Substitution of His-181 by alanine in yeast phosphoglycerate mutase leads to cofactor-induced dissociation of the tetrameric structure

The tetrameric structure of Plasmodium falciparum phosphoglycerate mutase is critical for optimal enzymatic activity

The glycolytic enzyme phosphoglycerate mutase (PGM) is of utmost importance for overall cellular metabolism and has emerged as a novel therapeutic target in cancer cells. This enzyme is also conserved in the rapidly proliferating malarial parasite Plasmodium falciparum, which have a similar metabolic framework as cancer cells and rely on glycolysis as the sole energy-yielding process during intraerythrocytic development. There is no redundancy among the annotated PGM enzymes in Plasmodium, and PfPGM1 is absolutely required for the parasite survival as evidenced by conditional knockdown in our study. A detailed comparison of PfPGM1 with its counterparts followed by in-depth structure-function analysis revealed unique attributes of this parasitic protein. Here, we report for the first time the importance of oligomerization for the optimal functioning of the enzyme in vivo, as earlier studies in eukaryotes only focused on the effects in vitro. We show that single point mutation of the amino acid residue W68 led to complete loss of tetramerization and diminished catalytic activity in vitro. Additionally, ectopic expression of the WT PfPGM1 protein enhanced parasite growth, whereas the monomeric form of PfPGM1 failed to provide growth advantage. Furthermore, mutation of the evolutionarily conserved residue K100 led to a drastic reduction in enzymatic activity. The indispensable nature of this parasite enzyme highlights the potential of PfPGM1 as a therapeutic target against malaria, and targeting the interfacial residues critical for oligomerization can serve as a focal point for promising drug development strategies that may not be restricted to malaria only.

A Cooperative Folding Unit as the Structural Link for Energetic Coupling within a Protein

Previously, we demonstrated that binding of a ligand to Escherichia coli cofactor-dependent phosphoglycerate mutase (dPGM), a homodimeric protein, is energetically coupled with dimerization. The equilibrium unfolding of dPGM occurs with a stable, monomeric intermediate. Binding of several nonsubstrate metabolites stabilizes the dimeric native form over the monomeric intermediate, reducing the population of the intermediate. Both the active site and the dimer interface appear to be unfolded in the intermediate. We hypothesized that a loop containing residues 118-152 was responsible for the energetic coupling between the dimer interface and the distal active site and was unfolded in the intermediate. Here, we investigated the structure of the dPGM intermediate by probing side-chain interactions and solvent accessibility of the peptide backbone. By comparing the effect of a mutation on the global stability and the stability of the intermediate, we determine an equilibrium φ value (φ_eq value), which provides information about whether side-chain interactions are retained or lost in the intermediate. Hydrogen/deuterium exchange coupled with mass spectrometry (HDX-MS) was used to investigate differences in the solvent accessibility of the peptide backbone in the intermediate and native forms of dPGM. The results of φ_eq value analysis and HDX-MS reveal the least stable folding unit of dPGM, which is unfolded in the intermediate and links the active site to the dimer interface. The structure of the intermediate reveals how the cooperative network of residues in dPGM gives rise to the observed energetic coupling between dimerization and ligand binding.

Gene expression and molecular characterization of a chaperone protein HtpG from Bacillus licheniformis

Heat shock protein 90 (Hsp90/HtpG) is a highly abundant and ubiquitous ATP-dependent molecular chaperone consisting of three flexibly linked regions, an N-terminal nucleotide-binding domain, middle domain, and a C-terminal domain. Here the putative htpG gene of Bacillus licheniformis was cloned and heterologously expressed in Escherichia coli M15 cells. Native-gel electrophoresis, size exclusion chromatography, and cross-linking analysis revealed that the recombinant protein probably exists as a mixture of monomer, dimer and other oligomers in solution. The optimal conditions for the ATPase activity of B. licheniformis HtpG (BlHtpG) were 45°C and pH 7.0 in the presence of 0.5mM Mg(2+) ions. The molecular architecture of this protein was stable at higher temperatures with a transition point (Tm) of 45°C at neutral pH, whereas the Tm value was reduced to 40.8°C at pH 10.5. Acrylamide quenching experiment further indicated that the dynamic quenching constant (Ksv) of BlHtpG became larger at higher pH values. BlHtpG also experienced a significant change in the protein conformation upon the addition of ATP and organic solvents. Collectively, our experiment data may provide insights into the molecular properties of BlHtpG and identify the alteration of protein structure to forfeit the ATPase activity at alkaline conditions.

