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Tryptocidine C (TpcC, cyclo[D-Phe1-Pro2-Trp3-D-Trp4-Asn5-Gln6-Trp7-Val8-Orn9-Leu10]) is a broad-spectrum antimicrobial peptide in the tyrothricin complex produced by a soil bacterium, Brevibacillus parabrevis. Electrospray mass spectrometric studies reveal the oligomerisation of TpcC into dimers and higher oligomers, analogous to tyrocidine C (TrcC, Trp7 replaced by Tyr7). Ion mobility mass spectrometry (IMMS) further confirms the formation of stable peptide dimers and tetramers with diameters of 2.7 nm and 3.3 nm, respectively, calculated from collisional cross section (CCS). Molecular dynamic simulations and docking studies support the formation of amphipathic dimers, with a diameter of 2.5 ± 0.07 nm calculated from low energy model CCS. Circular dichroism and IMMS studies point towards dynamic hydrogen-bonded conformational changes up to 28–33 μM after which the structures become more static (or in equilibrium). Fluorescence studies indicate aromatic stacking of Trp residues with a CMC of 18 μM in aqueous solutions. The concentration and time dependent interaction of Trp in oligomers indicate cooperativity in the TpcC oligomerisation that leads to the formation of higher order microscopic structures. Scanning electron microscopy studies unequivocally shows that TpcC forms nanospheres with a mean diameter of 25 nm. Repeated smaller oligomeric units, possibly dimers and tetramers, self-assemble to form these nanospheres. Image 1 • Tryptocidine C (TpcC), an antimicrobial cyclodecapeptide from tyrothricin readily oligomerises into nano-assemblies. • Dimers, trimers, tetramers and higher oligomers of TpcC were observed with electrospray – and ion mobility mass spectrometry. • Circular dichroism and fluorescence studies indicated a H-bond network and cooperative aromatic stacking in TpcC oligomers. • Molecular modelling revealed beta-sheet type TpcC monomers and dimers with CCS values correlating with those determined by IMMS. [ABSTRACT FROM AUTHOR]

The ground-state properties of (NO)2 and (NO)4 have been investigated using multireference second-order perturbation theory (MRMP2) and include a two-tier extrapolation to the complete basis set (CBS) limit. For the NO dimer the MRMP2(18,14)/CBS predicted structure, binding energy (with respect to 2NO; De = 3.46 kcal/mol), and dipole moment (ue = 0.169 D) are in excellent agreement with experimental measurements (De = 2.8-3.8 kcal/mol; ue = 0.171 D). Additionally, three of four intermolecular anharmonic MRMP2(18,14)/CBS-estimated frequencies (143 cm-1, 238 cm-1, 421 cm-1) are in excellent agreement with recent gas-phase experimental measurements (135 cm-1, 239 cm-1, 429/428 cm-1); however, the predicted value of 151 cm-1 for the out-of-plane torsion (A2) is elevated compared to recent experimental estimates of 97-117 cm-1. Our finding that this infrared-forbidden vibration is also predicted to have an extremely low Raman activity (0.04 Å/amu at the MP2/QZ level of theory) conflicts with Raman measurements of a strong intensity for a low frequency band; however, these studies were performed for low temperature solid and liquid phases. Probing the possibility of the presence of higher order clusters we investigated the stability of (NO)4 and discovered three isomers, each resembling pairs of dimers, that were stable to dissociation to 2(NO)2, with the lowest-energy isomer (Ci structure) having a predicted binding energy almost identical to that of the dimer. Computed vibrational frequencies of the Ci isomer indicate that the 12 highest-frequency modes resemble barely shifted NO dimer-combined bands while the 13th highest-frequency mode at ∼100 cm-1 is exclusive to (NO)4. Moreover, this tetramer-unique vibration is infrared inactive but has a very high predicted Raman activity of some 24 Å/amu. Guided by the theoretical results, we reexamined and reassigned experimental Raman and infrared data going back to 1951 and determined that our best predictions of vibrational frequencies of (NO)2 and (NO)4 are consistent with experimental observations. We thus postulate the existence and observation of (NO)4. [ABSTRACT FROM AUTHOR]

The quaternary structures of the cholinesterases, acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), are essential for their localization and function. Of practical importance, BChE is a promising therapeutic candidate for intoxication by organophosphate nerve agents and insecticides, and for detoxification of addictive substances. Efficacy of the recombinant enzyme hinges on its having a long circulatory half-life; this, in turn, depends strongly on its ability to tetramerize. Here, we used cryoelectron microscopy (cryo-EM) to determine the structure of the highly glycosylated native BChE tetramer purified from human plasma at 5.7 Å. Our structure reveals that the BChE tetramer is organized as a staggered dimer of dimers. Tetramerization is mediated by assembly of the C-terminal tryptophan amphiphilic tetramerization (WAT) helices from each subunit as a superhelical assembly around a central lamellipodin-derived oligopeptide with a proline-rich attachment domain (PRAD) sequence that adopts a polyproline II helical conformation and runs antiparallel. The catalytic domains within a dimer are asymmetrically linked to the WAT/PRAD. In the resulting arrangement, the tetramerization domain is largely shielded by the catalytic domains, which may contribute to the stability of the human BChE (HuBChE) tetramer. Our cryo-EM structure reveals the basis for assembly of the native tetramers and has implications for the therapeutic applications of HuBChE. This mode of tetramerization is seen only in the cholinesterases but may provide a promising template for designing other proteins with improved circulatory residence times. [ABSTRACT FROM AUTHOR]

Background Phosphodiesterases (PDEs) are a superfamily of evolutionary conserved cyclic nucleotides (cAMP/cGMP) hydrolysing enzymes, components of transduction pathways regulating crucial aspects of cell life. PDE5, one of these families, is the molecular target of several drugs used to treat erectile dysfunction and pulmonary hypertension. Despite its medical relevance, PDE5 macromolecular structure has only been solved for the isolated regulatory and catalytic domains. The definition of the quaternary structure of the full length PDE5 (MmPDE5A1), produced in large amounts in the yeast Kluyveromyces lactis , could greatly enhance the knowledge on its assembly/allosteric regulation and the development of new inhibitors for clinical-therapeutic applications. Methods Small-angle X-ray scattering (SAXS), analytical ultracentrifugation (AUC), size exclusion chromatography (SEC), native polyacrylamide gel electrophoresis (PAGE) and western blot (WB) were used to assess the assembly of PDE5A1. Results The full length MmPDE5A1 isoform is a mixture of dimers and tetramers in solution. We also report data showing that dimers and tetramers also coexist in vivo in platelets, blood components naturally containing high levels of PDE5. Conclusions This is the first time that structural studies on the full length protein evidenced the assembly of PDE5 in tetramers in addition to the expected dimers. General Significance The assembly of PDE5 in tetramers in platelets, beside the dimers, opens the possibility to alternative assembly/allosteric regulation of this enzyme, as component of large signaling complexes, in all cellular districts in which PDE5 is present. [ABSTRACT FROM AUTHOR]

