Responsiveness of sphingosine phosphate lyase insufficiency syndrome to vitamin B6 cofactor supplementation

Sphingosine-1-phosphate (S1P) lyase is a vitamin B6-dependent enzyme that degrades sphingosine-1-phosphate in the final step of sphingolipid metabolism. In 2017, a new inherited disorder was described caused by mutations in SGPL1, which encodes sphingosine phosphate lyase (SPL). This condition is referred to as SPL insufficiency syndrome (SPLIS) or alternatively as nephrotic syndrome type 14 (NPHS14). Patients with SPLIS exhibit lymphopenia, nephrosis, adrenal insufficiency, and/or neurological defects. No targeted therapy for SPLIS has been reported. Vitamin B6 supplementation has therapeutic activity in some genetic diseases involving B6-dependent enzymes, a finding ascribed largely to the vitamin's chaperone function. We investigated whether B6 supplementation might have activity in SPLIS patients. We retrospectively monitored responses of disease biomarkers in patients supplemented with B6 and measured SPL activity and sphingolipids in B6-treated patient-derived fibroblasts. In two patients, disease biomarkers responded to B6 supplementation. S1P abundance and activity levels increased and sphingolipids decreased in response to B6. One responsive patient is homozygous for an SPL R222Q variant present in almost 30% of SPLIS patients. Molecular modeling suggests the variant distorts the dimer interface which could be overcome by cofactor supplementation. We demonstrate the first potential targeted therapy for SPLIS and suggest that 30% of SPLIS patients might respond to cofactor supplementation.

Charge neutralization in the active site of the catalytic trimer of aspartate transcarbamoylase promotes diverse structural changes

A classical model for allosteric regulation of enzyme activity posits an equilibrium between inactive and active conformations. An alternative view is that allosteric activation is achieved by increasing the potential for conformational changes that are essential for catalysis. In the present study, substitution of a basic residue in the active site of the catalytic (C) trimer of aspartate transcarbamoylase with a non-polar residue results in large interdomain hinge changes in the three chains of the trimer. One conformation is more open than the chains in both the wild-type C trimer and the catalytic chains in the holoenzyme, the second is closed similar to the bisubstrate-analog bound conformation and the third hinge angle is intermediate to the other two. The active-site 240s loop conformation is very different between the most open and closed chains, and is disordered in the third chain, as in the holoenzyme. We hypothesize that binding of anionic substrates may promote similar structural changes. Further, the ability of the three catalytic chains in the trimer to access the open and closed active-site conformations simultaneously suggests a cyclic catalytic mechanism, in which at least one of the chains is in an open conformation suitable for substrate binding whereas another chain is closed for catalytic turnover. Based on the many conformations observed for the chains in the isolated catalytic trimer to date, we propose that allosteric activation of the holoenzyme occurs by release of quaternary constraint into an ensemble of active-site conformations.

Human CTP synthase filament structure reveals the active enzyme conformation

The universally conserved enzyme CTP synthase (CTPS) forms filaments in bacteria and eukaryotes. In bacteria, polymerization inhibits CTPS activity and is required for nucleotide homeostasis. Here we show that for human CTPS, polymerization increases catalytic activity. The cryo-EM structures of bacterial and human CTPS filaments differ considerably in overall architecture and in the conformation of the CTPS protomer, explaining the divergent consequences of polymerization on activity. The structure of human CTPS filament, the first structure of the full-length human enzyme, reveals a novel active conformation. The filament structures elucidate allosteric mechanisms of assembly and regulation that rely on a conserved conformational equilibrium. The findings may provide a mechanism for increasing human CTPS activity in response to metabolic state and challenge the assumption that metabolic filaments are generally storage forms of inactive enzymes. Allosteric regulation of CTPS polymerization by ligands likely represents a fundamental mechanism underlying assembly of other metabolic filaments.