Biophysical characterization of Bacillus licheniformis and Escherichia coli γ-glutamyltranspeptidases: A comparative analysis

The oligomeric states of Bacillus licheniformis and Escherichia coli γ-glutamyltranspeptidases (BlGGT and EcGGT) in solution have been investigated by analytical ultracentrifugation. The results showed that BlGGT has a sedimentation coefficient of 5.12S, which can be transformed into an experimental molecular mass of approximately 62,680Da. The monomeric conformation is conserved in EcGGT. SDS-PAGE analysis and cross-linking studies further proved that the autocatalytically processed BlGGT and EcGGT form a heterodimeric association. Unfolding analyses using circular dichroism and tryptophan emission fluorescence revealed that these two proteins had a different sensitivity towards temperature- and guanidine hydrochloride (GdnHCl)-induced denaturation. BlGGT and EcGGT had a T(m) value of 59.5 and 49.2°C, respectively, and thermal unfolding of both proteins was found to be highly irreversible. Chemical unfolding of BlGGT was independent to the pH value ranging from 5 to 10, whereas the pH environment was found to significantly influence the GdnHCl-induced denaturation of EcGGT. Both enzymes did not reactivate from the completely unfolded states, accessible at 6M GdnHCl. BlGGT was active in the presence of 4M NaCl, whereas the activity of EcGGT was significantly decreased at the high-salt condition. Taken together, these findings suggest that the biophysical properties of the homologous GGTs from two mesophilic sources are quite different.

Seeing the Process of Histidine Phosphorylation in Human Bisphosphoglycerate Mutase

Bisphosphoglycerate mutase is an erythrocyte-specific enzyme catalyzing a series of intermolecular phosphoryl group transfer reactions. Its main function is to synthesize 2,3-bisphosphoglycerate, the allosteric effector of hemoglobin. In this paper, we directly observed real-time motion of the enzyme active site and the substrate during phosphoryl transfer. A series of high resolution crystal structures of human bisphosphoglycerate mutase co-crystallized with 2,3-bisphosphoglycerate, representing different time points in the phosphoryl transfer reaction, were solved. These structures not only clarify the argument concerning the substrate binding mode for this enzyme family but also depict the entire process of the key histidine phosphorylation as a "slow movie". It was observed that the enzyme conformation continuously changed during the different states of the reaction. These results provide direct evidence for an "in line" phosphoryl transfer mechanism, and the roles of some key residues in the phosphoryl transfer process are identified.

Refolding of Escherichia coli outer membrane protein F in detergent creates LPS-free trimers and asymmetric dimers

The Escherichia coli OmpF (outer-membrane protein F; matrix porin) is a homotrimeric beta-barrel and a member of the bacterial porin superfamily. It is the best characterized porin protein, but has resisted attempts to refold it efficiently in vitro. In the present paper, we report the discovery of detergent-based folding conditions, including dodecylglucoside, which can create pure samples of trimeric OmpF. Whereas outer membrane LPS (lipopolysaccharide) is clearly required for in vivo folding, the artificially refolded and LPS-free trimer has properties identical with those of the outer-membrane-derived form. Thus LPS is not required either for in vitro folding or for structural integrity. Dimeric forms of OmpF have been observed in vivo and are proposed to be folding intermediates. In vitro, dimers occur transiently in refolding of trimeric OmpF and, in the presence of dodecylmaltoside, pure dimer can be prepared. This form has less beta-structure by CD and shows lower thermal stability than the trimer. Study of these proteins at the single-molecule level is possible because each OmpF subunit forms a distinct ion channel. Whereas each trimer contains three channels of equal conductance, each dimer always contains two distinct channel sizes. This provides clear evidence that the two otherwise identical monomers adopt different structures in the dimer and indicates that the asymmetric interaction, characteristic of C3 symmetry, is formed at the dimer stage. This asymmetric dimer may be generally relevant to the folding of oligomeric proteins with odd numbers of subunits such as aspartate transcarbamoylase.

