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Like an alpinist continuously seeking virgin peaks to climb, Kenji Soda has investigated a variety of unique enzymes for which there was little or no information available; and by doing so he opened up a variety of new fields in enzyme science and technology. In particular, he has promoted the study of enzymes requiring vitamin B-derived cofactors such as FAD, NAD(P) and pyridoxal 5'-phosphate, shedding light on their reaction mechanisms, enzymological properties, crystal structures and potential practical applications. Highlighted in this review are the studies of enzymes acting on d-amino acids and sulphur/selenium-containing amino acids and those from thermophilic and psychrophilic bacteria.

When overexpressed as an immature enzyme in the mesophilic bacterium Escherichia coli, recombinant homoserine dehydrogenase from the hyperthermophilic archaeon Sulfurisphaera tokodaii (StHSD) was markedly activated by heat treatment. Both the apo- and holo-forms of the immature enzyme were successively crystallized, and the two structures were determined. Comparison among the structures of the immature enzyme and previously reported structures of mature enzymes revealed that a conformational change in a flexible part (residues 160–190) of the enzyme, which encloses substrates within the substrate-binding pocket, is smaller in the immature enzyme. The immature enzyme, but not the mature enzyme, formed a complex that included NADP+, despite its absence during crystallization. This indicates that the opening to the substrate-binding pocket in the immature enzyme is not sufficient for substrate-binding, efficient catalytic turnover or release of NADP+. Thus, specific conformational changes within the catalytic region appear to be responsible for heat-induced activation. Heat-induced maturation of homoserine dehydrogenase from a hyperthermophilic archaeon requires conformational changes within the catalytic region. [ABSTRACT FROM AUTHOR]

Crystal structures of the thermostable meso-diaminopimelate dehydrogenase (DAPDH) from Ureibacillus thermosphaericus were determined for the enzyme in the apo form and in complex with NADP+ and N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid. The main-chain coordinates of the enzyme showed notable similarity to those of Symbiobacterium thermophilum DAPDH. However, the subunit arrangement of U. thermosphaericus DAPDH (a dimer) was totally different from that of the S. thermophilum enzyme (a hexamer). Structural comparison with the dimeric enzyme from the mesophile Corynebacterium glutamicum revealed that the presence of large numbers of intrasubunit and intersubunit hydrophobic interactions, as well as the extensive formation of intersubunit ion-pair networks, were likely to be the main factors contributing to the higher thermostability of U. thermosphaericus DAPDH. This differs from S. thermophilum DAPDH, within which the unique hexameric assembly is likely to be responsible for its high thermostability. Analysis of the active site of U. thermosphaericus DAPDH revealed the key factors responsible for the marked difference in substrate specificity between DAPDH and the D-amino acid dehydrogenase recently created from DAPDH by introducing five point mutations [Akita et al. (2012). Biotechnol. Lett. 34, 1693-1699; 1701-1702]. [ABSTRACT FROM AUTHOR]

Two types of hetero-oligomeric dye-linked l-proline dehydrogenases (αβ and αβγδ types) are expressed in the hyperthermophilic archaea belonging to Thermococcales. In both enzymes, the β subunit (PDHβ) is responsible for catalyzing l-proline dehydrogenation. The genes encoding the two enzyme types form respective clusters that are completely conserved among Pyrococcus and Thermococcus strains. To compare the enzymatic properties of PDHβs from αβ- and αβγδ-type enzyme complexes, eight PDHβs (four of each type) from Pyrococcus furiosus DSM3638, Pyrococcus horikoshii OT-3, Thermococcus kodakaraensis KOD1 JCM12380 and Thermococcus profundus DSM9503 were expressed in Escherichia coli cells and purified to homogeneity using one-step Ni-chelating chromatography. The αβ-type PDHβs showed greater thermostability than most of the αβγδ-type PDHβs: the former retained more than 80 % of their activity after heating at 70 °C for 20 min, while the latter showed different thermostabilities under the same conditions. In addition, the αβ-type PDHβs utilized ferricyanide as the most preferable electron acceptor, whereas αβγδ-type PDHβs preferred 2, 6-dichloroindophenol, with one exception. These results indicate that the differences in the enzymatic properties of the PDHβs likely reflect whether they were from an αβγδ- or αβ-type complex, though the wider divergence observed within αβγδ-type PDHβs based on the phylogenetic analysis may also be responsible for their inconsistent enzymatic properties. By contrast, differences in the kinetic parameters among the PDHβs did not reflect the complex type. Interestingly, the k value for free αβ-type PDHβ from P. horikoshii was much larger than the value for the same subunit within the αβ-complex. This indicates that the isolated PDHβ could be a useful element for an electrochemical system for detection of l-proline. [ABSTRACT FROM AUTHOR]

The article focuses on a study related to the evaluation of enzymes activity isolated from hyperthermophile Pyrobaculum islandicum. As per the study, the enzymes isolated from the species have various application in bioprocess engineering. The study highlights the molecular mechanism behind the stability of unboiled Nicotinamide Adenine Dinucleotide (NAD) dependent glutamate dehydrogenase. The study further concludes that the enzyme is extremely thermostable.