Crystal structures reveal a new and novel FoxO1 binding site within the human glucose-6-phosphatase catalytic subunit 1 gene promoter

Human glucose-6-phosphatase plays a vital role in blood glucose homeostasis and holds promise as a therapeutic target for diabetes. Expression of its catalytic subunit gene 1 (G6PC1) is tightly regulated by metabolic-response transcription factors such as FoxO1 and CREB. Although at least three potential FoxO1 binding sites (insulin response elements, IREs) and one CREB binding site (cAMP response element, CRE) within the proximal region of the G6PC1 promoter have been identified, the interplay between FoxO1 and CREB and between FoxO1 bound at multiple IREs has not been well characterized. Here we present the crystal structures of the FoxO1 DNA binding domain in complex with the G6PC1 promoter. These complexes reveal the presence of a new non-consensus FoxO1 binding site that overlaps the CRE, suggesting a mutual exclusion mechanism for FoxO1 and CREB binding at the G6PC1 promoter. Additional findings include (i) non-canonical FoxO1 recognition sites, (ii) incomplete FoxO1 occupancies at the available IRE sites, and (iii) FoxO1 dimeric interactions that may play a role in stabilizing DNA looping. These findings provide insight into the regulation of G6PC1 gene transcription by FoxO1, and demonstrate a high versatility of target gene recognition by FoxO1 that correlates with its diverse roles in biology.

Crystal structure of 3WJ core revealing divalent ion-promoted thermostability and assembly of the Phi29 hexameric motor pRNA

The bacteriophage phi29 DNA packaging motor, one of the strongest biological motors characterized to date, is geared by a packaging RNA (pRNA) ring. When assembled from three RNA fragments, its three-way junction (3WJ) motif is highly thermostable, is resistant to 8 M urea, and remains associated at extremely low concentrations in vitro and in vivo. To elucidate the structural basis for its unusual stability, we solved the crystal structure of this pRNA 3WJ motif at 3.05 Å. The structure revealed two divalent metal ions that coordinate 4 nt of the RNA fragments. Single-molecule fluorescence resonance energy transfer (smFRET) analysis confirmed a structural change of 3WJ upon addition of Mg²⁺. The reported pRNA 3WJ conformation is different from a previously published construct that lacks the metal coordination sites. The phi29 DNA packaging motor contains a dodecameric connector at the vertex of the procapsid, with a central pore for DNA translocation. This portal connector serves as the foothold for pRNA binding to procapsid. Subsequent modeling of a connector/pRNA complex suggests that the pRNA of the phi29 DNA packaging motor exists as a hexameric complex serving as a sheath over the connector. The model of hexameric pRNA on the connector agrees with AFM images of the phi29 pRNA hexamer acquired in air and matches all distance parameters obtained from cross-linking, complementary modification, and chemical modification interference.

Identification of a Novel Noncatalytic Bicarbonate Binding Site in Eubacterial β-Carbonic Anhydrase

The structures of beta class carbonic anhydrases (beta-CAs) determined so far fall into two distinct subclasses based on the observed coordination of the catalytic zinc (Zn2+) ion. The subclass of beta-CAs that coordinate Zn2+ tetrahedrally with four protein-derived ligands is represented by the structures of orthologues from Porphyridium purpureum, Escherichia coli, and Mycobacterium tuberculosis. Here we present the structure of an additional member of that subclass, that from Haemophilus influenzae, as well as detailed kinetic analysis, revealing the correspondence between structural classification and kinetic profile for this subclass. In addition, we identify a unique, noncatalytic binding mode for the substrate bicarbonate that occurs in both the H. influenzae and E. coli enzymes. The kinetic and structural analysis indicates that binding of bicarbonate in this site of the enzyme may modulate its activity by influencing a pH-dependent, cooperative transition between active and inactive forms. We hypothesize that the two structural subclasses of beta-CAs may provide models for the proposed active and inactive forms of the H. influenzae and E. coli enzymes.

Mechanisms of product feedback regulation and drug resistance in cytidine triphosphate synthetases from the structure of a CTP-inhibited complex

Cytidine triphosphate synthetases (CTPSs) synthesize CTP and regulate its intracellular concentration through direct interactions with the four ribonucleotide triphosphates. In particular, CTP product is a feedback inhibitor that competes with UTP substrate. Selected CTPS mutations that impart resistance to pyrimidine antimetabolite inhibitors also relieve CTP inhibition and cause a dramatic increase in intracellular CTP concentration, indicating that the drugs act by binding to the CTP inhibitory site. Resistance mutations map to a pocket that, although adjacent, does not coincide with the expected UTP binding site in apo Escherichia coli CTPS [EcCTPS; Endrizzi, J. A., et al. (2004) Biochemistry 43, 6447-6463], suggesting allosteric rather than competitive inhibition. Here, bound CTP and ADP were visualized in catalytically active EcCTPS crystals soaked in either ATP and UTP substrates or ADP and CTP products. The CTP cytosine ring resides in the pocket predicted by the resistance mutations, while the triphosphate moiety overlaps the putative UTP triphosphate binding site, explaining how CTP competes with UTP while CTP resistance mutations are acquired without loss of catalytic efficiency. Extensive complementarity and interaction networks at the interfacial binding sites provide the high specificity for pyrimidine triphosphates and mediate nucleotide-dependent tetramer formation. Overall, these results depict a novel product inhibition strategy in which shared substrate and product moieties bind to a single subsite while specificity is conferred by separate subsites. This arrangement allows for independent adaptation of UTP and CTP binding affinities while efficiently utilizing the enzyme surface.