Crystal Structure of Human Bisphosphoglycerate Mutase

Bisphosphoglycerate mutase is a trifunctional enzyme of which the main function is to synthesize 2,3-bisphosphoglycerate, the allosteric effector of hemoglobin. The gene coding for bisphosphoglycerate mutase from the human cDNA library was cloned and expressed in Escherichia coli. The protein crystals were obtained and diffract to 2.5 A and produced the first crystal structure of bisphosphoglycerate mutase. The model was refined to a crystallographic R-factor of 0.200 and R(free) of 0.266 with excellent stereochemistry. The enzyme remains a dimer in the crystal. The overall structure of the enzyme resembles that of the cofactor-dependent phosphoglycerate mutase except the regions of 13-21, 98-117, 127-151, and the C-terminal tail. The conformational changes in the backbone and the side chains of some residues reveal the structural basis for the different activities between phosphoglycerate mutase and bisphosphoglycerate mutase. The bisphosphoglycerate mutase-specific residue Gly-14 may cause the most important conformational changes, which makes the side chain of Glu-13 orient toward the active site. The positions of Glu-13 and Phe-22 prevent 2,3-bisphosphoglycerate from binding in the way proposed previously. In addition, the side chain of Glu-13 would affect the Glu-89 protonation ability responsible for the low mutase activity. Other structural variations, which could be connected with functional differences, are also discussed.

Structure-function analysis of recombinant substrate protein 22 kDa (SP-22). A mitochondrial 2-CYS peroxiredoxin organized as a decameric toroid

Bovine mitochondrial SP-22 is a member of the peroxiredoxin family of peroxidases. It belongs to the peroxiredoxin 2-Cys subgroup containing three cysteines at positions 47, 66, and 168. The cloning and overexpression in Escherichia coli of recombinant wild type SP-22 and its three cysteine mutants (C47S, C66S, and C168S) are reported. Purified His-tagged SP-22 was fully active with Cys-47 being confirmed as the catalytic residue. The enzyme forms a stable decameric toroid consisting of five basic dimeric units containing intermolecular disulfide bonds linking the catalytically active Cys-47 of one subunit and Cys-168 of the adjacent monomer. The disulfide bonds are not required for overall structural integrity. The toroidal units have average external and internal diameters of 15 and 7 nm, respectively, and can form stacks in a lateral arrangement of two or three rings. C47S had a pronounced tendency to stack in long tubular structures containing up to 60 rings. Further unusual structural features are the presence of radial spikes projecting from the external surface and ordered electron-dense material within the central cavity of the toroid.

19 A solution structure of the filarial nematode immunomodulatory protein, ES-62

ES-62, a protein secreted by filarial nematodes, parasites of vertebrates including humans, has an unusual posttranslational covalent addition of phosphorylcholine to an N-type glycan. Studies on ES-62 from the rodent parasite Acanthocheilonema viteae ascribe it a dominant role in ensuring parasite survival by modulating the host immune system. Understanding this immunomodulation at the molecular level awaits full elucidation but distinct components of ES-62 may participate: the protein contributes aminopeptidase-like activity whereas the phosphorylcholine is thought to act as a signal transducer. We have used biophysical and bioinformatics-based structure prediction methods to define a low-resolution model of ES-62. Sedimentation equilibrium showed that ES-62 is a tightly bound tetramer. The sedimentation coefficient is consistent with this oligomer and the overall molecular shape revealed by small angle x-ray scattering. A 19 A model for ES-62 was restored from the small-angle x-ray scattering data using the program DAMMIN which uses simulated annealing to find a configuration of densely packed scattering elements consistent with the experimental scattering curve. Analysis of the primary sequence with the position-specific iterated basic local alignment search tool, PSI-BLAST, identified six closely homologous proteins, five of which are peptidases, consistent with observed aminopeptidase activity in ES-62. Differences between the secondary structure content of ES-62 predicted using the consensus output from the secondary structure prediction server JPRED and measured using circular dichroism are discussed in relation to multimeric glycosylated proteins. This study represents the first attempt to understand the multifunctional properties of this important parasite-derived molecule by studying its structure.

