Hydrophobic formate dehydrogenase from Pseudomonas sp. 101 in the system of reverse micelles of aerosol OT in octane

NAD(+)-dependent formate dehydrogenase (FDH) was hydrophobized with palmitoyl chloride to give the samples with various modification degrees (2-10). The native and modified FDHs were comparatively studied in the system of reverse micelles of Aerosol OT in octane. Like the native, the modified enzyme displayed three maxima in the curve of dependence of its catalytic activity on the degree of surfactant hydration (the micelle size), which reflect the enzyme functioning in the form of a monomer, dimer, or octamer. The peak corresponding to the functioning of the FDH dimer was found to decrease along with an increase in the modification degree. Thus, the modified enzyme mainly functions in the form of monomer and octamer. The modified FDH displayed membranotropy and revealed the dependence of catalytic activity on surfactant concentration.     

Effect of chemical modification of lipase on the regulation of its lipolytic activity in reversed micelles

Hydrophilized and hydrophobized forms of the lipase from Mucor miehei were obtained by its chemical modification with cellobiose and N-hydroxysuccinimidyl palmitate with a modification degree of 4 in both cases. A comparative analysis of the regulation of the catalytic activities of the native and modified lipases was carried out in the system of reversed micelles of OT aerosol (AOT) in isooctane. The level of catalytic activity of all the lipase preparations in the micellar medium was found to be higher than that in aqueous solution. The chemical modification of lipase did not result in a change in the regulation of the oligomeric composition of the enzyme controlled by the degree of micelle hydration omega0 (micelle size). The kcat dependences on omega0 for each lipase preparation exhibit two maxima, corresponding to the functioning of lipase monomers and tetramers. The changes in the hydrophilic-lipophilic balance of the lipase surface significantly affect the character of the regulation of enzyme activity due to changes in the surfactant concentration (the number of micelles). The lipase hydrophobization results in a decrease in the enzyme activation effect with an increase in the AOT concentration in comparison with the native lipase. The lipase hydrophilization dramatically decreases the activity of lipase tetramer when the AOT concentration is increased. The catalytic activity of the monomer of hydrophilized lipase is practically independent of the AOT concentration. Kinetic data indicate a mixed type of activation of both oligomeric forms of the native and the hydrophobized lipase by AOT molecules and the noncompetitive type of the activation and AOT inhibition of the monomer and the tetramer of the hydrophilized lipase, respectively. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2005, vol. 31, no. 6; see also http://www.maik.ru.     

Effect of Chemical Modification of Lipase on the Regulation of Its Lipolytic Activity in Reversed Micelles

Hydrophilized and hydrophobized forms of the lipase from Mucor miehei were obtained by its chemical modification with cellobiose and N-succinimidyl palmitate with a modification degree of 4 in both cases. A comparative analysis of the regulation of the catalytic activities of the native and modified lipases was carried out in the system of reversed micelles of OT aerosol (AOT) in isooctane. The level of catalytic activity of all the lipase preparations in the micellar medium was found to be higher than that in aqueous solution. The chemical modification of lipase did not result in a change in the regulation of the oligomeric composition of the enzyme controlled by the degree of micelle hydration Ω₀ (micelle size). The k _cat dependences on Ω₀ for each lipase preparation exhibit two maxima, corresponding to the functioning of lipase monomers and tetramers. The changes in the hydrophilic-lipophilic balance of the lipase surface significantly affect the character of the regulation of enzyme activity due to changes in the surfactant concentration (the number of micelles). The lipase hydrophobization results in a decrease in the enzyme activation effect with an increase in the AOT concentration in comparison with the native lipase. The lipase hydrophilization dramatically decreases the activity of lipase tetramer when the AOT concentration is increased. The catalytic activity of the monomer of hydrophilized lipase is practically independent of the AOT concentration. Kinetic data indicate a mixed type of activation of both oligomeric forms of the native and the hydrophobized lipase by AOT molecules and the noncompetitive type of the activation and AOT inhibition of the monomer and the tetramer of the hydrophilized lipase, respectively.     

