Serine transhydroxymethylase. Relaxation and transient kinetic study of the formation and interconversion of the enzyme-glycine complexes

A DFT study of the active role of the phosphate group of an internal aldimine in a transamination reaction

A transamination reaction from an internal aldimine ([PLP]) and (S)-alanine to pyridoxamine phosphate (PMP) and pyruvic acid was investigated by DFT calculations. As [PLP], a model where the lysine (-Lys) part was approximated by -CH[-NH-C(O)-CH₃]-C(O)-NH-CH₃ was adopted. (H₂O)₄ was also included to trace reaction paths involving proton transfers. 13 elementary processes were obtained. For (the external aldimine → quinoid), (quinoid → ketimine) and (ketimine → carbinol amine) processes, the water dimer was found to connect a phosphate-group oxygen with the moving proton. The connection promoted the Grotthuss-type proton transfer in transition states. It was revealed that the phosphate group is not a mere substituent but has the central role in the transfer.

Inactive mutants of human pyridoxine 5'-phosphate oxidase: a possible role for a noncatalytic pyridoxal 5'-phosphate tight binding site

Pyridoxal 5'-phosphate (PLP) is a cofactor for many vitamin B6-requiring enzymes that are important for the synthesis of neurotransmitters. Pyridoxine 5'-phosphate oxidase (PNPO) is one of two enzymes that produce PLP. Some 16 known mutations in human PNPO (hPNPO), including R95C and R229W, lead to deficiency of PLP in the cell and have been shown to cause neonatal epileptic encephalopathy (NEE). This disorder has no effective treatment, and is often fatal unless treated with PLP. In this study, we show that R95C hPNPO exhibits a 15-fold reduction in affinity for the FMN cofactor, a 71-fold decrease in affinity for the substrate PNP, a 4.9-fold decrease in specific activity, and a 343-fold reduction in catalytic activity, compared to the wild-type enzyme. We have reported similar findings for R229W hPNPO. This report also shows that wild-type, R95C and R229W hPNPO bind PLP tightly at a noncatalytic site and transfer it to activate an apo-B6 enzyme into the catalytically active holo-form. We also show for the first time that hPNPO forms specific interactions with several B6 enzymes with dissociation constants ranging from 0.3 to 12.3 μm. Our results suggest a possible in vivo role for the tight binding of PLP in hPNPO, whether wild-type or variant, by protecting the very reactive PLP, and transferring this PLP directly to activate apo-B6 enzymes.

Molecular basis of E. coli L-threonine aldolase catalytic inactivation at low pH

L-Threonine aldolases (TAs), a family of enzymes belonging to the fold-type I pyridoxal 5'-phosphate (PLP) dependent enzymes, play a role in catalyzing the reversible cleavage of l-3-hydroxy-α-amino acids to glycine and the corresponding aldehydes. Threonine aldolases have great biotechnological potential for the syntheses of pharmaceutically relevant drug molecules because of their stereospecificity. The pH-dependency of their catalytic activity, affecting reaction intermediates, led us to study the effect of low-pH on Escherichia coli TA (eTA) structure. We report here a low-pH crystal structure of eTA at 2.1 Å resolution, with a non-covalently bound uncleaved l-serine substrate, and a PLP cofactor bound as an internal aldimine. This structure contrasts with other eTA structures obtained at physiological pH that show products or substrates bound as PLP-external aldimines. The non-productive binding at low-pH is due to an unusual substrate serine binding orientation in which the α-amino group and carboxylate group are in the wrong positions (relative to the active site residues) as a result of protonation of the α-amino group of the serine, as well as the active site histidines, His83 and His126. Protonation of these residues prevents the characteristic nucleophilic attack of the α-amino group of substrate serine on C4' of PLP to form the external aldimine. Our study shows that at low pH the change in charge distribution at the active site can result in substrates binding in a non-productive orientation.

