The threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli K 12. Binding of threonine and of pyridine nucleotides: stoichiometry and optical effects

Leishmania major pteridine reductase 1 belongs to the short chain dehydrogenase family: stereochemical and kinetic evidence

Pteridine reductase 1 (PTR1) is a novel broad spectrum enzyme of pterin and folate metabolism in the protozoan parasite Leishmania. Overexpression of PTR1 confers methotrexate resistance to these protozoa, arising from the enzyme's ability to reduce dihydrofolate and its relative insensitivity to methotrexate. The kinetic mechanism and stereochemical course for the catalyzed reaction confirm PTR1's membership within the short chain dehydrogenase/reductase (SDR) family. With folate as a substrate, PTR1 catalyzes two rounds of reduction, yielding 5,6,7, 8-tetrahydrofolate and oxidizing 2 equiv of NADPH. Dihydrofolate accumulates transiently during folate reduction and is both a substrate and an inhibitor of PTR1. PTR1 transfers the pro-S hydride of NADPH to carbon 6 on the si face of dihydrofolate, producing the same stereoisomer of THF as does dihydrofolate reductase. Product inhibition and isotope partitioning studies support an ordered ternary complex mechanism, with NADPH binding first and NADP+ dissociating after the reduced pteridine. Identical kinetic mechanisms and NAD(P)H hydride chirality preferences are seen with other SDRs. An observed tritium effect upon V/K for reduction of dihydrofolate arising from isotopic substitution of the transferred hydride was suppressed at a high concentration of dihydrofolate, consistent with a steady-state ordered kinetic mechanism. Interestingly, half of the binary enzyme-NADPH complex appears to be incapable of rapid turnover. Fluorescence quenching results also indicate the existence of a nonproductive binary enzyme-dihydrofolate complex. The nonproductive complexes observed between PTR1 and its substrates are unique among members of the SDR family and may provide leads for developing antileishmanial therapeutics.

Comparison of crotoxin isoforms reveals that stability of the complex plays a major role in its pharmacological action

Crotoxin from the venom of the South American rattlesnake Crotalus durissus terrificus is a potent neurotoxin consisting of a weakly toxic phospholipase-A2 subunit (CB) and a non-enzymic, non-toxic subunit (CA). Crotoxin complex (CACB) dissociates upon interaction with membranes: CB binds while CA does not. Moreover, CA enhances the toxicity of CB by preventing its non-specific adsorption. Several crotoxin isoforms have been identified. Multiple variants of each subunit give different crotoxin complexes that can be subdivided into two classes: those of high toxicity and low enzymic activity and those of moderate toxicity and a high phospholipase-A2 activity. In this study, we demonstrate that the more-toxic isoforms block neuromuscular transmission of chick biventer cervicis preparations more efficiently than weakly toxic isoforms. The less-toxic crotoxin complexes have the same Km and Vmax as CB alone. In contrast, the more-toxic isoforms are enzymically less active than CB. These differences correlate with the stability of the complexes: less-toxic isoforms are less stable (Kd = 25 nM) and dissociate rapidly (half-life about 1 min), whereas the more-toxic isoforms are more stable (Kd = 4.5 nM) and dissociate more slowly (half-life 10-20 min). The rate of interaction of crotoxin complexes with vesicles of negatively charged phospholipids paralleled the rate of dissociation of the complexes in the absence of vesicles. The differences of pharmacological and biochemical properties of crotoxin isoforms indicate that the stability of crotoxin complexes plays a major role in the synergistic action of crotoxin subunits: a stronger association between the two crotoxin subunits would account for their slower dissociation rate, a weaker enzymic activity, a slower interaction with phosphatidylglycerol vesicles, a faster blockade of neuromuscular transmission and a higher lethal potency.

Allosteric transition of aspartokinase I-homoserine dehydrogenase I studied by time-resolved fluorescence

The allosteric transition of threonine-sensitive aspartokinase I-homoserine dehydrogenase I from Escherichia coli has been studied by time-resolved fluorescence spectroscopy. Fluorescence decay can be resolved into 2 distinct classes of tryptophan emitters: a fast component, with a lifetime of about 1.5 ns; and a slow component, with a lifetime of about 4.5 ns. The fluorescence properties of the slow component are modified by the allosteric transition. In the T-form of the enzyme stabilized by threonine, the lifetime of the slow component is longer, with a red-shifted spectrum; its accessibility to quenching by acrylamide becomes slightly higher without any decrease of fluorescence anisotropy. These results indicate a change in polarity of the slow component environment. The quaternary structure change associated with the allosteric transition probably involves global movements of structural domains without leading to any local mobility on the nanosecond time-scale. We suggest that the slow component corresponds to the unique tryptophan of the buried kinase domain.