Identification of fructose 6-phosphate- and fructose 1-phosphate-binding residues in the regulatory protein of glucokinase

Glucokinase is inhibited in the liver by a regulatory protein (GKRP) whose effects are increased by Fru-6-P and suppressed by Fru-1-P. To identify the binding site of these phosphate esters, we took advantage of the homology of GKRP to the isomerase domain of GlmS (glucosamine-6-phosphate synthase) and created 12 different mutants of rat GKRP. Mutations of three residues predicted to bind to Fru-6-P resulted in proteins that were approximately 5-fold (S110A) and 50-fold (S179A and K514A) less potent as inhibitors of glucokinase and had an at least 100-fold reduced affinity for the effectors. Mutation of another residue of the putative binding site (T109A) resulted in a 10-fold decrease in the inhibitory power and an inversion of the effect of sorbitol-6-P, a Fru-6-P analog. The replacement of Gly(107), a residue close to the binding site, by cysteine (as in GlmS and Xenopus GKRP) resulted in a protein that had 20 times more affinity for Fru-6-P and 30 times less affinity for Fru-1-P. These results are consistent with GKRP having one single binding site for phosphate esters. They also show that a missense mutation of GKRP can lead to a gain of function.

A cofactor-dependent phosphoglycerate mutase homolog from Bacillus stearothermophilus is actually a broad specificity phosphatase

The distribution of phosphoglycerate mutase (PGM) activity in bacteria is complex, with some organisms possessing both a cofactor-dependent and a cofactor-independent PGM and others having only one of these enzymes. Although Bacillus species contain only a cofactor-independent PGM, genes homologous to those encoding cofactor-dependent PGMs have been detected in this group of bacteria, but in at least one case the encoded protein lacks significant PGM activity. Here we apply sequence analysis, molecular modeling, and enzymatic assays to the cofactor-dependent PGM homologs from B. stearothermophilus and B. subtilis, and show that these enzymes are phosphatases with broad substrate specificity. Homologs from other gram-positive bacteria are also likely to possess phosphatase activity. These studies clearly show that the exploration of genomic sequences through three-dimensional modeling is capable of producing useful predictions regarding function. However, significant methodological improvements will be needed before such analysis can be carried out automatically.

Characterization of active-site mutants of Schizosaccharomyces pombe phosphoglycerate mutase. Elucidation of the roles of amino acids involved in substrate binding and catalysis

The roles of a number of amino acids present at the active site of the monomeric phosphoglycerate mutase from the fission yeast Schizosaccharomyces pombe have been explored by site-directed mutagenesis. The amino acids examined could be divided broadly into those presumed from previous related structural studies to be important in the catalytic process (R14, S62 and E93) and those thought to be important in substrate binding (R94, R120 and R121). Most of these residues have not previously been studied by site-directed mutagenesis. All the mutants except R14 were expressed in an engineered null strain of Saccharomyces cerevisiae (S150-gpm:HIS) in good yield. The R14Q mutant was expressed in good yield in the transformed AH22 strain of S. cerevisiae. The S62A mutant was markedly unstable, preventing purification. The various mutants were purified to homogeneity and characterized in terms of kinetic parameters, CD and fluorescence spectra, stability towards denaturation by guanidinium chloride, and stability of phosphorylated enzyme intermediate. In addition, the binding of substrate (3-phosphoglycerate) to wild-type, E93D and R120,121Q enzymes was measured by isothermal titration calorimetry. The results provide evidence for the proposed roles of each of these amino acids in the catalytic cycle and in substrate binding, and will support the current investigation of the structure and dynamics of the enzyme using multidimensional NMR techniques.