Structure-based mechanism for early PLP-mediated steps of rabbit cytosolic serine hydroxymethyltransferase reaction

Serine hydroxymethyltransferase catalyzes the reversible interconversion of L-serine and glycine with transfer of one-carbon groups to and from tetrahydrofolate. Active site residue Thr254 is known to be involved in the transaldimination reaction, a crucial step in the catalytic mechanism of all pyridoxal 5'-phosphate- (PLP-) dependent enzymes, which determines binding of substrates and release of products. In order to better understand the role of Thr254, we have expressed, characterized, and determined the crystal structures of rabbit cytosolic serine hydroxymethyltransferase T254A and T254C mutant forms, in the absence and presence of substrates. These mutants accumulate a kinetically stable gem-diamine intermediate, and their crystal structures show differences in the active site with respect to wild type. The kinetic and crystallographic data acquired with mutant enzymes permit us to infer that conversion of gem-diamine to external aldimine is significantly slowed because intermediates are trapped into an anomalous position by a misorientation of the PLP ring, and a new energy barrier hampers the transaldimination reaction. This barrier likely arises from the loss of the stabilizing hydrogen bond between the hydroxymethyl group of Thr254 and the ε -amino group of active site Lys257, which stabilizes the external aldimine intermediate in wild type SHMTs.

Role of a conserved active site cation-pi interaction in Escherichia coli serine hydroxymethyltransferase

Serine hydroxymethyltransferase is a pyridoxal 5'-phosphate-dependent enzyme that catalyzes the interconversion of serine and glycine using tetrahydropteroylglutamate as the one-carbon carrier. In all pyridoxal phosphate-dependent enzymes, amino acid substrates are bound and released through a transaldimination process, in which an internal aldimine and an external aldimine are interconverted via gem-diamine intermediates. Bioinformatic analyses of serine hydroxymethyltransferase sequences and structures showed the presence of two highly conserved residues, a tyrosine and an arginine, engaged in a cation-pi interaction. In Escherichia coli serine hydroxymethyltranferase, the hydroxyl group of this conserved tyrosine (Tyr55) is located in a position compatible with a role as hydrogen exchanger in the transaldimination reaction. Because of the location of Tyr55 at the active site, the enhancement of its acidic properties caused by the cation-pi interaction with Arg235, and the hydrogen bonds established by its hydroxyl group, a role of this residue as acid-base catalyst in the transaldimination process was envisaged. The role played by this cation-pi interaction in the E. coli serine hydroxymethyltransferase was investigated by crystallography and site-directed mutagenesis using Y55F and three R235 mutant forms. The crystal structure of the Y55F mutant suggests that the presence of Tyr55 is indispensable for a correct positioning of the cofactor and for the maintenance of the structure of several loops involved in substrate and cofactor binding. The kinetic properties of all mutant enzymes are profoundly altered. Substrate binding and rapid kinetic experiments showed that both Y55 and R235 are required for a correct progress of the transaldimination reaction.

Detection of a gem-diamine and a stable quinonoid intermediate in the reaction catalyzed by serine-glyoxylate aminotransferase from Hyphomicrobium methylovorum

BACKGROUND

The enzyme L-serine-glyoxylate aminotransferase (SGAT) from Hyphomicrobium methylovorum is a PLP-containing enzyme that catalyzes the conversion of L-serine and glyoxylate to hydroxypyruvate and glycine. The cloned enzyme expressed in Escherichia coli is isolated as a mixture of the E:PLP and E:PMP forms. The PLP form of the enzyme has a maximum absorbance at 413 nm.

METHODS

Uv-visible spectra of SGAT were obtained using an HP-8453 diode array spectrophotometer in the absence and presence of substrates and substrate analogs. Pre-steady state kinetic studies were carried out using an OLIS rapid scanning spectrophotometer in the rapid scanning mode.

RESULTS

Incubation of the enzyme with a saturating concentration D-serine leads to a shift in the 413 nm peak to 421 nm that is ascribed to the external aldimine. The reverse stereochemistry of D-serine does not allow for abstraction of the C alpha proton by the epsilon-amine of the active site lysine residue leading to an abortive external aldimine intermediate. Pre-steady state studies pushing SGAT against D-serine leads to a rapid decrease in the 413 nm peak and an increase at approximately 330 nm with an associated rate constant of 47 s(-1) at pH 7.6. This is followed by a slower decrease (0.26 s(-1)) at 330 nm and an increase and shift of the 413 nm peak to 421 nm. The intermediate species that absorbs at approximately 330 nm is attributed to the gem-diamine intermediate. The rate of the fast phase increases with pH and increase in rate is likely due to the deprotonation of an enzymatic group that accepts a proton from the alpha-amine of D-serine. In the presence of hydroxypyruvate and ammonia the enzyme spectra display an increase in absorbance at 521 nm that occurs on the order of minutes. The shape and position of the 521 nm species is consistent with a quinonoid intermediate.