Affinity labeling of the Escherichia coli aspartate-beta-semialdehyde dehydrogenase with an alkylating coenzyme analogue. Half-site reactivity and competition with the substrate alkylating analogue

3-Chloroacetylpyridine-adenine dinucleotide phosphate (clac3PdADP+, a NADP+ alkylating analogue, irreversibly inactivates aspartate-beta-semialdehyde dehydrogenase with pseudo-first-order kinetics. NADP+ and NADPH, but not the substrate, protected the enzyme from inactivation. The pH dependence of the inactivation kinetics was determined. Incorporation of 1 mol cl[14C]-ac3PdADP+/dimer totally inactivates the enzyme. Successive alkylation by the coenzyme analogue and by the substrate analogue, L-2-amino-4-oxo-5-chloropentanoic acid, was studied. After inactivation with the coenzyme analogue, no incorporation of the substrate analogue was detected. However, when the enzyme was first inactivated with the substrate analogue, the protein could subsequently be alkylated with the coenzyme analogue. The binding of NADP+ and NADPH to aspartate-beta-semialdehyde dehydrogenase was determined by fluorescence.

Effects of monovalent cations on AMP nucleosidase from Azotobacter vinelandii

The effect of monovalent cations on the purified AMP nucleosidase (AMP phosphoribohydrolase, EC 3.2.2.4) from Azotobacter vinelandii was investigated. All the monovalent cations were activators of the enzyme: Rb+ and Cs+ were the most effective, followed by K+, Na+, NH4+ and Li+ in that order. The apparent Ka for MgATP and nH values (Hill's interaction coefficient) decreased from 0.9 to 0.1 mM, and from 4 to 1, respectively, with the increase in K+ concentration, suggesting that the cation effects are on MgATP binding rather than catalysis. Gel filtration studies have revealed that the enzyme forms a non-dissociable enzyme species with a Stokes radius of 6.0--6.2 nm in the presence of saturating concentrations of monovalent cations, which can be distinguished from the 5.5-nm enzyme species showing temperature-dependent dissociation of the molecule in sulfate or phosphate. These results suggest that these ligands affect the association of the subunits through changes in the environment of the hydrophobic side chains of the enzyme molecules.

Anticooperative binding of L-tryptophan to tryptophanyl-tRNA synthetase from beef pancreas. Study at equilibrium by dialysis and changes in spectroscopic properties

Equilibrium dialysis and gel filtration studies show that tryptophanyl-tRNA synthetase from beef pancreas binds two molecules of L-tryptophan per dimer in an anticooperative way. The binding of tryptophan ellicits a series of spectroscopic changes in the protein as seen by absorbance, fluorescence and circular dichroism. The molar absorption change of the protein-tryptophan system upon formation of the complex is delta epsilon292 = 10 400 +/- 1000 M(-1) cm(-1) per dimer. Taking an initial symmetrical dimeric protein the two dissociation constants for tryptophan at pH 8, 25 degrees C are respectively K1 = 2.0 +/- 0.5 muM and K2 = 10 +/- 4 muM. They are respectively K1 = 1 +/- 0.25 muM and K2 = 20 +/- 8 muM if one considers a sequenced binding of the two tryptophan molecules. The dichroic band at 290 nm of the free protein disappears when tryptophan is bound. All observed changes are characteristic of tryptophan perturbation and none of tyrosine perturbation. They all exceed the effect that can be expected from the change in environment of the bound tryptophan molecules and modifications of the tertiary structure of the protein have to be taken into account to explain the observed spectroscopic data.

Fluorescence energy transfer between heterologous active sites of affinity-labeled aspartokinase of Escherichia coli

The distance between aspartokinase and homoserine dehydrogenase active sites was determined using fluorescence energy transfer between modified substrates. The fluorescent 1,N(6)-ethenoadenosine 5'-triphosphate was bound at the kinase active site by Co(III) affinity labeling. Reduced thionicotinamide adenine dinucleotide phosphate quenched the fluorescence of bound nucleotide. Fluorescence depolarization measurements led to a delimitation of the value of the dipolar orientation factor to the range 0.3 to 2.8. The distance between the fluorescent probe and the quencher was 29 +/- 4 A. In the presence of threonine, this distance increased to 36 +/- 5 A. Threonine binding either increased the intersite distance by ca. 7 A or caused a reorientation of the probe at the dehydrogenase site.

