerization helix, while other bacterial and eukaryotic PGMs are likely monomers. These structural, biochemical, and bioinformatic studies of StPGM provide insights into the large α-D-phosphohexomutase enzyme superfamily to which it belongs, and are also relevant to the design of inhibitors specific to the bacterial PGMs., (Copyright © 2010 Wiley-Liss, Inc.)

The aim of this article is to analyze conformational changes by comparing 10 different structures of Pseudomonas aeruginosa phosphomannomutase/phosphoglucomutase (PMM/PGM), a four-domain enzyme in which both substrate binding and catalysis require substantial movement of the C-terminal domain. We focus on changes in interdomain and active site crevices using a method called computational solvent mapping rather than superimposing the structures. The method places molecular probes (i.e., small organic molecules containing various functional groups) around the protein to find hot spots. One of the most important hot spots is in the active site, consistent with the ability of the enzyme to bind both glucose and mannose phosphosugar substrates. The protein has eight additional hot spots at domain-domain interfaces and hinge regions. The locations and nature of six of these hot spots vary between the open, half-open, and closed conformers of the enzyme, in good agreement with the ligand-induced conformational changes. In the closed structures the number of probe clusters at the hinge region significantly depends on the position of the phosphorylated oxygen in the substrate (e.g., glucose 1-phosphate versus glucose 6-phosphate), but the protein remains almost unchanged in terms of the overall RMSD, indicating that computational solvent mapping is a more sensitive approach to detect changes in binding sites and interdomain crevices. Focusing on multidomain proteins we show that the subresolution conformational differences revealed by the mapping are in fact significant, and present a general statistical method of analysis to determine the significance of rigid body domain movements in X-ray structures., (Copyright © 2010 The Protein Society.)

Fragment complementation has been used to investigate the role of chain connectivity in the catalytic reaction of phosphomannomutase/phosphoglucomutase (PMM/PGM) from Pseudomonas aeruginosa, a human pathogen. A heterodimer of PMM/PGM, created from fragments corresponding to its first three and fourth domains, was constructed and enzyme activity reconstituted. NMR spectra demonstrate that the fragment corresponding to the fourth (C-terminal) domain exists as a highly structured, independent folding domain, consistent with its varied conformation observed in enzyme-substrate complexes. Steady-state kinetics and thermodynamics studies reported here show that complete conformational freedom of Domain 4, because of the break in the polypeptide chain, is deleterious to catalytic efficiency primarily as a consequence of increased entropy. This extends observations from studies of the intact enzyme, which showed that the degree of flexibility of a hinge region is controlled by the precise sequence of amino acids optimized through evolutionary constraints. This work also sheds light on the functional advantage gained by combining separate folding domains into a single polypeptide chain.

The enzyme phosphoglucosamine mutase catalyzes the conversion of glucosamine 6-phosphate to glucosamine 1-phosphate, an early step in the formation of the nucleotide sugar UDP-N-acetylglucosamine, which is involved in peptidoglycan biosynthesis. These enzymes are part of the large alpha-D-phosphohexomutase enzyme superfamily, but no proteins from the phosphoglucosamine mutase subgroup have been structurally characterized to date. Here, the crystallization of phosphoglucosamine mutase from Bacillus anthracis in space group P3(2)21 by hanging-drop vapor diffusion is reported. The crystals diffracted to 2.7 A resolution under cryocooling conditions. Structure determination by molecular replacement was successful and refinement is under way. The crystal structure of B. anthracis phosphoglucosamine mutase should shed light on the substrate-specificity of these enzymes and will also serve as a template for inhibitor design.

The enzyme phosphomannomutase/phosphoglucomutase (PMM/PGM) from the bacterium Pseudomonas aeruginosa is involved in the biosynthesis of several complex carbohydrates, including alginate, lipopolysaccharide, and rhamnolipid. Previous structural studies of this protein have shown that binding of substrates produces a rotation of the C-terminal domain, changing the active site from an open cleft in the apoenzyme into a deep, solvent inaccessible pocket where phosphoryl transfer takes place. We report herein site-directed mutagenesis, kinetic, and structural studies in examining the role of residues in the hinge between domains 3 and 4, as well as residues that participate in enzyme-substrate contacts and help form the multidomain "lid" of the active site. We find that the backbone flexibility of residues in the hinge region (e.g., mutation of proline to glycine/alanine) affects the efficiency of the reaction, decreasing k cat by approximately 10-fold and increasing K m by approximately 2-fold. Moreover, thermodynamic analyses show that these changes are due primarily to entropic effects, consistent with an increase in the flexibility of the polypeptide backbone leading to a decreased probability of forming a catalytically productive active site. These results for the hinge residues contrast with those for mutants in the active site of the enzyme, which have profound effects on enzyme kinetics (10 (2)-10 (3)-fold decrease in k cat/ K m) and also show substantial differences in their thermodynamic parameters relative to those of the wild-type (WT) enzyme. These studies support the concept that polypeptide flexibility in protein hinges may evolve to optimize and tune reaction rates.