FAD insertion is essential for attaining the assembly competence of the dihydrolipoamide dehydrogenase (E3) monomer from Escherichia coli

Dihydrolipoamide dehydrogenase (E3) from Escherichia coli, an FAD-linked homodimer, can be fully reconstituted in vitro following denaturation in 6 m guanidinium chloride. Complete restoration of activity occurs within 1-2 h in the presence of FAD, dithiothreitol, and bovine serum albumin. In the absence of FAD, the dihydrolipoamide dehydrogenase monomer forms a stable folding intermediate, which is incapable of dimerization. This intermediate displays a similar tryptic resistance to the native enzyme but is less heat-stable, because its ability to form native E3 is lost after incubation at 65 degrees C for 15 min. The presence of FAD promotes slow, additional conformational rearrangements of the E3 subunit as observed by cofactor-dependent decreases in intrinsic tryptophan fluorescence. However, after 2 h, the tryptophan fluorescence spectrum and far UV CD spectrum of E3, refolded in the absence of FAD, are similar to that of the native enzyme, and full activity can still be recovered on addition of FAD. Cross-linking studies show that FAD insertion is necessary for the monomeric folding intermediate to attain an assembly competent state leading to dimerization. Thus cofactor insertion represents a key step in the assembly of this enzyme, although its initial presence appears not to be required to promote the correct folding pathway.

Structure and mechanism of action of a novel phosphoglycerate mutase from Bacillus stearothermophilus

Bacillus stearothermophilus phosphoglycerate mutase (PGM), which interconverts 2- and 3-phosphoglyceric acid (PGA), does not require 2,3-diphosphoglyceric acid for activity. However, this enzyme does have an absolute and specific requirement for Mn(2+) ions for catalysis. Here we report the crystal structure of this enzyme complexed with 3PGA and manganese ions to 1.9 A resolution; this is the first crystal structure of a diphosphoglycerate-independent PGM to be determined. This information, plus the location of the two bound Mn(2+) ions and the 3PGA have allowed formulation of a possible catalytic mechanism for this PGM. In this mechanism Mn(2+) ions facilitate the transfer of the substrate's phosphate group to Ser62 to form a phosphoserine intermediate. In the subsequent phosphotransferase part of the reaction, the phosphate group is transferred from Ser62 to the O2 or O3 positions of the reoriented glycerate to yield the PGA product. Site-directed mutagenesis studies were used to confirm our mechanism and the involvement of specific enzyme residues in Mn(2+) binding and catalysis.

Polyanionic inhibitors of phosphoglycerate mutase: combined structural and biochemical analysis

The effects that the inhibitors inositol hexakisphosphate and benzene tri-, tetra- and hexacarboxylates have on the phosphoglycerate mutases from Saccharomyces cerevisiae and Schizosaccharomyces pombe have been determined. Their Kivalues have been calculated, and the ability of the inhibitors to protect the enzymes against limited proteolysis investigated. These biochemical data have been placed in a structural context by the solution of the crystal structures of S. cerevisiae phosphoglycerate mutase soaked with inositol hexakisphosphate or benzene hexacarboxylate. These large polyanionic compounds bind to the enzyme so as to block the entrance to the active-site cleft. They form multiple interactions with the enzyme, consistent with their low Kivalues, and afford good protection against limited proteolysis of the C-terminal region by thermolysin. The inositol compound is more efficacious because of its greater number of negative charges. The S. pombe phosphoglycerate mutase that is inherently lacking a comparable C-terminal region has higher Kivalues for the compounds tested. Moreover, the S. pombe enzyme is less sensititive to proteolysis, and the presence or absence of the inhibitor molecules has little effect on susceptibility to proteolysis.