Abstract: Although it is universally accepted that dopamine transporters (DATs) exist in monomers, dimers and tetramers (i.e. dimers of dimers), it is not known whether the oligomeric organization of DAT is a prerequisite for its ability to take up dopamine (DA), or whether each DAT protomer, the subunit of quaternary structure, functions independently in terms of DA translocation. In this study, copper phenanthroline (CuP) was used to selectively target surface DAT: increasing concentrations of CuP gradually cross‐linked natural DAT dimers in LLC‐PK1 cells stably expressing hDAT and thereby reduced DA uptake functionality until all surface DATs were inactivated. DATs that were not cross‐linked by CuP showed normal DA uptake with DA Km at ~ 0.5 μM and DA efflux with basal and amphetamine‐induced DA efflux as much as control values. The cocaine analog 2β‐carbomethoxy‐3β‐[4‐fluorophenyl]‐tropane (CFT) was capable to bind to copper‐cross‐linked DATs, albeit with an affinity more than fivefold decreased (Kd of CFT = 109 nM after cross‐linking vs 19 nM before). A kinetic analysis is offered describing the changing amounts of dimers and monomers with increasing [CuP], allowing the estimation of dimer functional activity compared with a DAT monomer. Consonant with previous conclusions for serotonin transporter and NET that only one protomer of an oligomer is active at the time, the present data indicated a functional activity of the DAT dimer of 0.74 relative to a monomer. [ABSTRACT FROM AUTHOR]

On the basis of the data of ESR and magnetic susceptibility of BiBaNbMnO solid solutions with perovskite-like structure ferro- and aniferromagnetic bonded dimers, trimers, and tetramers of Mn(II), Mn(III), and Mn(IV) atoms in various valence states were found. The distribution of aggregates and exchange parameters in the clusters were calculated as a function of fraction x of manganese atoms in the solid solutions. In the ESR spectra of diluted solid solutions there are absorption bands with g-factors ~4.3 and 2.1 assigned to Mn(II) and Mn(IV) atoms. [ABSTRACT FROM AUTHOR]

Coherent exciton delocalization in dye aggregate systems gives rise to a variety of intriguing optical phenomena, including J- and H-aggregate behavior and Davydov splitting. Systems that exhibit coherent exciton delocalization at room temperature are of interest for the development of artificial light-harvesting devices, colorimetric detection schemes, and quantum computers. Here, we report on a simple dye system templated by DNA that exhibits tunable optical properties. At low salt and DNA concentrations, a DNA duplex with two internally functionalized Cy5 dyes (i.e., dimer) persists and displays predominantly J-aggregate behavior. Increasing the salt and/or DNA concentrations was found to promote coupling between two of the DNA duplexes via branch migration, thus forming a four-armed junction (i.e., tetramer) with H-aggregate behavior. This H-tetramer aggregate exhibits a surprisingly large Davydov splitting in its absorbance spectrum that produces a visible color change of the solution from cyan to violet and gives clear evidence of coherent exciton delocalization. [ABSTRACT FROM AUTHOR]

This paper describes the supramolecular assembly of a macrocyclic ß-sheet containing residues 1622 of the ß-amyloid peptide, Aß. X-ray crystallography reveals that the macrocyclic ß-sheet assembles to form double-walled nanotubes, with an inner diameter of 7 nm and outer diameter of 11 nm. The inner wall is composed of an extended network of hydrogen-bonded dimers. The outer wall is composed of a separate extended network of ß-barrel-like tetramers. These large peptide nanotubes pack into a hexagonal lattice that resembles a honeycomb. The complexity and size of the peptide nanotubes rivals some of the largest tubular biomolecular assemblies, such as GroEL and microtubules. These observations demonstrate that small amyloidogenic sequences can be used to build large nanostructures. [ABSTRACT FROM AUTHOR]

Using first principles density functional theory based calculations, we have studied hydrogen dissociation on sub nanometer bimetallic clusters formed from d-block (Pd) and p-block (Ga) elements in gas phase to explore the feasibility of using them as cheap catalysts for hydrogen dissociation. Our calculations show that the dimers, trimers and tetramers of these clusters are thermodynamically more stable than the pure ones for all Ga concentrations. For a given cluster size, we find that the clusters containing equal amount of Pd and Ga are the most stable ones. In contrast to bulk PdGa, the contribution of Pd-d states to the highest occupied molecular orbitals of the bimetallic clusters are either very small or absent. Study of adsorption of hydrogen molecule on these clusters show that hydrogen binds in an activated form only on the Pd rich clusters. From the calculations of hydrogen dissociation barriers on tetramers of pure Pd, 25% Ga (Pd3Ga) and 50% Ga (Pd2Ga2) we find that Pd3Ga is the most efficient catalyst for hydrogen dissociation with barriers even lower than that on the PdGa surfaces. [ABSTRACT FROM AUTHOR]

Magnetic susceptibility of BiNbFeO ( х ≤ 0.06) iron-containing solid solutions with perovskite-like structure was studied. Exchange-bound aggregates of Fe(III) atoms with antiferro- and ferromagnetic exchange types are formed in the solid solutions. The exchange parameters in the clusters and the distribution of tetramers and dimers of Fe(III) atoms over the solid solutions as functions of paramagnetic atom content were calculated. [ABSTRACT FROM AUTHOR]

The oligomerization of precocene I with Brönsted and Lewis acids has been studied. In this way, the reaction of this chromene with HCl/MeOH gave two dimers, a trimer, a linear tetramer and a mixture of pentamers, whilst with FeCl 3 /HOAc a dimer and six cyclic tetramers were obtained. The cyclization of linear tetramers occurs between C-4‴ and C-6 or, in lower yield, between C-4‴ and C-8. In the formation of linear tetramers the C-8 functionalization was not detected, which could indicate that it occurs during the cyclization process. Moreover, oxidative one-electron coupling reactions were also observed in the treatment of precocene I with FeCl 3 /HOAc. On the other hand, the reaction of precocene I with FeCl 3 /Ac 2 O produced 6-acylation leading to the natural chromene encecalin. [ABSTRACT FROM AUTHOR]