Crystal structure of Escherichia coli cytidine triphosphate synthetase, a nucleotide-regulated glutamine amidotransferase/ATP-dependent amidoligase fusion protein and homologue of anticancer and antiparasitic drug targets

Cytidine triphosphate synthetases (CTPSs) produce CTP from UTP and glutamine, and regulate intracellular CTP levels through interactions with the four ribonucleotide triphosphates. We solved the 2.3-A resolution crystal structure of Escherichia coli CTPS using Hg-MAD phasing. The structure reveals a nearly symmetric 222 tetramer, in which each bifunctional monomer contains a dethiobiotin synthetase-like amidoligase N-terminal domain and a Type 1 glutamine amidotransferase C-terminal domain. For each amidoligase active site, essential ATP- and UTP-binding surfaces are contributed by three monomers, suggesting that activity requires tetramer formation, and that a nucleotide-dependent dimer-tetramer equilibrium contributes to the observed positive cooperativity. A gated channel that spans 25 A between the glutamine hydrolysis and amidoligase active sites provides a path for ammonia diffusion. The channel is accessible to solvent at the base of a cleft adjoining the glutamine hydrolysis active site, providing an entry point for exogenous ammonia. Guanine nucleotide binding sites of structurally related GTPases superimpose on this cleft, providing insights into allosteric regulation by GTP. Mutations that confer nucleoside drug resistance and release CTP inhibition map to a pocket that neighbors the UTP-binding site and can accommodate a pyrimidine ring. Its location suggests that competitive feedback inhibition is affected via a distinct product/drug binding site that overlaps the substrate triphosphate binding site. Overall, the E. coli structure provides a framework for homology modeling of other CTPSs and structure-based design of anti-CTPS therapeutics.

Structure of Mycobacterium tuberculosis PknB supports a universal activation mechanism for Ser/Thr protein kinases

A family of eukaryotic-like Ser/Thr protein kinases occurs in bacteria, but little is known about the structures and functions of these proteins. Here we characterize PknB, a transmembrane signaling kinase from Mycobacterium tuberculosis. The intracellular PknB kinase domain is active autonomously, and the active enzyme is phosphorylated on residues homologous to regulatory phospho-acceptors in eukaryotic Ser/Thr kinases. The crystal structure of the PknB kinase domain in complex with an ATP analog reveals the active conformation. The predicted fold of the PknB extracellular domain matches the proposed targeting domain of penicillin-binding protein 2x. The structural and chemical similarities of PknB to metazoan homologs support a universal activation mechanism of Ser/Thr protein kinases in prokaryotes and eukaryotes.

Crystal structure of E. coli beta-carbonic anhydrase, an enzyme with an unusual pH-dependent activity

Carbonic anhydrases fall into three distinct evolutionary and structural classes: alpha, beta, and gamma. The beta-class carbonic anhydrases (beta-CAs) are widely distributed among higher plants, simple eukaryotes, eubacteria, and archaea. We have determined the crystal structure of ECCA, a beta-CA from Escherichia coli, to a resolution of 2.0 A. In agreement with the structure of the beta-CA from the chloroplast of the red alga Porphyridium purpureum, the active-site zinc in ECCA is tetrahedrally coordinated by the side chains of four conserved residues. These results confirm the observation of a unique pattern of zinc ligation in at least some beta-CAS: The absence of a water molecule in the inner coordination sphere is inconsistent with known mechanisms of CA activity. ECCA activity is highly pH-dependent in the physiological range, and its expression in yeast complements an oxygen-sensitive phenotype displayed by a beta-CA-deletion strain. The structural and biochemical characterizations of ECCA presented here and the comparisons with other beta-CA structures suggest that ECCA can adopt two distinct conformations displaying widely divergent catalytic rates.