Sulphate ions observed in the 2.12 A structure of a new crystal form of S. cerevisiae phosphoglycerate mutase provide insights into understanding the catalytic mechanism

The structure of a new crystal form of Saccharomyces cerevisiae phosphoglycerate mutase has been solved and refined to 2.12 A with working and free R-factors of 19.7 and 22.9 %, respectively. Higher-resolution data and greater non-crystallographic symmetry have produced a more accurate protein structure than previously. Prominent among the differences from the previous structure is the presence of two sulphate ions within each active site cleft. The separation of the sulphates suggests that they may occupy the same sites as phospho groups of the bisphosphate ligands of the enzyme. Plausible binding modes for 2,3-bisphosphoglycerate and 1, 3-bisphosphoglycerate are thereby suggested. These results support previous conclusions from mutant studies, highlight interesting new targets for mutagenesis and suggest a possible mechanism of enzyme phosphorylation.

Comparison of structural stabilities of prostaglandin H synthase-1 and -2

There are two known isoforms of prostaglandin H synthase (PGHS), a key enzyme in the conversion of arachidonic acid to bioactive prostanoids. The "constitutive" isoform, PGHS-1, is thought to have housekeeping functions, and the "inducible" isoform, PGHS-2, has been implicated in cellular responses to cytokines. The two isoforms have high sequence conservation in the cyclooxygenase active site and quite similar crystallographic structures, but differ markedly in their interactions with many cyclooxygenase substrates and inhibitors. We have evaluated the stability of the overall folding, and of the active sites of ovine PGHS-1 and human PGHS-2 using denaturation with guanidinium hydrochloride (GdmHCl). Changes in hydrodynamic and cross-linking properties indicated a dimer --> monomer transition for both isoforms between 0.5 and 2 M GdmHCl; the monomers unfolded at higher GdmHCl levels. Changes in overall secondary and tertiary structure, measured by tryptophan fluorescence and circular dichroism, occurred in two phases for each isoform, with the transition between the phases at 0.2-0.5 M GdmHCl. Disruption of active site functions (cyclooxygenase, peroxidase, and cyclooxygenase inhibitor binding activities) began at GdmHCl levels below 0.2 M. The structural and functional changes were completely reversible up to about 2 M GdmHCl, they were more pronounced at lower protein levels, and they required lower GdmHCl levels for PGHS-2 than for PGHS-1. The results are consistent with a four-state denaturation process for both isoforms: native dimers --> inactive dimers --> compact monomers --> unfolded monomers. The first two steps are reversible for both isoforms; PGHS-2 undergoes the first and last steps more readily than PGHS-1. Thus, the structural stability of PGHS-2, both in the active site regions and in the subunits overall, is distinctly less than that of PGHS-1. These differences in structural stability may contribute to the isoforms' active site ligand selectivity.

The 2.3 A X-ray crystal structure of S. cerevisiae phosphoglycerate mutase

The high resolution crystal structure of Saccharomyces cerevisiae phosphoglycerate mutase has been determined. This structure shows important differences from the lower resolution structure deposited in 1982. The crystal used to determine the new structure was of a different form, having spacegroup P2(1). The model was refined to a crystallographic R-factor of 18.9% and a free R-factor of 28.4% using all data between 25 and 2.3 A and employing a bulk solvent correction. The enzyme is a tetramer of identical, 246 amino acid subunits, whose structure is revealed to be a dimer of dimers, with four independent active sites located well away from the subunit contacts. Each subunit contains two domains, the larger with a typical nucleotide binding fold, although phosphoglycerate mutase has no physiological requirement to bind nucleotides. The catalytic-site histidine residues are no longer in a "clapping-hands" conformation, but more resemble the conformation seen in the distantly related enzymes prostatic acid phosphatase and fructose-2,6-bisphosphatase. However, the catalytic histidine residues in the mutase are found to be much closer to each other than in the phosphatase structures, perhaps due to the absence of bound ligands in the mutase crystal. An intricate web of H-bonds is found around the catalytic histidine residues, high-lighting residues probably important for maintaining their correct orientation and charge. The positions of certain other residues, including some found near the catalytic site and some lining the catalytic-site cleft, have been changed by the correction of registration errors between sequence and electron density in the original structure. Electron density was apparent for a portion of the functionally important C-terminal tail, which was absent from the earlier structure, showing it to adopt a mainly helical conformation.