GENERAL SIGNIFICANCE

The data suggest a non-enzymatic reaction between hydroxypyruvate and ammonia to form an imine which will be in equilibrium with the enamine. A mechanism is proposed by which the enamine reacts with the PLP form of SGAT to generate the stable highly conjugated quinonoid intermediate.

Investigating the Geminal Diamine Intermediate of Yersinia pestis Arginine Decarboxylase with Substrate, Product, and Inhibitors Using Single Wavelength Stopped-Flow Spectroscopy

The reaction mechanism of Yersinia pestis arginine decarboxylase has been investigated using a series of substrate, product, and inhibitors. Using single wavelength stopped-flow spectroscopy, novel mechanistic features were noted in the presence of the product, agmatine. By focusing on the excitation and emission wavelengths of the geminal diamine intermediate, we were able to monitor the formation and decay of two different geminal diamine species. Experiments revealed that the enzyme exists in two different conformational states--one that binds ligand and one that does not. The on and off rates for the conversion between the two conformational states was determined to be 390 s-1 and 880 s-1, respectively. The KD for agmatine binding was 6 mM. In addition, experiments revealed a pH-dependent conversion between two states of the enzyme. The deprotonated form of the enzyme binds ligand more slowly than the protonated form. The rates for the formation of the geminal diamine and external aldimine in this pathway were determined to be 25 and 4 s-1, respectively. There is also a slow interconversion between the protonated and deprotonated enzymes that has a pKa of approximately 8.0. Finally, the formation of the geminal diamine was determined to be Mg2+-dependent.

Serine hydroxymethyltransferase: role of glu75 and evidence that serine is cleaved by a retroaldol mechanism

Serine hydroxymethyltransferase (SHMT) catalyzes the reversible interconversion of serine and glycine with tetrahydrofolate serving as the one-carbon carrier. SHMT also catalyzes the folate-independent retroaldol cleavage of allothreonine and 3-phenylserine and the irreversible conversion of 5,10-methenyltetrahydrofolate to 5-formyltetrahydrofolate. Studies of wild-type and site mutants of SHMT have failed to clearly establish the mechanism of this enzyme. The cleavage of 3-hydroxy amino acids to glycine and an aldehyde occurs by a retroaldol mechanism. However, the folate-dependent cleavage of serine can be described by either the same retroaldol mechanism with formaldehyde as an enzyme-bound intermediate or by a nucleophilic displacement mechanism in which N5 of tetrahydrofolate displaces the C3 hydroxyl of serine, forming a covalent intermediate. Glu75 of SHMT is clearly involved in the reaction mechanism; it is within hydrogen bonding distance of the hydroxyl group of serine and the formyl group of 5-formyltetrahydrofolate in complexes of these species with SHMT. This residue was changed to Leu and Gln, and the structures, kinetics, and spectral properties of the site mutants were determined. Neither mutation significantly changed the structure of SHMT, the spectral properties of its complexes, or the kinetics of the retroaldol cleavage of allothreonine and 3-phenylserine. However, both mutations blocked the folate-dependent serine-to-glycine reaction and the conversion of methenyltetrahydrofolate to 5-formyltetrahydrofolate. These results clearly indicate that interaction of Glu75 with folate is required for folate-dependent reactions catalyzed by SHMT. Moreover, we can now propose a promising modification to the retroaldol mechanism for serine cleavage. As the first step, N5 of tetrahydrofolate makes a nucleophilic attack on C3 of serine, breaking the C2-C3 bond to form N5-hydroxymethylenetetrahydrofolate and an enzyme-bound glycine anion. The transient formation of formaldehyde as an intermediate is possible, but not required. This mechanism explains the greatly enhanced rate of serine cleavage in the presence of folate, and avoids some serious difficulties presented by the nucleophilic displacement mechanism involving breakage of the C3-OH bond.