High-resolution structure of the HNF-1alpha dimerization domain

The N-terminal dimerization domain of the transcriptional activator hepatocyte nuclear factor-1alpha (HNF-1alpha) is essential for DNA binding and association of the transcriptional coactivator, DCoH (dimerization cofactor of HNF-1). To investigate the basis for dimerization of HNF-1 proteins, we determined the 1.2 A resolution X-ray crystal structure of the dimerization domain of HNF-1alpha (HNF-p1). Phasing was facilitated by devising a simple synthesis for Fmoc-selenomethionine and substituting leucine residues with selenomethionine. The HNF-1 dimerization domain forms a unique, four-helix bundle that is preserved with localized conformational shifts in the DCoH complex. In three different crystal forms, HNF-p1 displays subtle shifts in the conformation of the interhelix loop and the crossing angle between the amino- and carboxyl-terminal helices. In all three crystal forms, the HNF-p1 dimers pair through an exposed hydrophobic surface that also forms the binding site for DCoH. Conserved core residues in the dimerization domain of the homologous transcriptional regulator HNF-1beta rationalize the functional heterodimerization of the HNF-1alpha and HNF-1beta proteins. Mutations in HNF-1alpha are associated with maturity-onset diabetes of the young type 3 (MODY3), and the structure of HNF-p1 provides insights into the effects of three MODY3 mutations.

Structural basis of dimerization, coactivator recognition and MODY3 mutations in HNF-1alpha

Maturity-onset diabetes of the young type 3 (MODY3) results from mutations in the transcriptional activator hepatocyte nuclear factor-1alpha (HNF-1alpha). Several MODY3 mutations target the HNF-1alpha dimerization domain (HNF-p1), which binds the coactivator, dimerization cofactor of HNF-1 (DCoH). To define the mechanism of coactivator recognition and the basis for the MODY3 phenotype, we determined the cocrystal structure of the DCoH-HNF-p1 complex and characterized biochemically the effects of MODY3 mutations in HNF-p1. The DCoH-HNF-p1 complex comprises a dimer of dimers in which HNF-p1 forms a unique four-helix bundle. Through rearrangements of interfacial side chains, a single, bifunctional interface in the DCoH dimer mediates both HNF-1alpha binding and formation of a competing, transcriptionally inactive DCoH homotetramer. Consistent with the structure, MODY3 mutations in HNF-p1 reduce activator function by two distinct mechanisms.

Cloning, crystallization and preliminary characterization of a beta-carbonic anhydrase from Escherichia coli

Carbonic anhydrases are zinc metalloenzymes that fall into three distinct evolutionary and structural classes, alpha, beta and gamma. Although alpha-class enzymes, particularly mammalian carbonic anhydrase II, have been the subject of extensive structural studies, for the beta class, consisting of a wide variety of prokaryotic and plant chloroplast carbonic anhydrases, the structural data is quite limited. A member of the beta class from E. coli (CynT2) has been crystallized in native and selenomethionine-labelled forms and multiwavelength anomalous dispersion techniques have been applied in order to determine the positions of anomalous scatterers. The resulting phase information is sufficient to produce an interpretable electron-density map. A crystal structure for CynT2 would contribute significantly to the emerging structural knowledge of a biologically important class of enzymes that perform critical functions in carbon fixation and prokaryotic metabolism.

Binding of bisubstrate analog promotes large structural changes in the unregulated catalytic trimer of aspartate transcarbamoylase: implications for allosteric regulation

A central problem in understanding enzyme regulation is to define the conformational states that account for allosteric changes in catalytic activity. For Escherichia coli aspartate transcarbamoylase (ATCase; EC) the active, relaxed (R state) holoenzyme is generally assumed to be represented by the crystal structure of the complex of the holoenzyme with the bisubstrate analog N-phosphonacetyl-L-aspartate (PALA). It is unclear, however, which conformational differences between the unliganded, inactive, taut (T state) holoenzyme and the PALA complex are attributable to localized effects of inhibitor binding as contrasted to the allosteric transition. To define the conformational changes in the isolated, nonallosteric C trimer resulting from the binding of PALA, we determined the 1.95-A resolution crystal structure of the C trimer-PALA complex. In contrast to the free C trimer, the PALA-bound trimer exhibits approximate threefold symmetry. Conformational changes in the C trimer upon PALA binding include ordering of two active site loops and closure of the hinge relating the N- and C-terminal domains. The C trimer-PALA structure closely resembles the liganded C subunits in the PALA-bound holoenzyme. This similarity suggests that the pronounced hinge closure and other changes promoted by PALA binding to the holoenzyme are stabilized by ligand binding. Consequently, the conformational changes attributable to the allosteric transition of the holoenzyme remain to be defined.