The crystal structure of an extremely thermostable multicopper oxidase (McoP) from the hyperthermophilic archaeon Pyrobaculum aerophilum was determined at a resolution of 2.0 Å. The overall fold was comprised of three cupredoxin-like domains and the main-chain coordinates of the enzyme were similar to those of multicopper oxidases from Escherichia coli (CueO) and Bacillus subtilis (CotA). However, there were clear topological differences around domain 3 between McoP and the other two enzymes: a methionine-rich helix in CueO and a protruding helix in CotA were not present in McoP. Instead, a large loop (PL-1) covered the T1 copper centre of McoP and a short α-helix in domain 3 extended near the N-terminal end of PL-1. In addition, the sizes of several surface loops in McoP were markedly smaller than the corresponding loops in CueO and CotA. Structural comparison revealed that the presence of extensive hydrophobic interactions and a smaller cavity volume are likely to be the main factors contributing to the hyperthermostability of McoP. [ABSTRACT FROM AUTHOR]

The activity of a dye-linked l-proline dehydrogenase (dye- l-proDH) was found in the crude extract of an aerobic hyperthermophilic archaeon, Pyrobaculum calidifontis JCM 11548, and was purified 163-fold through four sequential chromatography steps. The enzyme has a molecular mass of about 108 kDa and is a homodimer with a subunit molecular mass of about 46 kDa. The enzyme retained more than 90% of its activity after incubation at 100 °C for 120 min (pH 7.5) or after incubation at pHs 4.5-9.0 for 30 min at 50 °C. The enzyme catalyzed l-proline dehydrogenation to Δ-pyroline-5-carboxylate using 2,6-dichloroindophenol (DCIP) as the electron acceptor and the Michaelis constants for l-proline and DCIP were 1.67 and 0.026 mM, respectively. The prosthetic group on the enzyme was identified as flavin adenine dinucleotide by high-performance liquid chromatography. The subunit N-terminal amino acid sequence was MYDYVVVGAG. Using that sequence and previously reported genome information, the gene encoding the enzyme (Pcal_1655) was identified. The gene was then cloned and expressed in Escherichia coli and found to encode a polypeptide of 415 amino acids with a calculated molecular weight of 46,259. The dye- l-proDH gene cluster in P. calidifontis inherently differs from those in the other hyperthermophiles reported so far. [ABSTRACT FROM AUTHOR]

Peptide tags containing tyrosines (Y-tag) were introduced at the C-terminus of a hyperthermophilic enzyme, alkaline phosphatase from Pyrococcus furiosus (PfuAP). Immobilization of the recombinant PfuAPs onto water-in-oil-in-water (W/O/W) type microcapsules was performed by an in situ polymerization method. All the recombinant PfuAPs prepared in this study were quantitatively immobilized onto microcapsules. The PfuAP-immobilized microcapsules showed no significant loss of enzymatic activity until the 5th round of assays. This result implies that the recombinant PfuAPs were covalently immobilized onto microcapsules. Immobilized PfuAP tagged with a Y-tag having the sequence GGYYY exhibited approximately a twofold higher catalytic activity compared with the wild-type PfuAP. [ABSTRACT FROM AUTHOR]

The crystal structure of the highly thermostablel-aspartate dehydrogenase (l-aspDH; EC 1.4.1.21) from the hyperthermophilic archaeon Archaeoglobus fulgidus was determined in the presence of NAD and a substrate analog, citrate. The dimeric structure of A. fulgidusl-aspDH was refined at a resolution of 1.9 Å with a crystallographic R-factor of 21.7% ( Rfree = 22.6%). The structure indicates that each subunit consists of two domains separated by a deep cleft containing an active site. Structural comparison of the A. fulgidusl-aspDH/NAD/citrate ternary complex and the Thermotoga maritimal-aspDH/NAD binary complex showed that A. fulgidusl-aspDH assumes a closed conformation and that a large movement of the two loops takes place during substrate binding. Like T. maritimal-aspDH, the A. fulgidus enzyme is highly thermostable . But whereas a large number of inter- and intrasubunit ion pairs are responsible for the stability of A. fulgidusl-aspDH, a large number of inter- and intrasubunit aromatic pairs stabilize the T. maritima enzyme. Thus stabilization of these twol-aspDHs appears to be achieved in different ways. This is the first detailed description of substrate and coenzyme binding tol-aspDH and of the molecular basis of the high thermostability of a hyperthermophilicl-aspDH. [ABSTRACT FROM AUTHOR]