1:1[α/α- Nα-Bn-hydrazino] pseudopeptide−polymer bioconjugates were synthesized and investigated as additives in a reference gas separation membrane (Pebax®) for CO2 capture. Pebax® is a polyether block amide thermoplastic elastomer provided by Arkema and is already well known for its good performance for CO2 separations. First, dimer and tetramer pseudopeptides were synthesized and their terminal amine was modified into a 'clickable' alkyne group in view of coupling. Second, an α-azido acrylic poly(ethylene glycol)-based oligomer was obtained by single-electron transfer living radical polymerization and the two partners were coupled using copper(I) catalyzed alkyne-azide cycloaddition (CuAAC) 'click' chemistry. The pseudopeptides and their bioconjugates were then assessed as original additives in Pebax® membranes for CO2/CH4 and CO2/N2 separations. The permeation data were analyzed according to the solution-diffusion model. Compared to pseudopeptides, the pseudopeptide−polymer bioconjugates enabled the membrane properties to be greatly improved with better permeability (×1.5) and a good constant selectivity for CO2 capture. The best membrane properties were obtained with 3 eq. wt% of the tetramer-based bioconjugate with a CO2 permeability of 194 Barrer (+46% compared to that of Pebax®) and constant selectivity (αCO2/N2 = 44 and αCO2/CH4 = 13). © 2016 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Summary Histone chaperones are critical for guiding specific post-transcriptional modifications of histones, safeguarding the histone deposition (or disassociation) of nucleosome (dis)assembly, and regulating chromatin structures to change gene activities. HAT1-interacting factor 1 (Hif1) has been reported to be an H3-H4 chaperone and to be involved in telomeric silencing and nucleosome (dis)assembly. However, the structural basis for the interaction of Hif1 with histones remains unknown. Here, we report the complex structure of Hif1 binding to H2A-H2B for uncovering the chaperone specificities of Hif1 on binding to both the H2A-H2B dimer and the H3-H4 tetramer. Our findings reveal that Hif1 interacts with the H2A-H2B dimer and the H3-H4 tetramer via distinct mechanisms, suggesting that Hif1 is a pivotal scaffold on alternate binding of H2A-H2B and H3-H4. These specificities are conserved features of the Sim3-Hif1-NASP interrupted tetratricopeptide repeat proteins, which provide clues for investigating their potential roles in nucleosome (dis)assembly. [ABSTRACT FROM AUTHOR]

The Glaser-Hay coupling reaction of a tetraphenylporphyrin derivative having two acetylenes in the quasi-trans positions was carried out. Although this oxidative coupling reaction has high reactivity and produces various butadiynylene oligomers, the degree of oligomerization was low. A large substrate and/or product might decrease the frequency factor of the Arrhenius equation and the resultant rate constant. This finding is in agreement with that of the one-pot synthesis of structurally well-defined porphyrin oligomers, such as trimers and tetramers, which are used frequently in the field of advanced materials. [ABSTRACT FROM AUTHOR]

The synthesis of the guanidine MesN{C(NCy2)}N(H)Mes (LH; Mes = 2,4,6-Me3C6H2, Cy = cyclohexyl), and its use as a proligand for the synthesis of alkaline earth metal complexes are reported. Described herein are (i) an unusual Hauser base cubane, (ii) a homoleptic and a base-stabilized magnesium complex featuring the same guanidinate ligands, and (iii) the comparison of a series of alkaline earth (Mg, Ca, Sr, Ba) bis(guanidinate) complexes, which allows the opportunity to compare the changing trends in bonding as the Group is descended. The reaction between LH and MeMgI(OEt2)2 yields the Hauser base as a mixture of the tetramer [Mg4L4(μ3-I)4] (1a) and dimer [Mg2L2(μ-I)2(OEt2)2] (1b), and the reaction with two equivalents of MgnBu2 leads to the formation of four-coordinate [MgL2] (2), which features a square-planar geometry for the magnesium cation, or five-coordinate [MgL2(THF)] (3), depending on the solvent used. 1a is the first crystallographically-characterized cubane structure to consist of four LAeX (L = ligand, X = halide) units. The complexes [AeL2(THF)2] (Ae = Ca, 4; Ae = Sr, 5) and [BaL2] (6) were synthesized via redox transmetallation/ligand exchange reactions. Complex 6 is the first example of a homoleptic, monomeric barium complex of the NCN ligand family, with the structure stabilized by a number of barium-arene interactions in the solid state. [ABSTRACT FROM AUTHOR]

Pyropheophorbides, chlorophyll- a derivatives, were quadruply esterified at the 17-propionate residue with a tetraol to give the covalently-linked tetramers. The synthetic tetramers were mixed with a C 70 fullerene derivative for utilization as the active layer in bulk heterojunction organic solar cells. Their photovoltaic performances were comparable to those of the corresponding monomeric pigments. The similar power conversion efficiencies would be ascribable to the increase of carrier mobility in the tetramers and the partial suppression of photoexcited state of the chlorin π-systems by intramolecular quenching. Photophysical properties of synthetic covalently-linked mesopyropheophorbide- a dimer, trimer, and tetramer as well as its monomer in a solution supported the intramolecular interaction of chlorin moieties to quench their singlet excited states, which was especially observed in the trimer and tetramer. [ABSTRACT FROM AUTHOR]

G-Protein-coupled receptors (GPCRs) were classically described as monomers. We now appreciate that they also function as homo- and hetero-oligomers, for which structural information is lacking. Here, we use available 3D structures and biochemical considerations to present and evaluate experimentally testable structural models for GPCR oligomers and associated G proteins. [ABSTRACT FROM AUTHOR]

The molecular machinery of the cyanobacterial circadian clock oscillator consists of three proteins, KaiA, KaiB and KaiC, which interact with each other to generate circadian oscillations in the presence of ATP (the in vitro Kai ABC clock oscillator). KaiB comprises four subunits organized as a dimer of dimers. Our previous study suggested that, on interaction with KaiC, the tetrameric KaiB molecule dissociates into two molecules of dimeric KaiB. It is uncertain whether KaiB also exists as a monomer and whether the KaiB monomer can drive normal circadian oscillation. To address these questions, we constructed a new KaiB oligomer mutant with an N-terminal deletion, KaiB10-108. KaiB10-108 was a monomer at 4 °C but a dimer at 35 °C. KaiB10-108 was able to drive normal clock oscillation in an in vitro reconstituted Kai ABC clock oscillator at 25 °C, but it was not able to drive normal circadian gene expression rhythms in cyanobacterial cells at 41 °C. Wild-type KaiB existed in equilibrium between a dimer and tetramer at lower KaiB concentrations or in the presence of 1 m NaCl. Our findings suggest that KaiB is in equilibrium between a monomer, dimer and tetramer in cyanobacterial cells. [ABSTRACT FROM AUTHOR]