Structural studies of the catalytic reaction pathway of a hyperthermophilic histidinol-phosphate aminotransferase

In histidine biosynthesis, histidinol-phosphate aminotransferase catalyzes the transfer of the amino group from glutamate to imidazole acetol-phosphate producing 2-oxoglutarate and histidinol phosphate. In some organisms such as the hyperthermophile Thermotoga maritima, specific tyrosine and aromatic amino acid transaminases have not been identified to date, suggesting an additional role for histidinol-phosphate aminotransferase in other transamination reactions generating aromatic amino acids. To gain insight into the specific function of this transaminase, we have determined its crystal structure in the absence of any ligand except phosphate, in the presence of covalently bound pyridoxal 5'-phosphate, of the coenzyme histidinol phosphate adduct, and of pyridoxamine 5'-phosphate. The enzyme accepts histidinol phosphate, tyrosine, tryptophan, and phenylalanine, but not histidine, as substrates. The structures provide a model of how these different substrates could be accommodated by histidinol-phosphate aminotransferase. Some of the structural features of the enzyme are more preserved between the T. maritima enzyme and a related threonine-phosphate decarboxylase from S. typhimurium than with histidinol-phosphate aminotransferases from different organisms.

Crystal structure of histidinol phosphate aminotransferase (HisC) from Escherichia coli, and its covalent complex with pyridoxal-5'-phosphate and l-histidinol phosphate

The biosynthesis of histidine is a central metabolic process in organisms ranging from bacteria to yeast and plants. The seventh step in the synthesis of histidine within eubacteria is carried out by a pyridoxal-5'-phosphate (PLP)-dependent l-histidinol phosphate aminotransferase (HisC, EC 2.6.1.9). Here, we report the crystal structure of l-histidinol phosphate aminotransferase from Escherichia coli, as a complex with pyridoxamine-5'-phosphate (PMP) at 1.5 A resolution, as the internal aldimine with PLP, and in a covalent, tetrahedral complex consisting of PLP and l-histidinol phosphate attached to Lys214, both at 2.2 A resolution. This covalent complex resembles, in structural terms, the gem-diamine intermediate that is formed transiently during conversion of the internal to external aldimine.HisC is a dimeric enzyme with a mass of approximately 80 kDa. Like most PLP-dependent enzymes, each HisC monomer consists of two domains, a larger PLP-binding domain having an alpha/beta/alpha topology, and a smaller domain. An N-terminal arm contributes to the dimerization of the two monomers. The PLP-binding domain of HisC shows weak sequence similarity, but significant structural similarity with the PLP-binding domains of a number of PLP-dependent enzymes. Residues that interact with the PLP cofactor, including Tyr55, Asn157, Asp184, Tyr187, Ser213, Lys214 and Arg222, are conserved in the family of aspartate, tyrosine and histidinol phosphate aminotransferases. The imidazole ring of l-histidinol phosphate is bound, in part, through a hydrogen bond with Tyr110, a residue that is substituted by Phe in the broad substrate specific HisC enzymes from Zymomonas mobilis and Bacillus subtilis. Comparison of the structures of the HisC internal aldimine, the PMP complex and the HisC l-histidinol phosphate complex reveal minimal changes in protein or ligand structure. Proton transfer, required for conversion of the gem-diamine to the external aldimine, does not appear to be limited by the distance between substrate and lysine amino groups. We propose that the tetrahedral complex has resulted from non-productive binding of l-histidinol phosphate soaked into the HisC crystals, resulting in its inability to be converted to the external aldimine at the HisC active site.

Structure and function of the tryptophan synthase alpha(2)beta(2) complex. Roles of beta subunit histidine 86

To probe the structural and functional roles of active-site residues in the tryptophan synthase alpha(2)beta(2) complex from Salmonella typhimurium, we have determined the effects of mutation of His(86) in the beta subunit. His(86) is located adjacent to beta subunit Lys(87), which forms an internal aldimine with the pyridoxal phosphate and catalyzes the abstraction of the alpha-proton of L-serine. The replacement of His(86) by leucine (H86L) weakened pyridoxal phosphate binding approximately 20-fold and abolished the circular dichroism signals of the bound coenzyme and of a reaction intermediate. Correlation of these results with previous crystal structures indicates that beta-His(86) plays a structural role in binding pyridoxal phosphate and in stabilizing the correct orientation of pyridoxal phosphate in the active site of the beta subunit. The H86L mutation also altered the pH profiles of absorbance and fluorescence signals and shifted the pH optimum for the synthesis of L-tryptophan from pH 7.5 to 8.8. We propose that the interaction of His(86) with the phosphate of pyridoxal phosphate and with Lys(87) lowers the pK(a) of Lys(87) in the wild-type alpha(2)beta(2) complex and thereby facilitates catalysis by Lys(87) in the physiological pH range.