Abstract: Nicotinic acid adenine dinucleotide phosphate (NAADP) has been shown to mobilize Ca2+ from intracellular stores in a wide variety of organisms, ranging from plants to humans. We have developed a novel enzyme cycling assay for NAADP that involves coupled reactions catalyzed by four enzymes. In this system, NAADP is first converted into nicotinic acid adenine dinucleotide (NAAD) by alkaline phosphatase, after which the NAAD is converted to NAD, AMP, and PPi by NAD synthetase (NADS) in the presence of ATP and ammonia. The NAD is then amplified using an enzyme cycling system driven by glucose dehydrogenase and diaphorase. The resultant formation of formazan dye is measured spectrophotometrically based on the increase in absorbance at 450nm. Using this method, NAADP (20–400nM) was assayed, and a highly linear correlation was obtained between the NAADP concentration and the increase in absorbance at 450nm. The cycling rate was approximately 95cycles/min. In addition, the within-run coefficients of variation (CVs) for 25, 50, and 100nM NAADP solutions were 9.33, 4.86, and 3.13%, respectively. Interference by NAD analogs (e.g., NAAD, NADP) in the sample was eliminated prior to running the assay by treating the sample with NADS and NAD nucleosidase (NADase). In sum, our findings indicate this enzyme cycling assay to be readily applicable for determination for NAADP in a variety of biological samples and to be particularly appropriate for use with an autoanalyzer. [Copyright &y& Elsevier]

A gene encoding the L-threonine dehydrogenase homologue has been identified in a hyperthermophlic archaeon Pyrococcus horikoshii OT3 via genome sequencing. The gene was cloned and expressed in Escherichia coli. The purified enzyme from the recombinant E. coli was extremely thermostable; the activity was not lost after incubation at 100°C for 20 min. The enzyme (molecular mass: 192 kDa) is composed of a tetrameric structure with a type of subunit (41 kDa). The enzyme is specific for NAD and utilizes L-threonine, L-serine and DL- threo-3-phenylserine as the substrate. The enzyme required divalent cations such as Zn2+, Mn2+ and Co2+ for the activity, and contained one zinc ion/subunit. The Km values for L-threonine and NAD at 50°C were 0.20 mM and 0.024 mM, respectively. Kinetic analyses indicated that the L-threonine oxidation reaction proceeds via a random mechanism with regard to the binding of L-threonine and NAD. The enzyme showed pro- R stereospecificity for hydrogen transfer at the C4 position of the nicotinamide moiety of NADH. This is the first description of the characteristics of an L-threonine dehydrogenase from the archaea domain. [ABSTRACT FROM AUTHOR]

Dye-linked l-proline dehydrogenase catalyzes the oxidation of l-proline in the presence of artificial electron acceptors such as 2, 6-dichloroindophenol and ferricyanide. The enzyme from the hyperthermophilic archaeon Thermococcus profundus was purified and characterized for the first time in archaea by Sakuraba et al. in 2001. In this study, cloning and sequencing analyses of the gene encoding the enzyme and functional analysis of the subunits were performed. The gene formed an operon that consisted of four genes, pdhA, pdhB, pdhF, and pdhX, which are tandemly arranged in the order of pdhA-F-X-B. SDS-PAGE analysis of the purified recombinant enzyme showed four different bands corresponding to α (54 kDa), β (43 kDa), γ (19 kDa), and δ (8 kDa) subunits encoded by pdhA, pdhB, pdhF, and pdhX, respectively, and the molecular ratio of these subunits was determined to be equal. This indicates that the enzyme consists of a heterotetrameric αβγδ structure. Functional analysis of each subunit revealed that the β subunit catalyzed the dye-linked l-proline dehydrogenase reaction by itself and that, unexpectedly, the α subunit exhibited dye-linked NADH dehydrogenase activity. This is the first example showing the existence of a bifunctional dye-linked l-proline/NADH dehydrogenase complex. On the basis of genome analysis, similar gene clusters were observed in the genomes of Pyrococcus horikoshii, Pyrococcus abyssi, Pyrococcus furiosus, and Archaeoglobus fulgidus. These results indicate that the dye-linked l-proline dehydrogenase is a novel type of heterotetrameric amino acid dehydrogenase that might be widely distributed in the hyperthermophilic archaeal strain. [ABSTRACT FROM AUTHOR]