Investigation of two yeast genes encoding putative isoenzymes of phosphoglycerate mutase

Our previous data indicated that GPM1 encodes the only functional phosphoglycerate mutase in yeast. However, in the course of the yeast genome sequencing project, two homologous sequences, designated GPM2 and GPM3, were detected. They have been further investigated in this work. Key residues in the deduced amino acid sequence, shown to be involved in catalysis for Gpm1 (i.e. His8, Arg59, His181) are conserved in both enzymes. Overexpression of the genes under control of their own promoters in a gpm1 deletion mutant did not complement for any of the phenotypes. This could in part be attributed to a lack of expression due to their weak promoters. Higher level expression under the control of the yeast PFK2 promoter partially complemented the gpm1 defects, without restoring detectable enzymatic activity. Nevertheless, deletion of either GPM2 or GPM3, or the two deletions in concert, did not produce any obvious lesions for growth on a variety of different carbon sources, nor did they change the levels of key intermediary metabolites. We conclude that both genes evolved from duplication events and that they probably constitute non-functional homologues in yeast.

Critical role of human bisphosphoglycerate mutase Cys22 in the phosphatase activator-binding site

The enzymatic activities catalyzed by bisphosphoglycerate mutase (BPGM, EC 5.4.2.4) have been shown to occur at a unique active site, with distinct binding sites for diphosphoglycerates and monophosphoglycerates. The physiological phosphatase activator (2-phosphoglycolate) binds to BPGM at an undetermined site. BPGM variants were constructed by site-directed mutagenesis of three amino acid residues in the active site to identify residues specifically involved in the binding of the monophosphoglycerates and 2-phosphoglycolate. Substitution of Cys22 by functionally conservative residues, Thr or Ser, caused a great decrease in 2-phosphoglycolate-stimulated phosphatase activity and in the Ka value of the activator, whereas it caused no change in other catalytic activities or in the Km values of 2,3-diphosphoglycerate (2,3-DPG) and glycerate 3-phosphate (3-PG, EC 1.1.1.12), indicating that Cys22 is specifically involved either directly or indirectly in 2-phosphoglycolate binding. Kinetic experiments showed that the Ka of the cofactor and the Km of 3-PG were affected by the substitution of Ser23 indicating that this residue is necessary for the fixation of both 3-PG and 2-phosphoglycolate. The R89K variant has previously been shown to have a modified Km value for monophosphoglycerates, however, its affinity for 2-phosphoglycolate is unaltered, suggesting that Arg89 is specifically involved in monophosphoglycerates binding. CD spectroscopic studies of substrates and cofactor binding showed that 2,3-DPG induced structural modifications of normal and mutated enzymes which could be due to protein phosphorylation. Addition of 2-phosphoglycolate to phosphorylated proteins with normal affinity for the cofactor produced spectra with the same characteristics as unphosphorylated species. In summary, monophosphoglycerates and 2-phosphoglycolate have partially distinct binding sites in human BPGM. The specific implication of the Cys22 residue in 2-phosphoglycolate binding is of great significance in the design of analogs of therapeutic benefit.

Phosphoglycerate mutase from Schizosaccharomyces pombe: development of an expression system and characterisation of three histidine mutants of the enzyme

The small, monomeric, phosphoglycerate mutase (PGAM) from Schizosaccharomyces pombe has been overexpressed in a strain of Saccharomyces cerevisiae in which the gene encoding PGAM has been deleted, with a yield of purified enzyme of 10-15 mg per litre cell culture. Three mutants in which histidine residues in S. pombe PGAM have been substituted by glutamine have been purified and characterised. Two mutants (H151Q and H196Q) have kinetic and structural properties very similar to wild-type enzyme, consistent with the proposed location of these (non-conserved) histidines on the surface of the enzyme. The third mutant (H163Q) involving a histidine thought to be part of the active site has greatly reduced mutase and phosphatase activities. Mass spectrometry shows that the phosphorylated form of the H163Q is several 100-times more stable towards hydrolysis than the phosphorylated form of wild-type enzyme. The H163Q mutant appears to be structurally quite distinct from wild-type enzyme. 600 MHz 1D proton NMR spectra of good quality have been obtained for wild-type enzyme and the H151Q and H196Q mutants.