The effect of tunicamycin on secreted glycosidases of Aspergillus niger

The catalytic activity of the two stable aggregation states of arginine decarboxylase, a dimer and a decamer, has been examined under a variety of conditions. The specific activity of the dimer was determined at pH 5.2, the optimum pH, over a very broad range of protein concentrations. It was found to be independent of concentration only above 5 μg/ml, and decreased to 0 at 0.02 μg/ml, suggesting that a concentration-dependent reassociation was occurring in the assay. At 0.58 μg/ml, the restoration of activity was time dependent. We conclude that, at pH 5.2, the decamer is active and the dimer is essentially inactive. The activities of the dimer and the decamer were also compared at neutral pH, by using increasing concentrations of either Na⁺, arginine, or protein to induce reassociation. All of these experiments are consistent with the idea that both species are equally active at this pH. Furthermore, the dependence of activity on arginine concentration is not hyperbolic at pH 7.2. The arginine decarboxylase dimer was modified by allowing one sulfhydryl group per monomer to react with 5,5′-dithiobis(2-nitrobenzoic acid). The modified dimer reassociates less readily than the native form, requiring higher concentrations of any of the three associating agents tried. The modified decamer, at both pH 5.2 and 7.4, and the modified dimer, at pH 7.4, retain approximately 60% of the activity of the untreated enzyme.     

Trypsin-modified alkaline phosphatase. Formation of apoenzyme monomer and hybrid dimer

The cleavage of an amino-terminal decapeptide from Escherichia coli alkaline phosphatase has been previously described (Roberts, C. H., and Chlebowski, J. F. (1984) J. Biol. Chem. 259, 729-733) by this laboratory. The modest reduction in specific activity of the modified enzyme is paralleled by an apparent alteration in the Zn(II) affinity at one of the three active center metal ion binding sites. In contrast to the behavior of the native enzyme, formation of the metal-free apoprotein results in an irreversible loss of catalytic activity; phosphohydrolase activity is not restored on addition of Zn(II) and Mg(II). Differential scanning calorimetry and velocity sedimentation data indicate that the apo form of the modified enzyme exists as a monomer form which, while capable of binding Zn(II) does not readily reassociate to active dimer. Processive cleavage of the amino termini of the dimer by trypsin results in the transient formation of a hybrid dimer consisting of cleaved and uncleaved subunits. This species can be directly observed and isolated by taking advantage of the differential chromatographic mobility of the native "isozymes" and the resulting products. Coupled with improved procedures for the preparation of the modified protein, these data indicate that the amino-terminal modification results in alterations in the subunit interface domain and provides a species (the hybrid dimer) for the investigation of the propagation of these effects.     

The principal difference in regulation of the catalytic activity of water-soluble and membrane forms of enzymes in reversed micelles. Gamma-glutamyltransferase and aminopeptidase

The regularities of their functioning of enzyme, water-soluble and membrane forms, in the systems of the reversed micelles of surfactants in organic solvents are compared. Using as examples gamma-glutamyltransferase (in AOT reversed micelles in octane) and aminopeptidase (in Brij 96 reversed micelles in cyclohexane), the principal difference in the catalytic activity regulation of water-soluble and membrane forms is demonstrated. The catalytic activity of the membrane form depends considerably on the surfactant concentration at the constant degree of hydration, whereas the activity of the water-soluble form is constant under these conditions. The catalytic activity dependence on the surfactant concentration is regarded as a test for enzyme membrane activity.     