Alanine dehydrogenase of Vibrio proteolyticus DSM 30189 shows high activity toward β-hydroxypyruvate, and the enzyme is applicable to the production of l-serine. We have cloned the enzyme gene from the bacterium into Escherichia coli TG1 with a vector plasmid, pUC118. The enzyme was overproduced by the transformed cells and purified to homogeneity with a yield of 46%. The molecular mass of the enzyme was about 230 kDa and consisted of six identical subunits. The enzyme showed broad specificity toward α-keto acids in the reductive amination. The relative activities of the enzyme for pyruvate, β-fluoropyruvate, and β-hydroxypyruvate were 100%, 74%, and 54%, respectively. The enzyme retained more than 90% of the activity after incubation at 65 °C for 60 min in the presence of 2.0 M NaCl, but 98% of its original activity was lost in the absence of NaCl. RbCl, as well as NaCl, significantly stabilized the enzyme. On the other hand, LiCl and KCl were not as effective as stabilizers such as RbCl and NaCl. [Copyright &y& Elsevier]

Presents a study which characterized the NAD-dependent glutamate dehydrogenase of the archaeon Pyrobaculum islandicum. Purification of the dehydrogenase; Identification of the enzyme as the most thermostable NAD-dependent dehydrogenase; Methodology used to conduct the study; Results of the study.

Lecture dehydrogenase has been purified to homogeneity from a moderate thermophilic actinomycete, Thermoactinomyces intermedius IFO 14230. The enzyme can be stored without loss of its activity at a low temperature (e.g. 4°C) for over two years. The enzyme was more thermostable at higher concentrations of salts such as NaCl and KCl. It retained about 90% of activity on incubation at 70%C for at least 40 min in the presence of 3 M NaCl. The Michaelis constants for NAD, L-leucine, NADH, 2-oxoisocaproate and ammonia were determined to be 0.36, 2.0, 0.042, 0.63 and 118 mM, respectively, from initial-velocity analyses. The enzyme showed gene was cloned into Escherichia coli and its complete DNA sequence was determined. The leucine dehydrogenase gene (leudh) consists of a 1098-bp open reading frame and encodes 366 amino acid residues corresponding to a subunit (Mr 40586) of the octameric enzyme. The amino acid sequence of the enzyme showed 80.7% similarity with that of the Bacillus stearothermophilus enzyme. The enzyme was overproduced in E. coli JM 109 having a recombinant plasmid, pULDH2, which was constructed from pUC18 and the leudh gene. The enzyme was purified from the cell extract to homogeneity in one day, with 78% recovery. [ABSTRACT FROM AUTHOR]

The distribution of dye-linked L-malate dehydrogenase (L-malate: acceptor oxidoreductase, EC 1.1.9.16) was investigated in many thermophilic bacteria. The enzyme occurred widely in thermophilic spore-forming bacteria like bacilli and thermoactinomycetes. The enzyme was purified to homogeneity from a thermophile, Bacillus sp. DSM 465, with a 2.7% overall recovery by DEAEToyopearl column chromatography. Sephacryl S-400 column chromatography and preparative slab PAGE. The enzyme had a molecular mass of about 660 kDa and consisted of about ten subunits ail with a molecular mass of 66 kDa. The enzyme retained its full activity upon heating at 55 °C for at least 60 min and with incubation at pH 5.0-10.0, 55°C, for 10 min. The enzyme exclusively catalyzed L-malate dehydrogenation in the presence of an electron acceptor such as 2,6-dichloroindopheno!. The Michaelis constants for L-malate and 2,6-dichloroindophenol were determined to be 1.67 mM and 0.050 mM, respectively. FAD was identified as a prosthetic group of the enzyme by HPLC. [ABSTRACT FROM AUTHOR]