Histidine residues 139, 363 and 500 are essential for catalytic activity of cofactor-independent phosphoglyceromutase from developing endosperm of the castor plant

Cofactor-independent phosphoglyceromutase (PGM) from castor is inactivated by diethyl pyrocarbonate, implicating histidine residues in the catalytic mechanism. Treatment of the inhibited enzyme with 1 M hydroxylamine at pH 7.0 restores the enzyme activity. Spectroscopic data indicate that the inactivation of PGM with diethyl pyrocarbonate is the result of formation of carbethoxyhistidine derivatives. The substrate, 3-phosphoglycerate, substantially protects the enzyme against diethyl pyrocarbonate inactivation, indicating that the histidine residues important in catalysis are at or near the active site of the enzyme. There are 12 conserved histidine residues in all plant PGMs that have been sequenced. In the castor PGM, these conserved histidine residues were changed to either valine (H12V) or alanine (H41A, H65A, H84A, H127A, H139A, H163A, H363A H433A, H471A, H500A and H540A) by in vitro mutagenesis. Expression of these mutant proteins in Escherichia coli produced seven soluble mutant proteins (mutations H41A, H65A, H84A, H139A, H363A, H500A and H540A) and five insoluble mutant proteins (mutations H12V, H127A, H163A, H433A and H471A). Among the seven soluble proteins, four possessed normal PGM activity (mutations H41A, H65A, H84A and H540A) and three (mutations H139A, H363A and H500A) had no catalytic activity. Along with the in-vitro-expressed wild-type enzyme, mutant enzymes [H139A]PGM, [H363A]PGM and [H500A]PGM were purified to homogeneity. Purified wild-type PGM expressed in E. coli was active and had a Km value very close to that of the enzyme purified from castor endosperm, while the three mutant enzymes remained inactive throughout purification. Therefore, histidine residues 139, 363 and 500 appear to be essential for the catalytic activity of the cofactor-independent enzyme, and may be located at the active site. Hence, although the cofactor-dependent and cofactor-independent PGMs have no homology in their primary amino acid sequences, both enzymes appear to utilize histidine residues to mediate the transfers of proton and phospho groups in the reaction, and thus may be functionally and mechanistically convergent.

Dissociation of the tetrameric phosphoglycerate mutase from yeast by a mutation in the subunit contact region

Phosphoglycerate mutases from different sources exhibit a variety of quaternary structures (tetramer, dimer and monomer). To perturb the tetrameric structure of yeast phosphoglycerate mutase we have prepared a mutant enzyme in which Lys-168 in the subunit-contact region has been replaced by proline. The K168P mutant enzyme undergoes dissociation to dimers at low concentrations; thus on lowering the concentration from 200 micrograms/ml to 5 micrograms/ml the proportion of tetramer falls from 85% to 53%. The tetrameric structure of the wild-type enzyme remains intact over this range of concentrations. The mutant enzyme has similar kinetic properties to the wild-type enzyme, with kcat. being reduced by 26%. Far-u.v. c.d. studies show that there has been a small loss of helical structure in the mutant. Compared with wild-type enzyme, the K168P mutant enzyme is slightly less stable towards proteolysis by trypsin, but significantly less stable towards denaturation by guanidinium chloride, with the midpoint concentration of guanidinium chloride some 50% lower. After denaturation, the mutant enzyme could regain activity and quaternary structure when the guanidinium chloride concentration was lowered to 0.05 M. The properties of the mutant enzyme are discussed in terms of other dimeric phosphoglycerate and bisphosphoglycerate mutases which contain proline at position 168.