Properties of a cGMP-dependent monomeric protein kinase from bovine aorta

A form of cGMP-dependent protein kinase (cGK) that was different from previously described cGK was purified from bovine aorta smooth muscle. The partial amino-terminal sequencing of this enzyme indicated that it was derived by endogenous proteolysis of the type I beta isozyme of cGK. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, this form migrated as a smaller protein (Mr = 70,000) than the parent cGK (Mr = 80,000), and since the calculated nondenatured Mr was approximately 89,000 compared to Mr = 170,000 for the dimeric native enzyme, it represented a monomeric form of cGK. The monomer bound approximately 2 mol of [3H]cGMP per mol of monomer, although it had only one rapid component in [3H]cGMP dissociation assays as compared to one rapid and one slow component for the native cGK. The specific catalytic activity of the kinase was similar to that of the native enzyme, suggesting that the catalytic domain was essentially intact. The monomeric cGK incorporated significant 32P when incubated with Mg2+ and [gamma-32P]ATP in the presence of cGMP, although the phosphorylation proceeded at a slower rate than that obtained with native cGK. In contrast to previous reports of monomeric forms of cGK, this monomer was highly cGMP-dependent, although it had a slightly higher Ka (0.8 microM) for cGMP than that of the native enzyme (0.4 microM) and a low Hill coefficient of 1.0 (1.6 for the native enzyme). The cGMP dependence of the monomer did not decrease with dilution, implying that the cGMP dependence was not due to monomer-monomer interactions in the assay. The results indicated that the catalytic domain, cGMP binding domain(s), and inhibitory domain of cGK interact primarily within the same subunit rather than between subunits of the dimer as previously hypothesized for dimeric cGK.     

Regulation of the catalytic activity and supermolecular structure of penicillin acylase from Escherichia coli in an aerosol OT reversed micelle system in octane]

The properties of penicillin acylase from E. coli solubilized by hydrated reversed micelles of Aerozol OT (AOT) in octane were studied. The catalytic activity dependence on the hydration degree, a parameter which determines the size of the micelle inner cavity, represents a curve with three optima, each corresponding to the enzyme functioning either in a dimer form (omega 0 = 23) or in the form of separate subunits--heavy, beta, and light, alpha, at omega 0 = 20 and 14, respectively. Reversible dissociation of the enzyme was confirmed by ultracentrifugation followed by electrophoresis. Preparative isolation of penicillin acylase subunits, their catalytic activity being retained, was shown to be possible.     

The role of dissimilar subunits of NAD-specific isocitrate dehydrogenase from pig heart. Evaluation using affinity labeling

NAD-specific isocitrate dehydrogenase from pig heart is composed of three dissimilar subunits present in the native enzyme as 2 alpha:1 beta: 1 gamma, with a tetramer being the smallest form of complete enzyme. The role of these subunits has been explored using affinity labeling. Specifically labeled subunits are separated and then recombined with unmodified subunits to form dimers. Recombination of beta or gamma subunits modified by the isocitrate analogues, 3-bromo-2-ketoglutarate and 3,4-didehydro-2-ketoglutarate, with unmodified alpha subunit led to the same activity in the dimer as when unmodified beta or gamma was combined with alpha. Contrastingly, modification of alpha with these isocitrate analogues led to loss in activity either alone or when recombined with beta or gamma. Hence, the isocitrate site on alpha is required for catalytic activity but the isocitrate sites on beta or gamma are not necessary for the activity of the functional dimer. Reaction of isolated subunits with 3-bromo-2-ketoglutarate shows that alpha and the alpha beta dimer are modified at about the same rate as holoenzyme, suggestive of similarity of the isocitrate site in native enzyme and in isolated active entities containing alpha subunit; in contrast, beta and gamma subunits react more slowly. Modification by the 2',3'-dialdehyde derivative of the allosteric effector, ADP, led to loss of activity in reconstituted dimers, independent of which subunit was modified. Reaction of isolated subunits with the dialdehyde derivative of ADP is slow compared to the initial reaction with native enzyme, indicating differences in the effects of ADP on intact enzyme and subunits. The ADP sites on all subunits may thus be important in intersubunit interactions, which in turn modulate catalytic activity.     

Purification and properties of a homogeneous aryl sulfatase A from rabbit liver.