Alanine dehydrogenase (L-alanine:NAD+ oxidoreductase, deaminating) was simply purified to homogeneity from a thermophile. Bacillus sphaericus DSM 461, b ammonium sulfate fractionation, red-Sepharose 4B chromatography and preparative slab gel electrophoresis. The enzyme has a molecular mass of about 230 kDa and consisted of six subunits with an identical molecular mass of 38 kDa. The enzyme was much more thermostable than that from a mesophile, B. sphaericus, and retained its full activity upon heating at 75°C for at least 60 min and with incubation in pH 5.5-9.5 at 75°C for 10 min. The enzyme can be stored without loss of its activity in a frozen state (-20°C, at pH 7.2) for over 5 months. The optimum pH for the L-alanine deamination and pyruvate amination were around 10.5 and 8.2, respectively. The enzyme exclusively catalyzed the oxidative deamination of L-alanine in the presence of NAD+, but showed low amino acceptor specificity; hydroxypyruvate, oxaloacetate. 2-oxobutryrate and 3-fluoropyruvate are also aminated as well as pyruvate in the presence of NADH and ammonia. Initial velocity and product inhibition studies showed that the reductive amination proceeded through a sequential mechanism containing partially random binding. NADH binds first to the enzyme, and then pyruvate and ammonia bind in a random fashion. The products are sequentially released from the enzyme in the order L-alanine then NAD+. A dead-end inhibition by the formation of an abortive ternary complex which consists of the enzyme, NAD+ and pyruvate was included in the reaction. A possible role of the dead-end inhibition is to prevent the enzyme from functioning in the L-alanine synthesis. The Michaelis constants for the substrates were as follows: NADH, 0.10 mM; pyruvate, 0.50 mM; ammonia, 38.0 mM; L-alanine, 10.5 mM and NAD+, 0.26 mM. [ABSTRACT FROM AUTHOR]

4-α-Glucanotransferase was purified from cells of Thermococcus litoralis, a hyperthermophilic archaeon. The molecular mass of the enzyme was estimated to be approximately 87 kDa by gel filtration. The optimal temperature for its activity was 90°C. The enzyme catalyzed the transglycosylation of maltooligosaccharides, yielding maltooligosaccharides of various lengths and glucose. When maltoheptaose was used as the substrate, glucoamylase-resistant and glucoamylase-sensitive saccharides were produced. On incubation of amylose with the T. litoralis enzyme, glucoamylase-resistant but α-amylase—sensitive molecules were produced, but the amount of reducing sugar showed only slight increases. These results indicate that the T. litoralis enzyme catalyzes not only intermolecular transglycosylation to produce linear α-l,4-glucan, but also intramolecular transglycosylation to produce cyclic α-l,4-glucan (cycloamylose), similarly to potato 4-α-glucanotransferase (called disproportionating enzyme). The gene encoding the Z litoralis 4-α-glucanotransferase was cloned, sequenced and expressed in Escherichia coli. The nucleotide sequence of the gene encoded a 659-amino acid protein with a calculated molecular mass of 77 883 Da. The amino acid sequence of the T. titoralis enzyme showed high similarity with those of α-amylases of Pyrococcus furiosus, a hyperthermophilic archaeon, and Dictyoglomus thermophilum, an extremely thermophilic bacterium, but little similarity with those of other known 4-α-glucanotransferases. [ABSTRACT FROM AUTHOR]

The article focuses on a study that characterizes the uracil-disphosphate -glucose dehydrogenase (UDP-GDH) identified from the Hyperthermophilic Archaon, Pyrobaculum islandicum. It notes that the physiological functions of UDP-GDH have demonstrated its importance in polysaccharide biosynthesis, detoxification and embryonic development. Furthermore, the higher stability of P. islandicum UDP-GDH has been significant in preparing highly pure enzymes.

The gene (PH1074) encoding the NAB kinase of the hyperthermophilic archaeon Pyrococcus horikoshii was identified in the genome database, cloned, and functionally expressed in Escherichia coil. The recombinant enzyme was purified to homogeneity by heat treatment at 90°C for 20 min and one successive HiTrap affinity chromatography step. The purified enzyme was easily precipitated by dialysis against phosphate buffer without NaCl and imidazole and was usually stored in buffer containing 0.5 M NaCl and 0.5 M imidazole to avoid precipitation. The molecular mass of the active enzyme was determined to be 145 kDa by a gel filtration method, and the enzyme was composed of a tetramer of 37-kDa subunits. The archaeal enzyme utilized several nucleoside triphosphates, such as GTP, CTP, UTP, and ITP, as well as ATP and inorganic polyphosphates [poly(P)] as phosphoryl donors for NAB phosphorylation. The enzyme utilized poly(P)27 (the average length of the phosphoryl chain was 27) as the most active inorganic polyphosphate for NAD phosphorylation. Thus, this enzyme is categorized as an inorganic polyphosphate/ATPs dependent NAD kinase. The enzyme was the most thermostable NAD kinase found to date: its activity was not lost by incubation at 95°C for 10 mm. The enzyme showed classical Michaelis-Menten-type kinetics for NAD and ATP, but not for poly(P)27. The Km values for NAD were determined to be 030 and 0.40 mM when poly(P)27 and ATP, respectively, were used as the phosphoryl donors. The Km value for ATP was 0.29 mM, and the concentration of poly(P)27 which gave half of the maximum enzyme activity was 0.59 mM. The enzyme required several metal cations, such as Mg2+, Mn2+, or Ni2+, for its activity. The deduced amino acid sequence showed a low level of identity to those of E. coil ATP-dependent NAB kinase (31%) and the inorganic polyphosphate/ATP-dependent NAD kinase of Mycobacterium tuberculosis (29%). This is the first description of the characteristics of a poly(P)/ATP-dependent NAB kinase from a hyperthermophilic archaeon. [ABSTRACT FROM AUTHOR]