Crystal structure of Escherichia coli malate synthase G complexed with magnesium and glyoxylate at 2.0 A resolution: mechanistic implications

The crystal structure of selenomethionine-substituted malate synthase G, an 81 kDa monomeric enzyme from Escherichia coli has been determined by MAD phasing, model building, and crystallographic refinement to a resolution of 2.0 A. The crystallographic R factor is 0.177 for 49 242 reflections observed at the incident wavelength of 1.008 A, and the model stereochemistry is satisfactory. The basic fold of the enzyme is that of a beta8/alpha8 (TIM) barrel. The barrel is centrally located, with an N-terminal alpha-helical domain flanking one side. An inserted beta-sheet domain folds against the opposite side of the barrel, and an alpha-helical C-terminal domain forms a plug which caps the active site. Malate synthase catalyzes the condensation of glyoxylate and acetyl-coenzyme A and hydrolysis of the intermediate to yield malate and coenzyme A, requiring Mg(2+). The structure reveals an enzyme-substrate complex with glyoxylate and Mg(2+) which coordinates the aldehyde and carboxylate functions of the substrate. Two strictly conserved residues, Asp631 and Arg338, are proposed to provide concerted acid-base chemistry for the generation of the enol(ate) intermediate of acetyl-coenzyme A, while main-chain hydrogen bonds and bound Mg(2+) polarize glyoxylate in preparation for nucleophilic attack. The catalytic strategy of malate synthase appears to be essentially the same as that of citrate synthase, with the electrophile activated for nucleophilic attack by nearby positive charges and hydrogen bonds, while concerted acid-base catalysis accomplishes the abstraction of a proton from the methyl group of acetyl-coenzyme A. An active site aspartate is, however, the only common feature of these two enzymes, and the active sites of these enzymes are produced by quite different protein folds. Interesting similarities in the overall folds and modes of substrate recognition are discussed in comparisons of malate synthase with pyruvate kinase and pyruvate phosphate dikinase.

Assessment of the allosteric mechanism of aspartate transcarbamoylase based on the crystalline structure of the unregulated catalytic subunit

The lack of knowledge of the three-dimensional structure of the trimeric, catalytic (C) subunit of aspartate transcarbamoylase (ATCase) has impeded understanding of the allosteric regulation of this enzyme and left unresolved the mechanism by which the active, unregulated C trimers are inactivated on incorporation into the unliganded (taut or T state) holoenzyme. Surprisingly, the isolated C trimer, based on the 1.9-A crystal structure reported here, resembles more closely the trimers in the T state enzyme than in the holoenzyme:bisubstrate-analog complex, which has been considered as the active, relaxed (R) state enzyme. Unlike the C trimer in either the T state or bisubstrate-analog-bound holoenzyme, the isolated C trimer lacks 3-fold symmetry, and the active sites are partially disordered. The flexibility of the C trimer, contrasted to the highly constrained T state ATCase, suggests that regulation of the holoenzyme involves modulating the potential for conformational changes essential for catalysis. Large differences in structure between the active C trimer and the holoenzyme:bisubstrate-analog complex call into question the view that this complex represents the activated R state of ATCase.

High-resolution structures of the bifunctional enzyme and transcriptional coactivator DCoH and its complex with a product analogue

DCoH, the dimerization cofactor of hepatocyte nuclear factor 1 (HNF-1), functions as both a transcriptional coactivator and a pterin dehydratase. To probe the relationship between these two functions, the X-ray crystal structures of the free enzyme and its complex with the product analogue 7,8-dihydrobiopterin were refined at 2.3 A resolution. The ligand binds at four sites per tetrameric enzyme, with little apparent conformational change in the protein. Each active-site cleft is located in a subunit interface, adjacent to a prominent saddle motif that has structural similarities to the TATA binding protein. The pterin binds within an arch of aromatic residues that extends across one dimer interface. The bound ligand makes contacts to three conserved histidines, and this arrangement restricts proposals for the enzymatic mechanism of dehydration. The dihedral symmetry of DCoH suggests that binding to the dimerization domain of HNF-1 likely involves the superposition of two-fold rotation axes of the two proteins.