Aryl sulfatase A (aryl sulfate sulfohydrolase EC 3.1.6.1) has been purified > 10,000-fold from rabbit liver; by disc gel electrophoresis the enzyme appears homogeneous. Various properties of the enzyme have been determined and comparisons are made with other aryl sulfatases. Sodium dodecyl sulfate gel electrophoresis indicates that the enzyme is made up of monomers of molecular weight ～ 70,000. At pH 7.4 the enzyme exists as a dimer whereas a tetrameric form predominates at pH 4.8.The enzyme exhibits the anomalous kinetics often observed with aryl sulfatase A from mammalian tissues (the enzyme is modified to an inactive form while degrading substrate and the inactive form can be reactivated by sulfate ion). The enzyme activity has been studied under a variety of reaction conditions. Two pH optima are observed and neither enzyme concentration or changes in ionic strength appear to have an effect on the relative magnitudes of the optima. Aryl sulfatase A is competitively inhibited by potassium sulfate, potassium phosphate, and sodium sulfite (K_i = 2.9 × 10^?3 M, 3.4 × 10^?5 M, and 1.1 × 10^?6 M, respectively). Kinetic constants for some substituted phenyl sulfate esters have been determined. The variation in V is not consistent with a reaction mechanism involving a rate-limiting breakdown of a common intermediate.The inactive (modified) form of the enzyme has been isolated from reaction mixtures containing aryl sulfatase A and substrate. A procedure is presented for determining the relative amount of modified and native enzyme in these preparations. In the presence of substrate, sulfate displaces the equilibrium between native and modified enzyme in favor of native enzyme. In the absence of substrate neither sulfate or phosphate have an effect on the equilibrium. A study is made of the temperature dependence of the process in which the modified enzyme is converted back to native enzyme. The relatively small entropy of activation for the conversion of the modified to the native form (ΔS3 = ?8 cal/mole deg) does not seem to be consistent with a major modification of protein conformation.     

Factors affecting the oligomeric structure of yeast external invertase

It has been assumed that yeast external invertase is a dimer, with each subunit composed of a 60-kDa polypeptide chain. We now present evidence that at its optimal pH of 5.0, the predominant form of external invertase is an octamer with an average size of 8 X 10(5) Da. During ultracentrifugation the octamer dissociated to lower molecular weight forms, including a hexamer, tetramer, and dimer. All forms of the enzyme were shown to possess identical specific activities and to contain a similar carbohydrate to protein ratio. Although the monomer subunits (1 X 10(5) Da) were heterogenous in carbohydrate content, each subunit possessed nine oligosaccharide chains. When stained for protein and enzyme activity following sodium dodecyl sulfate-polyacrylamide gel electrophoresis, only the oligomeric form of the enzyme appeared to be active. Thus, on partially inactivating invertase with 4 M guanidine hydrochloride both octamer and monomer were evident on the gels but only the former was active. Similarly, incubating at pH 2.5 in the presence of sodium dodecyl sulfate yielded only inactive monomer. The monomer, unlike the active oligomeric aggregate, was unable to hydrolyze sucrose after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Consistent with the in vitro studies, freshly prepared yeast lysate was shown to contain the octameric species of external invertase as the major active form of this enzyme. From these studies and others which employed deglycosylated invertase, it is concluded that the carbohydrate component of external invertase contributes not only to stabilizing enzyme activity, but also to maintaining its oligomeric structure.     

Despite its high similarity with monomeric arginine kinase, muscle creatine kinase is only enzymatically active as a dimer

Although having highly similar primary to tertiary structures, the different guanidino kinases exhibit distinct quaternary structures: monomer, dimer or octamer. However, no evidence for communication between subunits has yet been provided, and reasons for these different levels of quaternary complexity that can be observed from invertebrate to mammalian guanidino kinases remain elusive. Muscle creatine kinase is a dimer and disruption of the interface between subunits has been shown to give rise to destabilized monomers with slight residual activity; this low activity could, however, be due to a fraction of protein molecules present as dimer. CK monomer/monomer interface involves electrostatic interactions and increasing salt concentrations unfold and inactivate this enzyme. NaCl and guanidine hydrochloride show a synergistic unfolding effect and, whatever the respective concentrations of these compounds, inactivation is associated with a dissociation of the dimer. Using an interface mutant (W210Y), protein concentration dependence of the NaCl-induced unfolding profile indicates that the active dimer is in equilibrium with an inactive monomeric state. Although highly similar to muscle CK, horse shoe crab (Limulus polyphemus) arginine kinase (AK) is enzymatically active as a monomer. Indeed, high ionic strengths that can monomerize and inactivate CK, have no effect on AK enzymatic activity or on its structure as judged from intrinsic fluorescence data. Our results indicate that expression of muscle creatine kinase catalytic activity is dependent on its dimeric state which is required for a proper stabilization of the monomers.     