NAD-dependent L-glutamate dehydrogenase (NAD-GDH) activity was detected in cell extract from the psychrophile Janthinobacterium lividum UTB1302, which was isolated from cold soil and purified to homogeneity. The native enzyme (1,065 kDa, determined by gel filtration) is a homohexamer composed of 170-kDa subunits (determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis). Consistent with these findings, gene cloning and sequencing enabled deduction of the amino acid sequence of the subunit, which proved to be comprised of 1,575 amino acids with a combined molecular mass of 169,360 Da. The enzyme from this psychrophile thus appears to belong to the GDH family characterized by very large subunits, like those expressed by Streptomyces clavuligerus and Pseudomonas aeruginosa (about 180 kDa). The entire amino acid sequence of the J. lividum enzyme showed about 40% identity with the sequences from S. clavuligerus and P. aeruginosa enzymes, but the central domains showed higher homology (about 65%). Within the central domain, the residues related to substrate and NAD binding were highly conserved, suggesting that this is the enzyme's catalytic domain. In the presence of NAD, but not in the presence of NADP, this GDH exclusively catalyzed the oxidative deamination of L-glutamate. The stereospecificity of the hydride transfer to NAD was pro-S, which is the same as that of the other known GDHs. Surprisingly, NAD-GDH activity was markedly enhanced by the addition of various amino acids, such as L-aspartate (1,735%) and L-arginine (936%), which strongly suggests that the N- and/or C-terminal domains play regulatory roles and are involved in the activation of the enzyme by these amino acids. [ABSTRACT FROM AUTHOR]

Abstract: The crystal structure of a highly thermostable UDP-galactose 4-epimerase (GalE) from the hyperthermophilic archaeon Pyrobaculum calidifontis was determined at a resolution of 1.8Å. The asymmetric unit contained one subunit, and the functional dimer was generated by a crystallographic two-fold axis. Each monomer consisted of a Rossmann-fold domain with NAD bound and a carboxyl terminal domain. The overall structure of P. calidifontis GalE showed significant similarity to the structures of the GalEs from Escherichia coli, human and Trypanosoma brucei. However, the sizes of several surface loops were markedly smaller in P. calidifontis GalE than the corresponding loops in the other enzymes. Structural comparison revealed that the presence of an extensive hydrophobic interaction at the subunit interface is likely the main factor contributing to the hyperthermostability of the P. calidifontis enzyme. Within the NAD-binding site of P. calidifontis GalE, a loop (NAD-binding loop) tightly holds the adenine ribose moiety of NAD. Moreover, a deletion mutant lacking this loop bound NAD in a loose, reversible manner. Thus the presence of the NAD-binding loop in GalE is largely responsible for preventing the release of the cofactor from the holoenzyme. [Copyright &y& Elsevier]

Dye-linked d-lactate dehydrogenase activity was found in the crude extract of a continental thermoacidophilic crenarchaeota, Sulfolobus tokodaii strain 7, and was purified 375-fold through four sequential chromatography steps. With a molecular mass of about 93 kDa, this enzyme was a homodimer comprised of identical subunits with molecular masses of about 48 kDa. The enzyme retained its full activity after incubation at 80°C for 10 min and after incubation at pHs ranging from 6.5 to 10.0 for 30 min at 50°C. The preferred substrate for this enzyme was d-lactate, with 2,6-dichloroindophenol serving as the electron acceptor. Using high-performance liquid chromatography (HPLC), the enzyme''s prosthetic group was determined to be flavin adenine dinucleotide (FAD). Its N-terminal amino acid sequence was MLEGIEYSQGEEREDFVGFKIKPKI. Using that sequence and previously reported genome information, the gene encoding the enzyme (ST0649) was identified. It was subsequently cloned and expressed in Escherichia coli and found to encode a polypeptide of 440 amino acids with a calculated molecular weight of 49,715. The amino acid sequence of this dye-linked d-lactate dehydrogenase showed higher homology (39% identity) with that of a glycolate oxidase subunit homologue from Archaeoglobus fulgidus, but less similarity (32% identity) to d-lactate dehydrogenase from A. fulgidus. Taken together, our findings indicate that the dye-linked d-lactate dehydrogenase from S. tokodaii is a novel type of FAD containing d-lactate dehydrogenase. [Copyright &y& Elsevier]