Properties of tetrahydropteroylpentaglutamate bound to 10-formyltetrahydrofolate dehydrogenase

A new rapid procedure for purifying 10-formyltetrahydrofolate dehydrogenase results in 90 mg of pure enzyme from two rabbit livers. This abundant liver enzyme is known to bind its product tetrahydropteroylpentaglutamate (H4PteGlu5) so tightly that it does not dissociate during size exclusion chromatography. 10-Formyltetrahydrofolate dehydrogenase is also known to exhibit strong product inhibition by H4PteGlu5. There is a several-fold excess of 10-formyltetrahydrofolate dehydrogenase subunits in liver relative to the concentration of H4PteGlun, suggesting that in vivo this enzyme may bind significant amounts of this coenzyme in a nearly irreversible enzyme. H4PteGlu5 complex. How this tightly bound H4PteGlun is transferred to the other two enzymes in the cytosol, serine hydroxymethyltransferase and C1-tetrahydrofolate synthase, which use H4PteGlu5 as a substrate, is the subject of this investigation. Analysis of the product inhibition curve for 10-formyltetrahydrofolate dehydrogenase shows that H4-PteGlu5 has a dissociation constant near 15 nM which is 60-fold lower than the Ks for 10-formyl-H4PteGlu5. Fluorescence titration studies also yield a Kd of about 20 nM for H4PteGlu5. Coupling the 10-formyltetrahydrofolate dehydrogenase reaction to an excess of either serine hydroxymethyltransferase or C1-tetrahydrofolate synthase not only abolishes product inhibition but also increases the initial rate of its activity by about 2-fold. Passage of a reaction mixture of 10-formyltetrahydrofolate dehydrogenase down a size exclusion column results in enzyme with 1 equiv of H4PteGlu5 bound per subunit. However, addition of either serine hydroxymethyltransferase or C1-tetrahydrofolate synthase results in a rapid transfer of this bound folate to these enzymes. Evidence is also presented that the tightly bound folate is in equilibrium with solvent H4PteGlu5.

Enzymatic mechanism for the hydrolysis of 5,10-methenyltetrahydropteroylglutamate to 5-formyltetrahydropteroylglutamate by serine hydroxymethyltransferase

Serine hydroxymethyltransferase in the presence of glycine catalyzes the hydrolysis of (6R)-5,10-methenyltetrahydropteroylpolyglutamate to (6S)-5-formyltetrahydropteroylpolyglutamate. The enzyme also catalyzes the formation of (6S)-5-formyltetrahydropteroylpolyglutamate from a compound in equilibrium with (6R)-5,10-methenyltetrahydropteroylpolyglutamate believed to be (6R,11R)-5,10-hydroxymethylenetetrahydropteroylpolyglutamate , a putative intermediate in the nonenzymatic hydrolysis of 5,10-methenyltetrahydropteroylglutamate to 5-formyltetrahydropteroylglutamate [Stover, P., & Schirch, V. (1992) Biochemistry (preceding paper in this issue)]. The enzymatic mechanism for the formation of (6S)-5-formyltetrahydropteroylpolyglutamate from these substrates and the role of glycine in the reaction was addressed. Evidence suggests that (6R,11R)-5,10-hydroxymethylenetetrahydropteroyltetraglutamate++ + is a catalytically competent intermediate in the enzyme-catalyzed hydrolysis of (6R)-5,10-methenyltetrahydropteroyltetraglutamate. The enzyme displays a high Km of 40 microM for (6R)-5,10-methenyltetrahydropteroyltetraglutamate, while the Km for (6R,11R)-5,10-hydroxymethylenetetrahydropteroyltetraglutamate++ + is below 0.5 microM. The kcat values for both reactions are identical and equal to the rate of formation of an enzyme ternary complex absorbing at 502 nm which is formed from glycine and (6S)-5-formyltetrahydropteroylpolyglutamate. The hydrolysis reaction proceeds with exchange of the C11 formyl proton of (6R)-5,10-methenyltetrahydropteroyltetraglutamate, suggesting that the enzyme-catalyzed reaction occurs by the same C11 carbanion inversion mechanism as the nonenzymatic reaction. Isotope exchange experiments using [2-3H]glycine and differential scanning calorimetry data suggest both a catalytic and a conformational role for glycine in the enzymatic reaction. The results are discussed in terms of the similarity in mechanisms of the SHMT-catalyzed retroaldol cleavage of serine and hydrolysis of (6R)-5,10-methenyltetrahydropteroylpolyglutamates.