Yeast glyceraldehyde-3-phosphate dehydrogenase. Evidence that subunit cooperativity in catalysis can be controlled by the formation of a complex with phosphoglycerate kinase

Sepharose-bound tetrameric, dimeric and monomeric forms of yeast glyceraldehyde-3-phosphate dehydrogenase were prepared, as well as immobilized hybrid species containing (by selective oxidation of an active center cysteine residue with H2O2) one inactivated subunit per tetramer or dimer. The catalytic properties of these enzyme forms were compared in the forward reaction (glyceraldehyde-3-phosphate oxidation) and reverse reaction (1,3-bisphosphoglycerate reductive dephosphorylation) under steady-state conditions. In the reaction of glyceraldehyde-3-phosphate oxidation, immobilized monomeric and tetrameric forms exhibited similar specific activities. The hybrid-modified dimer contributed on half of the total activity of a native dimer. The tetramer containing one modified subunit possessed 75% of the activity of an unmodified tetramer. In the reaction of 1,3-bisphosphoglycerate reductive dephosphorylation, the specific activity of the monomeric enzyme species was nearly twice as high as that of the tetramer, suggesting that only one-half of the active centers of the oligomer were acting simultaneously. Subunit cooperativity in catalysis persisted in an isolated dimeric species. The specific activity of a monomer associated with a peroxide-inactivated monomer in a dimer was equal to that of an isolated monomeric species and twice as high as that of a native immobilized dimer. The specific activity of subunits associated with a peroxide-inactivated subunit in a tetramer did not differ from that of a native immobilized tetramer; this indicates that interdimeric interactions are involved in catalytic subunit cooperativity. A complex was formed between the immobilized glyceraldehyde-3-phosphate dehydrogenase and soluble phosphoglycerate kinase. Three monomers of phosphoglycerate kinase were bound per tetramer of the dehydrogenase and one per dimer. Evidence is presented that if the reductive dephosphorylation of 1,3-bisphosphoglycerate proceeds in the phosphoglycerate kinase - glyceraldehyde-3-phosphate dehydrogenase complex, all active sites of the latter enzyme act independently, i.e. subunit cooperativity is abolished.     

Effect of oligomerization on the properties of essential SH-groups of mitochondrial creatine kinase

The properties of creatine kinase isolated from bovine heart mitochondria in dimeric (Mr = 84 +/- 6 kD) and octameric (Mr = 340 +/- 17 kD) forms were compared with those of the earlier described hexameric form of the enzyme (Mr = 240 +/- 12 kD). The kinetics of SH-group modification by DTNB, the inactivation kinetics as well as the number of modified SH-groups point to significant differences between the three oligomeric forms of the enzyme. Each subunit of creatine kinase was found to possess one "fast" essential cysteine residue whose modification by DTNB and iodoacetamide led to enzyme inactivation. The formation of an analog of the transition state complex (E--MgADP--NO3--creatine) was paralleled with partial protection of only the "fast" cysteine residue which manifested itself in the decrease of the rate of its interaction with DTNB in all the three oligomeric forms. Dimer association into a hexamer and octamer occurred in parallel with a decrease of the affinity of essential SH-groups of cysteine for DTNB in 50% of the oligomeric molecule subunits. Thus, in the dimer two essential SH-groups were rapidly modified by DTNB at the same rate: k1 = k2 = (23.9 +/- 5.6).10(4) M-1 min-1. Within the hexamer, the rate of modification of 3 out of 6 SH-groups was practically unchanged: k1 = (10.6 +/- 2.3).10(4) M-1 min-1. Another 3 SH-groups in the remaining 50% of the subunits were partly masked, which manifested itself in a 10-fold decrease of their modification rate: k2 = (1.12 +/- 0.28).10(4) M-1 min-1. Within the octamer, the SH-groups rapidly interacted with DTNB only on 4 subunits: k1 = (20.7 +/- 2.2).10(4) M-1 min-1, whereas in the remaining 4 octamer subunits a practically complete masking of essential SH-groups was observed, as a result of which these groups became inaccessible to DTNB. This manifested itself in a 1000-fold decrease of the rate of SH-group modification by DTNB which reached that of non-essential SH-group modification. In has been found that a complete loss of the octamer activity is due to the modification of only 4 SH-groups which interact with DTNB at a high rate. A model for subunit association into a dimer, hexamer and octamer has been proposed. Presumably, 50% of the active centers in the mitochondrial creatine kinase octamer are not involved in the catalytic act.     