Abstract: The crystal structure of the highly thermostable l-aspartate oxidase (LAO) from the hyperthermophilic archaeon Sulfolobus tokodaii was determined at a 2.09 Å resolution. The factors contributing to the thermostability of the enzyme were analyzed by comparing its structure to that of Escherichia coli LAO. Like E. coli LAO, the S. tokodaii enzyme consists of three domains: an FAD-binding domain, an α+β capping domain, and a C-terminal three-helix bundle. However, the situation of the linker between the FAD-binding domain and C-terminal three-helix bundle in S. tokodaii LAO is completely different from that in E. coli LAO, where the linker is situated near the FAD-binding domain and has virtually no interaction with the rest of the protein. In S. tokodaii LAO, this linker is situated near the C-terminal three-helix bundle and contains a β-strand that runs parallel to the C-terminal strand. This results in the formation of an additional β-sheet, which appears to reduce the flexibility of the C-terminal region. Furthermore, the displacement of the linker enables formation of a 5-residue ion-pair network between the FAD-binding and C-terminal domains, which strengthens the interdomain interactions. These features might be the main factors contributing to the high thermostability of S. tokodaii LAO. [Copyright &y& Elsevier]

Genes encoding 2-deoxy-D-ribose-5-phosphate aldolase (DERA) homologues from two hyperthermophiles, the archaeon Pyrobaculum aerophilum and the bacterium Thermotoga maritima, were expressed individually in Escherichia coli, after which the structures and activities of the enzymes produced were characterized and compared with those of E. coli DERA. To our surprise, the two hyperthermophilic DERAs showed much greater catalysis of sequential aldol condensation using three acetaldehydes as substrates than the E. coli enzyme, even at a low temperature (25°C), although both enzymes showed much less 2-deoxy-o-ribose-5-phosphate synthetic activity. Both the enzymes were highly resistant to high concentrations of acetaldehyde and retained about 50% of their initial activities after a 20-h exposure to 300 mM acetaldehyde at 25°C, whereas the E. coli DERA was almost completely inactivated after a 2-h exposure under the same conditions. The structure of the P. aerophilum DERA was determined by X-ray crystallography to a resolution of 2.0 A. The main chain coordinate of the P. aerophilum enzyme monomer was quite similar to those of the T. maritima and E. coli enzymes, whose crystal structures have already been solved. However, the quaternary structure of the hyperthermophilic enzymes was totally different from that of the E. coli DERA. The areas of the subunit-subunit interface in the dimer of the hyperthermophilic enzymes are much larger than that of the E. coli enzyme. This promotes the formation of the unique dimeric structure and strengthens the hydrophobic intersubunit interactions. These structural features are considered responsible for the extremely high stability of the hyperthermophilic DERAs. [ABSTRACT FROM AUTHOR]

Abstract: A gene encoding an l-aspartate dehydrogenase (EC 1.4.1.21) homologue was identified in the anaerobic hyperthermophilic archaeon Archaeoglobus fulgidus. After expression in Escherichia coli, the gene product was purified to homogeneity, yielding a homodimeric protein with a molecular mass of about 48 kDa. Characterization revealed the enzyme to be a highly thermostable l-aspartate dehydrogenase, showing little loss of activity following incubation for 1 h at up to 80 °C. The optimum temperature for l-aspartate dehydrogenation was about 80 °C. The enzyme specifically utilized l-aspartate as the electron donor, while either NAD or NADP could serve as the electron acceptor. The K m values for l-aspartate were 0.19 and 4.3 mM when NAD or NADP, respectively, served as the electron acceptor. The K m values for NAD and NADP were 0.11 and 0.32 mM, respectively. For reductive amination, the K m values for oxaloacetate, NADH and ammonia were 1.2, 0.014 and 167 mM, respectively. The enzyme showed pro-R (A-type) stereospecificity for hydrogen transfer from the C4 position of the nicotinamide moiety of NADH. This is the first report of an archaeal l-aspartate dehydrogenase. Within the archaeal domain, homologues of this enzyme occurred in many Methanogenic species, but not in Thermococcales or Sulfolobales species. [Copyright &y& Elsevier]