The sulfoneamidohelicene tetramer in solution exhibits different molecular responses to the same cooling stimulus delivered once and twice under thermal hysteresis conditions. Its random-coil state at a high temperature was cooled and maintained at a given temperature for which its molecules remained in a random coil (first cooling); the resulting solution was heated and cooled, after which a helix dimer formed (second cooling). Such a property can be regarded as a molecular function of counting the numbers 1 and 2. [ABSTRACT FROM AUTHOR]

Optically active through-space conjugated oligomers, namely, a dimer, trimer, tetramer, and cyclic trimer, consisting of a planar-chiral [2.2]paracyclophane skeleton were synthesized. In the ground state, observed similarities in the chiroptical properties of the oligomers were attributed to the equivalent orientations of two adjacent chromophores. In the excited state, the oligomers were folded into a form analogous to a one-handed helix by photoexcitation. All the compounds in dispersed solution exhibited intense circularly polarized luminescence with relatively large anisotropy factors on the order of 10−3. [ABSTRACT FROM AUTHOR]

This contribution reports solution-phase structural studies of oligomers of a family of peptides derived from the β-amyloid peptide (Aβ). We had previously reported the X-ray crystallographic structures of the oligomers and oligomer assemblies formed in the solid state by a macrocyclic β-sheet peptide containing the Aβ15-23 nonapeptide. In the current study, we set out to determine its assembly in aqueous solution. In the solid state, macrocyclic β-sheet peptide 1 assembles to form hydrogen-bonded dimers that further assemble in a sandwich-like fashion to form tetramers through hydrophobic interactions between the faces bearing V18 and F20. In aqueous solution, macrocyclic β-sheet peptide 1 and homologue 2a form hydrogen-bonded dimers that assemble to form tetramers through hydrophobic interactions between the faces bearing L17, F19, and A21. In the solid state, the hydrogen-bonded dimers are antiparallel, and the β-strands are fully aligned, with residues 17-23 of one of the macrocycles aligned with residues 23-17 of the other. In solution, residues 17-23 of the hydrogen-bonded dimers are shifted out of alignment by two residues toward the C-termini. The two hydrogen-bonded dimers are nearly orthogonal in the solid state, while in solution the dimers are only slightly rotated. The differing morphology of the solution-state and solid-state tetramers is significant, because it may provide a glimpse into some of the structural bases for polymorphism among βoligomers in Alzheimer's disease. [ABSTRACT FROM AUTHOR]

β-2-Microglobulin (β2m) forms amyloid fibrils in the joints of patients undergoing hemodialysis treatment as a result of kidney failure. In the presence of stoichiometric amounts of Cu(II), β2m self-associates into discrete oligomeric species, including dimers, tetramers, and hexamers, before ultimately forming amyloid fibrils that contain no copper. To improve our understanding of whether Cu(II) is unique in its ability to induce film amyloid formation and to delineate the coordinative interactions that allow Cu(II) to exert its effect, we have examined the binding of Ni(II) and Zn(II) to film and the resulting influence that these metals have on β2m aggregation. We find that, in contrast to Cu(II), Ni(II) does not induce the oligomerization or aggregation of film, while Zn(II) promotes oligomerization but not amyloid fibril formation. Using X-ray absorption spectroscopy and new mass spectrometry-related techniques, we find that different binding modes are responsible for the different effects of Ni(II) and Zn(II). By comparing the binding modes of Cu(II) with Ni(II), we find that Cu(II) binding to Asp59 and the backbone amide between the first two residues of β2m are important for allowing the formation of amyloid-competent oligomers, as Ni(II) appears not to bind these sites on the protein. The oligomers formed in the presence of Zn(II) are permitted by this metal's ability to bridge two β2m units via His51. These oligomers, however, are not able to progress to form amyloid fibrils because Zn(II) does not induce the required structural changes near the N-terminus and His31. [ABSTRACT FROM AUTHOR]

Highlights: [•] Angptl4 and LPL can both be released by heparin from 3T3-L1 adipocytes. [•] Covalent dimers and tetramers of Angptl4 are formed on the cell surface. [•] Knockdown of Angptl4 rescues LPL from inactivation. [•] Inactivation requires cells that produce Angptl4 and occurs on the cell surface. [•] Inactivation of LPL does not occur during intracellular maturation and transport. [ABSTRACT FROM AUTHOR]

Previously, we reported that little canonical (H3.1–H4)2 tetramers split to form “hybrid” tetramers consisted of old and new H3.1–H4 dimers, but approximately 10% of (H3.3–H4)2 tetramers split during each cell cycle. In this report, we mapped the H3.3 nucleosome occupancy, the H3.3 nucleosome turnover rate and H3.3 nucleosome splitting events at the genome-wide level. Interestingly, H3.3 nucleosome turnover rate at the transcription starting sites (TSS) of genes with different expression levels display a bimodal distribution rather than a linear correlation towards the transcriptional activity, suggesting genes are either active with high H3.3 nucleosome turnover or inactive with low H3.3 nucleosome turnover. H3.3 nucleosome splitting events are enriched at active genes, which are in fact better markers for active transcription than H3.3 nucleosome occupancy itself. Although both H3.3 nucleosome turnover and splitting events are enriched at active genes, these events only display a moderate positive correlation, suggesting H3.3 nucleosome splitting events are not the mere consequence of H3.3 nucleosome turnover. Surprisingly, H3.3 nucleosomes with high splitting index are remarkably enriched at enhancers in a cell-type specific manner. We propose that the H3.3 nucleosomes at enhancers may be split by an active mechanism to regulate cell-type specific transcription. [ABSTRACT FROM AUTHOR]