Subunit interaction enhances enzyme activity and stability of sweet potato cytosolic Cu/Zn-superoxide dismutase purified by a His-tagged recombinant protein method

The coding region of copper/zinc-superoxide dismutase (Cu/Zn-SOD) cDNA from sweet potato, Ipomoea batatas (L.) Lam. cv. Tainong 57, was introduced into an expression vector, pET-20b(+). The Cu/Zn-SOD purified by His-tagged technique showed two active forms (dimer and monomer). The amount of proteins of dimer and monomer appeared to be equal, but the activity of dimeric form was seven times higher than that of monomeric form. The enzyme was dissociated into monomer by imidazole buffer above 1.0 M, acidic pH (below 3.0), or SDS (above 1%). The enzyme is quite stable. The enzyme activity is not affected at 85 °C for 20 min, in alkali pH 11.2, or in 0.1 M EDTA and also quite resistant to proteolytic attack. Dimer is more stable than monomer. The thermal inactivation rate constant k _dcalculated for the monomer at 85 °C was 0.029 min^-1 and the half-life for inactivation was about 28 min. In contrast, there is no significant change of dimer activity after 40 min at 85 °C. The enzyme dimer and monomer retained 83% and 58% of original activity, respectively, after 3 h incubation with trypsin at 37 °C, while those retained 100% and 31% of original activity with chymotrypsin under the same condition. These results suggest subunit interaction might change the enzyme conformation and greatly improve the catalytic activity and stability of the enzyme. It is also possible that the intersubunit contacts stabilize a particular optimal conformation of the protein or the dimeric structure enhances catalytic activity by increasing the electrostatic steering of substrate into the active site.     

Comparative study of the regulation of catalytic activity of soluble and membrane forms of enzymes in reverse micellar systems. Gamma-glutamyltransferase and aminopeptidase]

The regulations of functioning of water soluble and membrane forms of enzymes in the systems of reversed micelles of surfactants in organic solvents are compared. By an examples of gamma-glutamyltransferase (in AOT reversed micelles in octane) and amino-peptidase (in Brij 96 reversed micelles in cyclohexane) the principal difference in the catalytic activity regulation of water soluble and membrane forms is demonstrated. The catalytic activity of the membrane form depends largely on the surfactant concentration at the constant hydration degree, whereas the activity of the water soluble form is constant under these conditions. The catalytic activity dependence on the surfactant concentration is regarded as a "test for the enzyme's membrane activity".     

Methionyl-tRNA synthetase of Escherichia coli. A zinc metalloprotein.

The native dimeric form of methionyl-tRNA synthetase of Escherichia coli contains two zinc atoms per dimer, one per subunit. The bound zinc is retained upon trypsin modification which yields a monomer with one zinc atom. The enzymatic activity of both the dimeric forms is reversibly inhibited by 1,10-phenanthroline but not by its non-chelating analogues. In addition, the native enzyme binds two Mn2+ per dimer with a binding constant of approx. 70 micron but no binding is observed with the trypsin-modified monomer.     