Abstract: Agmatinase is one of the key enzymes in the biosynthesis of polyamines such as putrescine and sperimidine from arginine in microorganisms. The gene (PH0083) encoding the putative agmatinase of hyperthermophilic archaeon Pyrococcus horikoshii was identified based on the genome database. The gene was cloned and expressed, and the product was mainly obtained as inactive inclusion body in Escherichia coli. The inclusion body was dissolved in 6 M guanidine–HCl and successively refolded to active enzyme by the dilution of the denaturant. The enzyme exclusively catalyzed the hydrolysis of agmatine, but not arginine. This indicates that PH0083 codes agmatinase. The enzyme required divalent cations such as Co2+, Ca2+ and Mn2+ for the activity. The highest activity was observed under fairly alkaline conditions, like pH 11. The purified recombinant enzyme consisted of four identical subunits with a molecular mass of 110–145 kDa. The enzyme was extremely thermostable: the full activity was retained on heating at 80 °C for 10 min, and a half of the activity was retained by incubation at 90 °C for 10 min. From a typical Michaelis–Menten type kinetics, an apparent K m value for agmatine was determined to be 0.53 mM. Phylogenic analysis revealed that the agmatinase from P. horikoshii does not belong to any clusters of enzymes found in bacteria and eukarya. This is the first description of the presence of archaeal agmatinase and its characteristics. [Copyright &y& Elsevier]

A novel alanine:glyoxylate aminotransferase was found in a hyperthermophilic archaeon, Thermococcus litoralis. The amino acid sequence of the enzyme did not show a similarity to any alanine:glyoxylate aminotransferases reported so far. Homologues of the enzyme appear to be present in almost all hyperthermophilic archaea whose whole genomes have been sequenced. [ABSTRACT FROM AUTHOR]

A halophilic NAD+-dependent 2-aminobutyrate dehydrogenase (EC1.4.1.1) was purified to homogeneity from a crude extract of an extreme halophile, Halobacterium saccharovorum DSM 1137, with a 30% yield. The enzyme had a molecular mass of about 160 kDa and consisted of four identical subunits. It retained more than 70% of the activity after heating at 60 °C for 1 h and kept it at 30 °C for 8 months in the presence of 2 M NaCl. The enzyme showed maximum activity in the presence of 2 M RbCl or KCl. The enzyme required NAD+ as a coenzyme and used l-2-aminobutyrate, l-alanine, and l-norvaline as substrates. The best substrate was l-2-aminobutyrate. The optimum pH was 9.3 for the oxidative deamination of l-2-aminobutyrate and 8.6 for the reductive amination of 2-ketobutyrate. The Michaelis constants were 1.2 mM for l-2-aminobutyrate, 0.16 mM for NAD+, 0.012 mM for NADH, 0.78 mM for 2-ketobutyrate, and 500 mM for ammonia in the presence of 2 M KCl. The Km values for the substrates depended on the concentration of KCl, and the Km values decreased under high salt conditions. [Copyright &y& Elsevier]

A gene encoding a nicotinamide mononucleotide (NMN) adenylyltransferase (NMNAT, EC 2.7.7.1) homologue was identified via genome sequencing in the anaerobic hyperthermophilic archaeon Pyrococcus horikoshii OT-3. The gene encoded a protein of 186 amino acids with a molecular weight of 21,391. The deduced amino acid sequence of the gene showed 59% identities to the NMNAT from Methanococcus jannaschii. The gene was overexpressed in Escherichia coli, and the produced enzyme was purified to homogeneity. Characterization of the enzyme revealed that it is an extremely thermostable NMNAT; the activity was not lost after incubation at 80 °C for 30 min. The native molecular mass was estimated to be 77 kDa. The Km values for ATP and NMN were calculated to be 0.056 and 0.061 mM, respectively. The optimum temperature of the reaction was estimated to be around 90 °C. The adenylyl group donor specificity was examined by high-performance liquid chromatography (HPLC). At 70 °C, ATP was a prominent donor. However, above 80 °C, a relatively small, but significant, NMNAT activity was detected when ATP was replaced by ADP or AMP in the reaction mixture. To date, an NMNAT that utilizes ADP or AMP as an adenylyl group donor has not been found. The present study provides interesting information in which a di- or mono-phosphate nucleotide can be utilized by adenylyltransferase at high temperature. [Copyright &y& Elsevier]

Describes the structure of hyperthermophilic 2-deoxy-D-ribose-5-phosphate aldolase (DERA) and of aldolase from the Archaea domain. Expression of the gene APE2347 that encodes a DERA homolog; Purification and characterization of Aeropyrum pernix DERA.