Human hemoglobin (Hb) and haptoglobin (Hp) exhibit an extremely high affinity for each other, and the dissociation of Hb tetramers into dimers is generally believed to be a prerequisite for complex formation. We have investigated Hp interactions with native Hb, αα, and ββ cross-linked Hb (ααXLHb and ββXLHb, respectively), and rapid kinetics of Hb ligand binding as well as the redox reactivity in the presence of and absence of Hp. The quaternary conformation of ββ subunit cross-linking results in a higher binding affinity than that of αα subunit cross-linked Hb. However, ββ cross-linked Hb exhibits a four fold slower association rate constant than the reaction rate of unmodified Hb with Hp. The Hp contact regions in the Hb dimer interfaces appear to be more readily exposed in ββXLHb than ααXLHb. In addition, apart from the functional changes caused by chemical modifications, Hp binding does not induce appreciable effects on the ligand binding and redox reactions of ββXLHb. Our findings may therefore be relevant to the design of safer Hb-based oxygen therapeutics by utilizing this preferential binding of ββXLHb to Hp. This may ultimately provide a safe oxidative inactivation and clearance pathway for chemically modified Hbs in circulation. [ABSTRACT FROM AUTHOR]

G-protein-coupled receptors (GPCRs) constitute the largest family of receptors and major pharmacological targets. Whereas many GPCRs have been shown to form di-/oligomers, the size and stability of such complexes under physiological conditions are largely unknown. Here, we used direct receptor labeling with SNAP-tags and total internal reflection fluorescence microscopy to dynamically monitor single receptors on intact cells and thus compare the spatial arrangement, mobility, and supramolecular organization of three prototypical GPCRs: the β1-adrenergic receptor (β1AR), the β2-adrenergic receptor (β2AR), and the γ-aminobutyric acid (GABAB) receptor. These GPCRs showed very different degrees of di-/oligomerization, lowest for β1ARs (monomers/dimers) and highest for GABAB receptors (prevalently dimers/tetramers of heterodimers). The size of receptor complexes increased with receptor density as a result of transient receptor-receptor interactions. Whereas β1-/β2ARs were apparently freely diffusing on the cell surface, GABAB receptors were prevalently organized into ordered arrays, via interaction with the actin cytoskeleton. Agonist stimulation did not alter receptor di-/oligomerization, but increased the mobility of GABAB receptor complexes. These data provide a spatiotemporal characterization of β1-/β2ARs and GABAB receptors at single-molecule resolution. The results suggest that GPCRs are present on the cell surface in a dynamic equilibrium, with constant formation and dissociation of new receptor complexes that can be targeted, in a ligand-regulated manner, to different cell-surface microdomains. [ABSTRACT FROM AUTHOR]

The cytosolic NADP+-dependent malic enzyme (c-NADP-ME) has a dimer-dimer quaternary structure in which the dimer interface associates more tightly than the tetramer interface. In this study, the urea-induced unfolding process of the c- NADP-ME interface mutants was monitored using fluorescence and circular dichroism spectroscopy, analytical ultracentrifugation and enzyme activities. Here, we demonstrate the differential protein stability between dimer and tetramer interface interactions of human c-NADP-ME. Our data clearly demonstrate that the protein stability of c-NADP-ME is affected predominantly by disruptions at the dimer interface rather than at the tetramer interface. First, during thermal stability experiments, the melting temperatures of the wild-type and tetramer interface mutants are 8-10°C higher than those of the dimer interface mutants. Second, during urea denaturation experiments, the thermodynamic parameters of the wild-type and tetramer interface mutants are almost identical. However, for the dimer interface mutants, the first transition of the urea unfolding curves shift towards a lower urea concentration, and the unfolding intermediate exist at a lower urea concentration. Third, for tetrameric WT c-NADP-ME, the enzyme is first dissociated from a tetramer to dimers before the 2 M urea treatment, and the dimers then dissociated into monomers before the 2.5 M urea treatment. With a dimeric tetramer interface mutant (H142A/D568A), the dimer completely dissociated into monomers after a 2.5 M urea treatment, while for a dimeric dimer interface mutant (H51A/D90A), the dimer completely dissociated into monomers after a 1.5 M urea treatment, indicating that the interactions of c-NADP-ME at the dimer interface are truly stronger than at the tetramer interface. Thus, this study provides a reasonable explanation for why malic enzymes need to assemble as a dimer of dimers. [ABSTRACT FROM AUTHOR]

We show a simple method to control both the stability and the self-assembly behavior of DNA structures. By connecting two adjacent duplexes with small synthetic linkers, factors such as linker rigidity and DNA strand orientation can increase the thermal denaturation temperature of 17 base-pair duplexes by up to 10 °C, and significantly increase the cooperativity of melting of the two duplexes. The same DNA sequence can thus be tuned to melt at vastly different temperatures by selecting the linker structure and DNA-to-linker connectivity. In addition, a small rigid m-triphenylene linker directly affects the self-assembly product distribution. With this linker, changes in the orientation of the linked strands (e.g., 5'3' vs 3'3') can lead to dramatic changes in the self-assembly behavior, from the formation of cyclic dimer and tetramer to higher-order oligomers. These variations can be readily predicted using a simple strand-end alignment model. [ABSTRACT FROM AUTHOR]

Human α-hemoglobin stabilizing protein (AHSP) is a conserved mammalian erythroid protein that facilitates the production of Hemoglobin A by stabilizing free α-globin. AHSP rapidly binds to ferrous α with association (k'AHSP) and dissociation (kAHSP) rate constants of ≈ 10 μM-1 s-1 and 0.2 s-1, respectively, at pH 7.4 at 22 °C. A small slow phase was observed when AHSP binds to excess ferrous αCO. This slow phase appears to be due to cis to trans prolyl isomerization of the Asp29-Pro30 peptide bond in wild-type AHSP because it was absent when αCO was mixed with P30A and P30W AHSP, which are fixed in the trans conformation. This slow phase was also absent when met(Fe3+)-α reacted with wild-type AHSP, suggesting that met-α is capable of rapidly binding to either Pro30 conformer. Both wildtype and Pro30-substituted AHSPs drive the formation of a met-α hemichrome conformation following binding to either met- or oxy(Fe2+)-α. The dissociation rate of the met-α·AHSP complex (kAHSP ≈ 0.002 s-1) is ∼100-fold slower than that for ferrous α·AHSP complexes, resulting in a much higher affinity of AHSP for met-α. Thus, in vivo, AHSP acts as a molecular chaperone by rapidly binding and stabilizing met-α hemichrome folding intermediates. The low rate of met-α dissociation also allows AHSP to have a quality control function by kinetically trapping ferric α and preventing its incorporation into less stable mixed valence Hemoglobin A tetramers. Reduction of AHSP-bound met-α allows more rapid release to β subunits to form stable fully, reduced hemoglobin dimers and tetramers. [ABSTRACT FROM AUTHOR]