Reversible dissociation of active octamer of cyanase to inactive dimer promoted by alteration of the sulfhydryl group

Cyanase is an inducible enzyme in Escherichia coli that catalyzes the reaction of cyanate with bicarbonate resulting in the decomposition of cyanate to ammonia and bicarbonate. In this study, the role of the single sulfhydryl group in each of the eight identical subunits of cyanase was investigated. Tetranitromethane, methyl methanethiosulfonate, N-ethylmaleimide, and Hg2+ all reacted with the sulfhydryl group to give derivatives which had reduced activities and which dissociated reversibly to inactive dimer. Association of inactive dimer to active octamer was facilitated by the presence of azide (cyanate analog) and bicarbonate, increased temperature and enzyme concentration, and presence of phosphate. Nitration of tyrosine residues by tetranitromethane occurred only in the absence of azide and bicarbonate, suggesting that at least some of the tyrosine residues become exposed when octamer dissociates to dimer. Site-directed mutagenesis was used to prepare a mutant enzyme in which serine was substituted for cysteine. The mutant enzyme was catalytically active and had properties very similar to native enzyme, except that it was less stable to treatment with urea and to high temperatures. These results establish that in native cyanase the sulfhydryl group per se is not required for catalytic activity, but it may play a role in stabilizing octameric structure, and that octameric structure is required for catalytic activity.     

4-Aminobutyrate aminotransferase. Conformational changes induced by reduction of pyridoxal 5-phosphate

Conformational changes induced in 4-aminobutyrate aminotransferase (4-aminobutyrate:2-oxoglutarate aminotransferase, EC 2.6.1.19) by conversion of pyridoxal-5-P to pyridoxyl-5-P were examined by two independent methods. The reactivity of the SH groups of the reduced enzyme is increased by chemical modification of the cofactor. 1.8 SH per dimer of modified enzyme react with DTNB, whereas 1.2 SH per dimer of the native enzyme react with the attacking reagent under identical experimental conditions. The modified and native forms of the enzyme bind the fluorescent probe ANS, but the number of binding sites for ANS is increased as result of conversion of P-pyridoxal to P-pyridoxyl. After the conformational changes onset by reduction of the cofactor, the modified enzyme binds one molecule of pyridoxal-5-P with a Kd of 0.1 microM to become catalytically competent. The catalytic site of the reduce enzyme was probed with P-pyridoxal analogs. Like resolved 4-aminobutyrate aminotransferase, the reduced species recognize the phosphorothioate analog and regain 40% of the total enzymatic activity. Since the catalytic parameters of reduced and native 4-aminobutyrate aminotransferase are indistinguishable, it is concluded that the additional catalytic site of the reduced enzyme is functionally identical to that of the native enzyme.     

Chemical modification of Lactobacillus casei thymidylate synthetase with 1-fluoro-2,4-dinitrobenzene and 1,5-difluoro-2,4-dinitrobenzene

Previous studies of the pH dependence of sulfhydryl group modification in thymidylate synthetase (W. A. Munroe, C. A. Lewis and R. B. Dunlap, 1978, Biochem. Biophys. Res. Commun.80, 355–360) suggested that a neighboring general base residue enhanced the nucleophilicity of the catalytic cysteinyl side chain. In an effort to identify the latter residue by active site crosslinking, chemical modification of the enzyme by 1,5-difluoro-2,4-dinitrobenzene was investigated and compared with results of modification by 1-fluoro-2,4-dinitrobenzene. Incubation of enzyme with 1-fluoro-2,4-dinitrobenzene led to rapid inactivation and loss of ability to form ternary complexes. Paper chromatography of the acid hydrolysate of enzyme modified with 1-fluoro-2,4-dinitrobenzene yielded two yellow spots, identified as dinitrophenylenecysteine and dinitrophenylenelysine. Specific active site labeling was indicated by substrate protection with dUMP, by the release of 1.65 of fluoride ion per enzyme dimer during inactivation, and by the fact that 70% of the activity was recovered after incubation of the inactivated enzyme with 2-mercaptoethanol, The results of a similar series of studies with 1,5-difluoro-2,4-dinitrobenzene indicated quite specific active site modification. The equivalents of fluoride ion released during modification, 3.5 per enzyme dimer, and the fact that thiolysis of the totally inactivated enzyme led to a recovery of only 18% of the original activity provided evidence for active site crosslinking with the catalytic cysteine as one of the modification sites. Characterization of the modified enzyme, its yellow acid hydrolysate fragments, and a variety of dinitrophenylene crosslinked models suggested that 1,5-difluoro-2,4-dinitrobenzene had modified the enzyme by crosslinking cysteine and serine residues.