Crystal structure of DCoH, a bifunctional, protein-binding transcriptional coactivator

DCoH, the dimerization cofactor of hepatocyte nuclear factor-1, stimulates gene expression by associating with specific DNA binding proteins and also catalyzes the dehydration of the biopterin cofactor of phenylalanine hydroxylase. The x-ray crystal structure determined at 3 angstrom resolution reveals that DCoH forms a tetramer containing two saddle-shaped grooves that comprise likely macromolecule binding sites. Two equivalent enzyme active sites flank each saddle, suggesting that there is a spatial connection between the catalytic and binding activities. Structural similarities between the DCoH fold and nucleic acid-binding proteins argue that the saddle motif has evolved to bind diverse ligands or that DCoH unexpectedly may bind nucleic acids.

2.8-A structure of yeast serine carboxypeptidase

The structure of monomeric serine carboxypeptidase from Saccharomyces cerevisiae (CPD-Y), deglycosylated by an efficient new procedure, has been determined by multiple isomorphous replacement and crystallographic refinement. The model contains 3333 non-hydrogen atoms, all 421 amino acids, 3 of 4 carbohydrate residues, 5 disulfide bridges, and 38 water molecules. The standard crystallographic R-factor is 0.162 for 10,909 reflections observed between 20.0- and 2.8-A resolution. The model has rms deviations from ideality of 0.016 A for bond lengths and 2.7 degrees for bond angles and from restrained thermal parameters of 7.9 A2. CPD-Y, which exhibits a preference for hydrophobic peptides, is distantly related to dimeric wheat serine carboxypeptidase II (CPD-WII), which has a preference for basic peptides. Comparison of the two structures suggests that substitution of hydrophobic residues in CPD-Y for negatively charged residues in CPD-WII in the binding site is largely responsible for this difference. Catalytic residues are in essentially identical configurations in the two molecules, including strained main-chain conformational angles for three active site residues (Ser 146, Gly 52, and Gly 53) and an unusual hydrogen bond between the carboxyl groups of Glu 145 and Glu 65. The binding of an inhibitor, benzylsuccinic acid, suggests that the C-terminal carboxylate binding site for peptide substrates is Asn 51, Gly 52, Glu 145, and His 397 and that the "oxyanion hole" consists of the amides of Gly 53 and Tyr 147. A surprising result of the study is that the domains consisting of residues 180-317, which form a largely alpha-helical insertion into the highly conserved cores surrounding the active site, are quite different structurally in the two molecules. It is suggested that these domains have evolved much more rapidly than other parts of the molecule and are involved in substrate recognition.

Interaction of mono- and dianions with cyanase: evidence for apparent half-site binding

Cyanase is an inducible enzyme in Escherichia coli that catalyzes bicarbonate-dependent hydrolysis of cyanate. The dianions oxalate, oxalacetate, and malonate are slow-binding inhibitors of cyanase, and some monoanions such as azide and chloride also inhibit cyanase activity [Anderson, P. M., & Little, R. M. (1986) Biochemistry 25, 1621-1626]. The purpose of this study was to investigate the interaction of selected dianions and monoanions by kinetic and equilibrium dialysis binding studies in an effort to obtain information about the active site and catalytic mechanism. Measurement of the effectiveness of 30 different dianions as inhibitors of cyanase showed a significant degree of structural and/or isomeric specificity and considerable variation with respect to the slow-binding nature of the inhibition. Oxalate and oxalacetate both show extreme slow-binding inhibition at very low concentrations. Kinetic studies of the rate of inhibition of cyanase by oxalate showed that the reaction is pseudo first order with respect to oxalate concentration and the results are consistent with a pathway in which oxalate forms a complex with the enzyme in a rapid initial reversible step followed by a slow isomerization step leading to a complex with a very low dissociation constant. The rate of inhibition is significantly reduced by the presence of relatively low concentrations of either azide (analogue of cyanate) or bicarbonate. Equilibrium dialysis binding studies showed that the stoichiometry of binding at saturation for oxalate, malonate, chloride, and bicarbonate is about 0.5 mol of ligand bound/mol of subunit for each compound.(ABSTRACT TRUNCATED AT 250 WORDS)