Abstract: The structure, energetics, and vibrational spectra of the (HXeCN)2 dimer were investigated at the CCSD(T), MP2 and B3LYP levels. Such properties of the (HXeCN)3 trimer and (HXeCN)4 tetramer were investigated at the B3LYP level. The dimer, trimer, and tetramer were predicted to have a C 2h , C 2v , and D 2d structure, respectively. In all of these oligomers, the N⋯Xe intermonomeric interaction is the most important one for holding the monomers together. Included with the ZPVE and BSSE, the stabilization energy of the dimer is 12.36kcal/mol at the CCSD(T) level, while those of the dimer, trimer, and tetramer are 10.42, 18.23, and 31.34kcal/mol, respectively, at the B3LYP level. At the B3LYP level, with respect to those of the isolated monomer, the C−Xe and Xe−H asymmetric stretching frequencies are shifted by −11.2 and +128.0cm−1 for the dimer, −51.6, +220.7 and −11.5, +96.6cm−1 for the trimer, and −14.1 and +201.8cm−1 for the tetramer. [Copyright &y& Elsevier]

Abstract: Serine recombinases, which generate double-strand breaks in DNA, must be carefully regulated to ensure that chemically active DNA complexes are assembled correctly. In the Hin-catalyzed site-specific DNA inversion reaction, two inversely oriented recombination sites on the same DNA molecule assemble into a synaptic complex that uniquely generates inversion products. The Fis-bound recombinational enhancer, together with topological constraints directed by DNA supercoiling, functions to regulate Hin synaptic complex formation and activity. We have isolated a collection of gain-of-function mutants in 22 positions within the catalytic and oligomerization domains of Hin using two genetic screens and by site-directed mutagenesis. One genetic screen measured recombination in the absence of Fis and the other assessed SOS induction as a readout of increased DNA cleavage. These mutations, together with molecular modeling, identify important sites of dynamic intrasubunit and intersubunit interactions that regulate assembly of the active tetrameric recombination complex. Of particular interest are interactions between the oligomerization helix (helix E) and the catalytic domain of the same subunit that function to hold the dimer in an inactive state in the absence of the Fis/enhancer system. Among these is a relay involving a triad of phenylalanines that are proposed to switch positions during the transition from dimers to the catalytically active tetramer. Novel Hin mutants that generate synaptic complexes that are blocked at steps prior to DNA cleavage are also described. [Copyright &y& Elsevier]

In spite of extensive research, the mechanism by which MutS initiates DNA mismatch repair (MMR) remains controversial. We use atomic force microscopy (AFM) to capture how MutS orchestrates the first step of E. coli MMR. AFM images captured two types of MutS/DNA complexes: single-site binding and loop binding. In most of the DNA loops imaged, two closely associated MutS dimers formed a tetrameric complex in which one of the MutS dimers was located at or near the mismatch. Surprisingly, in the presence of ATP, one MutS dimer remained at or near the mismatch site and the other, while maintaining contact with the first dimer, relocated on the DNA by reeling in DNA, thereby producing expanding DNA loops. Our results indicate that MutS tetramers composed of two non-equivalent MutS dimers drive E. coli MMR, and these new observations now reconcile the apparent contradictions of previous 'sliding' and 'bending/looping' models of interaction between mismatch and strand signal. [ABSTRACT FROM AUTHOR]

Summary: Sin resolvase is a site-specific serine recombinase that is normally controlled by a complex regulatory mechanism. A single mutation, Q115R, allows the enzyme to bypass the entire regulatory apparatus, such that no accessory proteins or DNA sites are required. Here, we present a 1.86 Å crystal structure of the Sin Q115R catalytic domain, in a tetrameric arrangement stabilized by an interaction between Arg115 residues on neighboring subunits. The subunits have undergone significant conformational changes from the inactive dimeric state previously reported. The structure provides a new high-resolution view of a serine recombinase active site that is apparently fully assembled, suggesting roles for the conserved active site residues. The structure also suggests how the dimer-tetramer transition is coupled to assembly of the active site. The tetramer is captured in a different rotational substate than that seen in previous hyperactive serine recombinase structures, and unbroken crossover site DNA can be readily modeled into its active sites. [Copyright &y& Elsevier]

Abstract: The comparative study of the interaction energy and its many-body decomposition for Be4, Mg4, and Ca4 at the all-electron CCSD(T)/aug-cc-pVQZ level is performed. For study of dependence of the binding energy and the orbital population on the cluster size the corresponding dimers and trimers were also calculated at the same level of theory. In comparison with weakly bound dimers, the binding energy in trimers and, especially, in tetramers drastically increases; e.g., Eb /N in Be3 is 7 times larger and in Be4 is 18.4 times larger than in Be2. This sharp increase is explained as a manifestation of many-body forces. As follows from the many-body decomposition, the tetramers, and trimers as well, are stabilized by the three-body forces, whereas the two- and four-body forces are repulsive. The attractive contribution to the three-body forces has a three-atom electron exchange origin. The latter benefits the promotion of ns-electrons to np-orbitals. The natural bond orbital (NBO) population analysis reveals a relatively large np-population in trimers and tetramers (in Be4 it is equal to 2p0.44). The population of the valence np-orbitals leads to the sp-hybridization providing the covalent bonding. [Copyright &y& Elsevier]

Saddle point searches have been employed to investigate the migration mechanisms of W clusters on W nanoparticles, and to determine the corresponding migration energies for the possible migration paths of these clusters. The tungsten clusters containing up to four adatoms are found to prefer 2D-compact structures with relatively low binding energies. The effect of interface and vertex regions on the migration behavior of the clusters is significantly strong, as compared to that of nanoparticle size. The migration mechanisms are quite different when the clusters are located at the center of the nanoparticle and near the interface or vertex areas. Near the interfaces and vertex areas, the substrate atoms tend to participate in the migration processes of the clusters, and can join the adatoms to form a larger cluster or lead to the dissociation of a cluster via the exchange mechanism, which results in the adatom crossing the facets. The lowest energy paths are used to be determined the energy barriers for W cluster migrations (from 1- to 4-atoms) on the facets, edges and vertex regions. The calculated energy barriers for the trimers suggest that the concerted migration is more probable than the successive jumping of a single adatom in the clusters. In addition, it of interest to note that the dimer shearing is a dominant migration mechanism for the tetramer, but needs to overcome a relatively higher migration energy than other clusters. [ABSTRACT FROM AUTHOR]