Serine hydroxymethyltransferase: origin of substrate specificity

All forms of serine hydroxymethyltransferase, for which a primary structure is known, have five threonine residues near the active-site lysyl residue (K229) that forms the internal aldimine with pyridoxal phosphate. For Escherichia coli serine hydroxymethyltransferase each of these threonine residues has been changed to an alanine residue. The resulting five mutant enzymes were purified and characterized with respect to kinetic and spectral properties. The mutant enzymes T224A and T227A showed no significant changes in kinetic and spectral properties compared to the wild-type enzyme. The T225A and T230A enzymes exhibited differences in Km and kcat values but exhibited the same spectral properties as the wild-type enzyme. The four threonine residues at positions 224, 225, 227, and 230 do not play a critical role in the mechanism of the enzyme. The T226A enzyme had nearly normal affinity for substrates and coenzymes but had only 3% of the catalytic activity of the wild-type enzyme. The spectrum of the T226A enzyme in the presence of amino acid substrates showed a large absorption maximum at 343 nm with only a small absorption band at 425 nm, unlike the wild-type enzyme whose enzyme-substrate complexes absorb at 425 nm. Rapid reaction studies showed that when amino acid substrates and substrate analogues were added to the T226A enzyme, the internal aldimine absorbing at 422 nm was rapidly converted to a complex absorbing at 343 nm in a second-order process. This was followed by a very slow first-order formation of a complex absorbing at 425 nm.(ABSTRACT TRUNCATED AT 250 WORDS)

Serine hydroxymethyltransferase. Effect of proteases on the activity and structure of the cytosolic enzyme

Homogeneous preparations of cytosolic serine hydroxymethyltransferase from rabbit liver were incubated with several different proteases. Chymotrypsin rapidly cleaves a tetradecapeptide from the NH2-terminal end of the enzyme with the enzyme retaining full catalytic activity. Trypsin digestion results in the release of several small peptides from the NH2-terminal end of the enzyme. The remaining core protein is reduced in molecular mass by about 3500 Da. With L-serine as substrate the core protein has 1.5 times the activity of the native enzyme. The difference in activity is due to a change in Vmax since the Km values for L-serine and tetrahydrofolate are unchanged. When allothreonine is used as the substrate the activity of the trypsin-treated enzyme is unchanged. Ks values for glycine and several folate compounds are also unchanged for the trypsin-digested enzyme. The relative distribution of three glycine-enzyme complexes shows only small differences between the native and trypsin-digested enzyme. Thermal denaturation studies show that the trypsin-digested enzyme has a thermal transition three degrees lower than the native enzyme but the same enthalpy of denaturation. These results suggest that the 25-30 amino acid residues from the NH2-terminal end of the enzyme are not important in determining the catalytic activity and structural stability of the purified enzyme. Several other proteases had no observable effect on the activity and size of the enzyme. All of the proteases tested inactivated the apoenzyme and digested it into small fragments. The loss of enzyme activity in frozen liver is probably the result of the enzyme slowly being converted to the apoenzyme form, which is susceptible to protease degradation.