Interaction of substrates and inhibitors with the homoserine dehydrogenase of kinase-inactivated aspartokinase I

The aspartokinase activity of the aspartokinase-homoserine dehydrogenase complex of Escherichia coli was affinity labeled with substrates ATP, aspartate, and feedback inhibitor threonine. Exchange-inert ternary adducts of Co(III)-aspartokinase and either ATP, aspartate or threonine were formed by oxidation of corresponding Co(II) ternary complexes with H2O2. The ternary enzyme-Co(III)-threonine adduct (I) had 3.8 threonine binding sites per tetramer, one-half that of the native enzyme. The binding of threonine to I was still cooperative as determined by equilibrium dialysis (nH = 2.2) or by studying inhibition of residual dehydrogenase activity (nH = 2.7). Threonine still protected the SH groups of I against 5,5'-dithiobis(2-nitrobenzoate) (DTNB) reaction but the number of SH groups reacting with thiol reagents (DTNB) was reduced by 1-2 per subunit in the absence of threonine. This suggests either that Co(III) is bound to the enzyme via sulfhydryl groups or that 1-2SH groups are buried or rendered inaccessible in I. The binding of threonine to sites not blocked by the affinity labeling produced changes in the circular dichroism of the complex comparable to changes produced by threonine binding to native enzyme and also protected against proteolytic digestion. The major conformational changes produced by threonine are thus ascribable to binding at this one class of regulatory sites. The interactions of kinase substrates with various aspartokinase-Co(III) complexes containing ATP, aspartate, or threonine and a threonine-insensitive homoserine dehydrogenase produced by mild proteolysis were studied. The inhibition of homoserine dehydrogenase by kinase substrates is not due to binding of these inhibitors at the kinase active site but was shown to be due to binding to sites within the dehydrogenase domain of the enzyme. L-alpha-Aminobutyrate, a presumed threonine analogue, also inhibits the dehydrogenase by binding at the same or similar sites in the dehydrogenase domain and not at threonine regulatory site.

Binding studies of NADPH to NADP-specific L-glutamate dehydrogenase from Saccharomyces cerevisiae

Optical characteristics of enzyme-reduced coenzyme complexes of yeast NADP-specific glutamate dehydrogenase have been investigated in the presence and absence of product (L-glutamate) and in the presence or absence of phosphate. The phosphate effect, pointed out in a previous work, is found again: inorganic phosphate (Pi) destabilizes the binary complex (E - NADPH), the dissociation constant of which is equal to 14 muM, a value much higher than that determined in Tris-HCl buffer: Kd = 0.9 muM. Concerning the role of phosphate some assumptions are drawn up with respect to a similar behaviour of Pi toward yeast glutamate dehydrogenase and ADP toward the beef liver enzyme. In the same way, L-glutamate induces a stabilization of the binary complex; this latter effect is unchanged in the presence of phosphate, yet it is less marked than in the case of beef liver glutamate dehydrogenase. Protein fluorescence, nucleotide fluorescence and circular dichroism measurements allowed the determination of three identical and independent NADPH binding sites per hexameric active unit. In analogy with beef liver enzyme, it seems that yeast glutamate dehydrogenase is a good model to study anticooperativity in ligand binding.

Unspecific arginine kinase of molecular weight 150 000. Amino acid composition, subunit structure and number of substrate binding sites

The amino acid composition of unspecific arginine kinase of molecular weight 150 000 of Sabella pavonina muscle has been determined. If was found to be very similar to that of the phosphagen kinases previously studied. The subunit structure of the enzyme has been investigated by physical and chemical means. The data obtained from ultracentrifugation studies in 6 M guanidine hydrochloride and from molecular sieving and disc electrophoresis in 8 M urea, as well as the tryptic peptide mapping, suggest that Sabella muscle kinase is composed of four non-covalently linked polypeptide chains, with similar molecular weights. The number of binding sites for the nucleotide substrate ADP-Mg2+ has been estimated, using differential spectrophotometry and gel filtration on Sephadex columns. By both methods it was demonstrated that the enzyme contains two catalytic sites per protein molecule of molecular weight 150 000. Thus, arginine kinase from Sabella muscle, of molecular weight 150 000, consists of four similar polypeptide chains, but possesses only two substrate binding sites per tetrameric molecule.

The threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli K12. Specific inactivation of the homoserine dehydrogenase activity by the affinity label, 2-amino-4-oxo-5-chloropentanoic acid

2-Amino-4-oxo-5-chloropentanoic acid inactivates specifically the homoserine dehydrogenase activity of the bifunctional enzyme, aspartokinase I--homoserine dehydrogenase I. The aspartokinase activity remains essentially untouched and retains its threonine sensitivity. The inactivation of the dehydrogenase requires the covalent binding of one equivalent of the analogue per subunit. Alkylation does not affect the tetrameric state of the protein. The alkylating agent, a substrate analogue, meets the qualitative and quantitative requirements of an affinity label.