Abstract: Solving structures of native oligomeric protein complexes using traditional high-resolution NMR techniques remains challenging. However, increased utilization of computational platforms, and integration of information from less traditional NMR techniques with data from other complementary biophysical methods, promises to extend the boundary of NMR-applicable targets. This article reviews several of the techniques capable of providing less traditional and complementary structural information. In particular, the use of orientational constraints coming from residual dipolar couplings and residual chemical shift anisotropy offsets are shown to simplify the construction of models for oligomeric complexes, especially in cases of weak homo-dimers. Combining this orientational information with interaction site information supplied by computation, chemical shift perturbation, paramagnetic surface perturbation, cross-saturation and mass spectrometry allows high resolution models of the complexes to be constructed with relative ease. Non-NMR techniques, such as mass spectrometry, EPR and small angle X-ray scattering, are also expected to play increasingly important roles by offering alternative methods of probing the overall shape of the complex. Computational platforms capable of integrating information from multiple sources in the modeling process are also discussed in the article. And finally a new, detailed example on the determination of a chemokine tetramer structure will be used to illustrate how a non-traditional approach to oligomeric structure determination works in practice. [Copyright &y& Elsevier]

Summary: TAp63α, a homolog of the p53 tumor suppressor, is a quality control factor in the female germline. Remarkably, already undamaged oocytes express high levels of the protein, suggesting that TAp63α''s activity is under tight control of an inhibitory mechanism. Biochemical studies have proposed that inhibition requires the C-terminal transactivation inhibitory domain. However, the structural mechanism of TAp63α inhibition remains unknown. Here, we show that TAp63α is kept in an inactive dimeric state. We reveal that relief of inhibition leads to tetramer formation with ∼20-fold higher DNA affinity. In vivo, phosphorylation-triggered tetramerization of TAp63α is not reversible by dephosphorylation. Furthermore, we show that a helix in the oligomerization domain of p63 is crucial for tetramer stabilization and competes with the transactivation domain for the same binding site. Our results demonstrate how TAp63α is inhibited by complex domain-domain interactions that provide the basis for regulating quality control in oocytes. [Copyright &y& Elsevier]

Tonsil Actisil FF, a commercial bentonitic clay, promotes the formation of a series of electrophilic-aromatic-substitution products from para-methoxybenzyl acetate in carbon disulfide. The molecules obtained correspond to linear isomeric dimers, trimers, tetramers and a pentamer, according to their spectroscopic data. A clear indication of the title mechanistic pathway for the oligomerization growth was obtained from the analysis of a set of computational-chemistry calculations using the density-functional-theory level B3LYP/6-311++G(d,p). The corresponding conclusions were based on the computed dipole moments, the HOMO/LUMO distributions, and a natural-populations analysis of the studied molecules. [ABSTRACT FROM AUTHOR]

The supercomplex organization of photosystem complexes was studied in various cyanobacteria, a glaucocystophyte and a primitive rhodophyte by blue-native PAGE with a wide range of detergent concentrations. In contrast to known cyanobacteria that produced the PSI trimer, a filamentous N2-fixing cyanobacterium Anabaena sp. PCC 7120 and a glaucocystophyte Cyanophora paradoxa NIES 547 had a PSI tetramer and dimer but no trimer at all. This was confirmed by sucrose density gradient centrifugation. A primitive rhodophyte Cyanidioschyzon merolae had two species of PSI monomeric complex with a light-harvesting Chl complex of a different composition. These results are discussed with regard to the evolution of the PSI supercomplex. [ABSTRACT FROM AUTHOR]

Abstract: Enoyl-[acyl carrier protein] reductase (ENR) is an essential enzyme in type II fatty-acid synthesis that catalyzes the last step in each elongation cycle. Thus far FabI, FabL and FabK have been reported to carry out the reaction, with FabI being the most characterized. Some bacteria have more than one ENR, and Bacillus cereus has two (FabI and FabL) reported. Here, we have determined the crystal structures of the later in the apo form and in the ternary complex with NADP+ and an indole naphthyridinone inhibitor. The two structures are almost identical, except for the three stretches that are disordered in the apo form. The apo form exists as a homo-dimer in both crystal and solution, while the ternary complex forms a homo-tetramer. The three stretches disordered in the apo structure are important in the cofactor and the inhibitor binding as well as in tetramer formation. [ABSTRACT FROM AUTHOR]

Reaction of bis(acetylacetonato)dioxomolybdenum(VI) (MoO(acac)) with benzoic acid, in ethanol, yielded an oxo, ethoxide and benzoate-bridged dinuclear Mo(V) species MoO(acac)(μ-O)(μ-OCH)(μ-OOCCH) ( 1), with two terminal “MoO” units and one acac ligand retained on each Mo atom. This complex was analysed by IR and H NMR spectroscopy, X-ray crystallography and elemental analyses. The same reaction with substituted benzoic acids ( o-OH)CHCOOH, ( p-Cl)CHCOOH, (2,4-(OH))CHCOOH and ( o-I)CHCOOH produced a series of dinuclear complexes, all of which were characterized by IR and H NMR spectroscopy and elemental analyses. Terephthalic acid (( p-COOH)CH(COOH)) on reaction with MoO(acac) resulted in a tetramer [MoO(acac)(μ-O)(μ-OCH)(μ-OC)CH( p-μ-OC)MoO(acac)(μ-O)(μ-OCH)] ( 6) which was evident from the H NMR spectrum and elemental analyses. The role of ethanol as a reducing agent has proved instrumental in the synthesis of these dimeric Mo(V) complexes. The synthetic details and characterization by spectroscopic and X-ray crystallographic techniques are discussed. [ABSTRACT FROM AUTHOR]

A 3,3′-orthogonally linked phenothiazine cation radical dimer with methyl substituents at the 2, 2′, 4, and 4′ positions was synthesized. The dimer was isolated as a stable product which exists in a triplet state due to a torsion angle of ca. 80° between the two phenothiazine units. The tetramer was also synthesized by linking the dimers using a 1,4-phenylene spacer. [ABSTRACT FROM AUTHOR]

In this tutorial review, an effort towards presentation of a comprehensive account of the recent developments on various kinds of artemisinin derivatives including artemisinin dimers, trimers and tetramers has been made and their efficacy towards malaria parasites and different cancer cells lines was compared with that of artemisinins, and various other anti-malarial and anti-cancer drugs. It is expected that this review will provide first-hand information on artemisinin chemistry to organic/medicinal chemists, and pharmacologists working on anticancer and anti-malarial drug development. [ABSTRACT FROM AUTHOR]