Serine hydroxymethyltransferase from Escherichia coli: purification and properties

Serine hydroxymethyltransferase from Escherichia coli was purified to homogeneity. The enzyme was a homodimer of identical subunits with a molecular weight of 95,000. The amino acid sequence of the amino and carboxy-terminal ends and the amino acid composition of cysteine-containing tryptic peptides were in agreement with the primary structure proposed for this enzyme from the structure of the glyA gene (M. Plamann, L. Stauffer, M. Urbanowski, and G. Stauffer, Nucleic Acids Res. 11:2065-2074, 1983). The enzyme contained no disulfide bonds but had one sulfhydryl group on the surface of the protein. Several sulfhydryl reagents reacted with this exposed group and inactivated the enzyme. Spectra of the enzyme in the presence of substrates and substrate analogs showed that the enzyme formed the same complexes and in similar relative concentrations as previously observed with the cytosolic and mitochondrial rabbit liver isoenzymes. Kinetic studies with substrates showed that the affinity and synergistic binding of the amino acid and folate substrates were similar to those obtained with the rabbit liver isoenzymes. The enzyme catalyzed the cleavage of threonine, allothreonine, and 3-phenylserine to glycine and the corresponding aldehyde in the absence of tetrahydrofolate. The enzyme was also inactivated by D-alanine caused by the transamination of the active site pyridoxal phosphate to pyridoxamine phosphate. This substrate specificity was also observed with the rabbit liver isoenzymes. We conclude that the reaction mechanism and the active site structure of E. coli serine hydroxymethyltransferase are very similar to the mechanism and structure of the rabbit liver isoenzymes.

The mechanism of tryptophan binding to tryptophan synthase from Escherichia coli

The kinetics of the binding of L-tryptophan to the alpha 2 holo beta 2 complex of tryptophan synthase from Escherichia coli have been measured by rapid-mixing techniques under conditions where tryptophan release is mainly rate-determining in tryptophan synthesis. The dependence of the three observable rate processes on the concentration of L-tryptophan suggests a mechanism in which a rapid binding step is followed by two isomerizations. The effect of the substrate analogue indolepropanol phosphate on the kinetics of binding and synthesis from L-serine and indole supports a branched mechanism with an unproductive enzyme-ligand complex being the major species. The productive enzyme-ligand complex absorbs light at 473 nm but not at 500 nm. These observations, and binding studies with D-tryptophan, suggest that at least two alterative modes of binding of L-tryptophan exist on the enzyme. The effects of protons, indole and indolepropanol phosphate on the three rate processes explain the dependence of kcat on the three non-competitive ligands.

Interaction of pyridoxal 5-phosphate with apo-serine hydroxymethyltransferase

The interaction of pyridoxal 5-phosphate with beef liver serine hydroxymethyltransferase (5,10-methylenetetrahydrofolate:glycine hydroxymethyltransferase, EC 2.1.2.1) has been investigated using sedimentation velocity, kinetic and equilibrium techniques. No evidence for an aggregating system could be found in sedimentation velocity experiments in the presence or absence of pyridoxal 5-phosphate. Reassociation of pyridoxal 5-phosphate with apoenzyme and reacquisition of enzymic activity follow identical kinetics. An initial fast step is followed by a second order process with a rate constant of 66 M-1. s-1. A dissociation constant of 27.5 micrometer was obtained from equilibrium studies. No interaction of binding sites was exposed by altering pH or in the presence of glycine or folate. Maxima observed in pH profiles with both binding and reactivation are interpreted as the composite fo two overlapping processes, one of which is ionization of the pyridinium nitrogen of pyridoxal 5-phosphate and the other a functional group on the apoenzyme. Evidence is presented to indicate the necessity for the formation of an enzyme . pyridoxal 5-phosphate Schiff's base complex during catalytic turnover.

Serine transhydroxymethylase. Equilibrium binding of folate analogs as active site probes

Formation of a quinoid-like structure within the glycyl-pyridoxal phosphate moiety of serine transhydroxymethylase (5,10-methylenetetrahydrofolate: glycine hydroxymethyltransferase, EC 2.1.2.1) is dependent upon the dissociation of the 2-S hydrogen of glycine which in turn requires the presence of tetrahydrofolate or analogs thereof. Equilibrium binding studies with the series folate, dihydrofolate, and tetrahydrofolate showed that reduction of the pteridine ring enhances both quinoid formation and binding. A 5,8-deazafolate series showed that modifications in the 4 position, 10 position and the glutamyl position yield interrelated alterations of quinoid formation which could not be correlated with binding.