Keap1 is a BTB-Kelch substrate adaptor protein that regulates steady-state levels of Nrf2, a bZIP transcription factor, in response to oxidative stress. We have determined the structure of the Kelch domain of Keap1 bound to a 16-mer peptide from Nrf2 containing a highly conserved DxETGE motif. The Nrf2 peptide contains two short antiparallel beta-strands connected by two overlapping type I beta-turns stabilized by the aspartate and threonine residues. The beta-turn region fits into a binding pocket on the top face of the Kelch domain and the glutamate residues form multiple hydrogen bonds with highly conserved residues in Keap1. Mutagenesis experiments confirmed the role of individual amino acids for binding of Nrf2 to Keap1 and for Keap1-mediated repression of Nrf2-dependent gene expression. Our results provide a detailed picture of how a BTB-Kelch substrate adaptor protein binds to its cognate substrate and will enable the rational design of novel chemopreventive agents.

Two complexes of the enzyme phosphomannomutase/phosphoglucomutase (PMM/PGM) from Pseudomonas aeruginosa with a slow substrate and with an inhibitor have been characterized by X-ray crystallography. Both ligands induce an interdomain rearrangement in the enzyme that creates a highly buried active site. Comparisons with enzyme-substrate complexes show that the inhibitor xylose 1-phosphate utilizes many of the previously observed enzyme-ligand interactions. In contrast, analysis of the ribose 1-phosphate complex reveals a combination of new and conserved enzyme-ligand interactions for binding. The ability of PMM/PGM to accommodate these two pentose phosphosugars in its active site may be relevant for future efforts towards inhibitor design.

The enzyme phosphomannomutase/phosphoglucomutase (PMM/PGM) from Pseudomonas aeruginosa catalyzes the reversible conversion of 1-phospho to 6-phospho-sugars. The reaction entails two phosphoryl transfers, with an intervening 180 degrees reorientation of the reaction intermediate (e.g. glucose 1,6-bisphosphate) during catalysis. Reorientation of the intermediate occurs without dissociation from the active site of the enzyme and is, thus, a simple example of processivity, as defined by multiple rounds of catalysis without release of substrate. Structural characterization of two PMM/PGM-intermediate complexes with glucose 1,6-bisphosphate provides new insights into the reaction catalyzed by the enzyme, including the reorientation of the intermediate. Kinetic analyses of site-directed mutants prompted by the structural studies reveal active site residues critical for maintaining association with glucose 1,6-bisphosphate during its unique dynamic reorientation in the active site of PMM/PGM.

The Kelch repeat is a common sequence motif in eukaryotic genomes and is approximately 50 amino acids in length. The structure of the Kelch domain of the human Keap1 protein has previously been determined at 1.85 Angstrom, showing that each Kelch repeat forms one blade of a six-bladed beta-propeller. Here, use of 1.35 Angstrom SAD data for de novo structure determination of the Kelch domain and for refinement at atomic resolution is described. The high quality and resolution of the diffraction data and phase information allows a detailed analysis of the role of solvent in the structure of the Kelch repeat. Ten structurally conserved water molecules are identified in each blade of the Kelch beta-propeller. These appear to play distinct structural roles that include lining the central channel of the propeller, interacting with residues in loops between strands of the blade and making contacts with conserved residues in the Kelch repeat. Furthermore, we identify a conserved C-H...pi hydrogen bond between two key residues in the consensus Kelch repeat. This analysis extends our understanding of the structural roles of conserved residues in the Kelch repeat and highlights the potential role of solvent in maintaining the fold of this common eukaryotic structural motif.

Keap1 is a substrate adaptor protein for an ubiquitin ligase complex that targets the Nrf2 transcription factor for degradation. Keap1 binds Nrf2 through its C-terminal Kelch domain, which contains six copies of the evolutionarily conserved kelch repeat sequence motif. The structure of the Kelch domain from human Keap1 has been determined by x-ray crystallography to a resolution of 1.85 A. The Kelch domain forms a 6-bladed beta-propeller structure, with residues at the C terminus forming the first strand in the first blade. Key structural roles have been identified for the highly conserved glycine, tyrosine, and tryptophan residues that define the kelch repeat sequence motif. In addition, we show that substitution of a single amino acid located within a loop that extends out from the bottom of the beta-propeller structure abolishes binding of Nrf2. The structure of the Kelch domain of Keap1 represents a high quality model for the superfamily of eukaryotic kelch repeat proteins and provides insight into how disease-causing mutations perturb the structural integrity of the Kelch domain.