Mono- and ditopic lithium ferrocenylhydridoborates Li[FcBH3] (2) and Li2[H3B-fc-BH3] (4) have been synthesized from FcB(OMe)2/(MeO)2B-fc-B(OMe)2 and Li[AlH4] (Fc = ferrocenyl; fc = 1,1′- ferrocenylene). X-ray quality crystals were grown from OEt2. Depending on the amount of Lit-coordinated solvent molecules, dimeric (2(OEt2)2) or tetrameric (2(OEt2)) aggregates are observed in the solid state. The ditopic derivative 4 crystallizes as two different macrocyclic dimers (4(OEt2)5 and 4(OEt2)6) in the unit cell. Each of the four aggregates is held together mainly by RBH3-η2-Li bonds. Addition of Me3SiCl to 2 or 4 generates the corresponding boranes FcBH2 (5) and H2B-fc-BH2 (6), which can be trapped by adduct formation with NMe2Et or SMe2. In contrast, when OEt2 is present as the sole Lewis basic donor, no stable ether adducts are obtained, but condensation takes place leading to Fc2BH (10) and the novel borane polymer [-fcB(H)-]n (9), respectively. In situ generation of FcBH2 (5) in the presence of cyclohexene gives Fc2BCy and BCy3 but no FcBCy2, thereby indicating that 5 undergoes condensation to 10 more quickly than hydroboration of an internal olef in can occur (Cy = cyclohexyl). Fc2BH (10) was further studied as a model system for the optimization of modification reactions of polymer [-fcB(H)-]n (9). Hydroboration of PhCCH or tBuCCH with 10 proceeds smoothly and quantitatively to give the corresponding vinylboranes Fc2B(CH=CHR) (11Ph, R = Ph; 11tBu, A = tBu), which were fully characterized. In a similar manner, the polymeric borane 9 was successfully transformed into ferrocenylborane polymers [-fcB(CH=CHR)-]n (12Ph, A = Ph; 12tBu R = tBu) that contain vinyl groups attached to boron. The structures of polymers 12 were confirmed by NMR and IR spectroscopy and mass spectrometry. The MALDI-TOF spectra of 12Ph and 12tBu showed patterns of equidistant peaks with peak separations that are consistent with the masses of the expected repeating units of each of the polymers. The absorption maxima in the UV-vis spectra of polymers 12 are significantly red-shifted in comparison to the dimeric model systems 11. [ABSTRACT FROM AUTHOR]

Oligomerization of members of the p53 family of transcription factors (p53, p63, and p73) is essential for their distinct functions in cell-cycle control and development. To elucidate the molecular basis for tetramer formation of the various family members, we solved the crystal structure of the human p73 tetramerization domain (residues 351-399). Similarly to the canonical p53 tetramer, p73 forms a tetramer with D2 symmetry that can be described as a dimer of dimers. The most striking difference between the p53 and p73 tetramerization domain is the presence of an additional C-terminal helix in p73. This helix, which is conserved in p63, is essential for stabilizing the overall architecture of the tetramer, as evidenced by the different oligomeric structures observed for a shortened variant lacking this helix. The helices act as clamps, wrapping around the neighboring dimer and holding it in place. In addition, we show by mass spectrometry that the tetramerization domains of p63 and p73, but not p53, fully exchange, with different mixed tetramers present at equilibrium, albeit at a relatively slow rate. Taken together, these data provide intriguing insights into the divergent evolution of the oligomerization domain within the p53 family, from the ancestral p63/p73-like protein toward smaller, less promiscuous monomeric building blocks in human p53, allowing functional separation of the p53 pathway from that of its family members. [ABSTRACT FROM AUTHOR]

Abstract: Interactions between Si ad-dimers on Si (001) have been studied by molecular dynamics simulations using the Stillinger–Weber potential. The interactions determine the formation of clusters from diffusing dimers. We show different pathways for the formation of multiple-dimer clusters and propose a new tetramer (TCC) structure formation by two diffusing dimers interacting. This tetramer structure has been found to be energetically stable with respect to isolated ad-dimers. Moreover, their local bonding configuration is very similar to the B-type step edge which is known to be the favoured adsorption site for epitaxial growth. The proposed tetramer may play a crucial role as the nucleus of the new epitaxial layer on Si (001). [Copyright &y& Elsevier]

Malic enzyme has a dimer of dimers quaternary structure in which the dimer interface associates more tightly than the tetramer interface, In addition, the enzyme has distinct active sites within each subunit. The mitochondrial NAD(P)[sup+]-dependent malic enzyme (m-NAD(P)-ME) isoform behaves cooperatively and allosterically and exhibits a quaternary structure in dimertetramer equilibrium. The cytosolic NADP[sup+]-dependent malic enzyme (c-NADP-ME) isoform is noncooperative and nonallosteric and exists as a stable tetramer. In this study, we analyze the essential factors governing the quaternary structure stability for human c-NADP-ME and m-NAD(P)-ME. Site-directed mutagenesis at the dimer and tetramer interfaces was employed to generate a series of dimers of c-NADP-ME and m-NAD(P)ME. Size distribution analysis demonstrated that human c-NADP-ME exists mainly as a tetramer, whereas human m-NAD(P)-ME exists as a mixture of dimers and tetramers. Kinetic data indicated that the enzyme activity of c-NADP-ME is not affected by disruption of the interface. There are no significant differences in the kinetic properties between AB and AD dimers, and the dimeric form of c-NADP-ME is as active as tetramers. In contrast, disrupting the interface of m-NAD(P)-ME causes the enzyme to be less active than wild type and to become less cooperative for malate binding; the kcat values of mutants decreased with increasing Kd,24 values, indicating that the dissociation of subunits at the dimer or tetramer interfaces significantly affects the enzyme activity. The above results suggest that the tetramer is required for a fully functional m-NAD(P)-ME. Taken together, the analytical ultracentrifugation data and the kinetic analysis of these interface mutants demonstrate the differential role of tetramer organization for the c-NADP-ME and m-NAD(P)-ME isoforms. The regulatory mechanism of m-NAD(P)-ME is closely related to the tetramer formation of this isoform. [ABSTRACT FROM AUTHOR]

The article presents a study which examines the continuous variation method used to characterize lithium enolate solution structures. It states that the method shows the cyclic dimers of 1―3 simple enolates in N,N,N',N'-tetramethylethylenediamine (TMEDA), cubic tetramers (5), and cubic tetramers (6). It also notes the statistical dimer and nonstatistical dimer distributions in lithium enolates as well as its higher oligomers.

Abstract: Supramolecular complexes between two cavity-appended porphyrin hosts and three bifunctional guests are described. The host with a single cavity exclusively forms dimers with the bifunctional guests, while the double cavity host yields tetramers and higher order assemblies. The role of allosteric interactions in the binding and assembly process is highlighted. [Copyright &y& Elsevier]