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P Truffa-Bachi

Publications and source records attributed to P Truffa-Bachi.

12 recordsLinked to original sources
PubMed1978

The threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli K12. Carboxymethylation of the enzyme: threonine binding and inhibition are functionally dissociable.

The inactivation of the aspartokinase I-homoserine dehydrogenase I by iodoacetic acid and the effect on the sensitivity to its inhibitor, L-threonine, were examined. Both aspartokinase and homoserine dehydrogenase inactivation, as well as the dehydrogenase desensitization toward L-threonine occur as a pseudo-first order process. During its inactivation, the aspartokinase remains sensitive to L-threonine. At 50% inactivation, the inhibition curve of the aspartokinase by L-threonine displays homotropic cooperative effects. This alkylated protein retains eight binding sites for L-threonine. During the carboxymethylation, the protein remains in the tetrameric form until half of the kinase activity is lost. At the end of the inactivation aggregate forms and dimers appear.

Alkylation↗
PubMed1977

Threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli K12. Kinetic and spectroscopic effects upon binding of serine and threonine.

The two threonine-sensitive activities aspartokinase and homoserine dehydrogenase are inhibited by L-serine. The inhibition of the aspartokinase by L-serine displays homotropic cooperative effects and is competitive versus aspartate. The inhibition by L-serine of the homoserine dehydrogenase displays Michaelis-Menten kinetics which are of a competitive nature versus homoserine. Characteristic effects of L-serine on the protein include a perturbation of its absorption and fluorescence spectra, with an increase in the fluorescence of the protein-NADPH complex. L-serine shifts the allosteric equilibrium of the protein to a "T-like" conformation to which L-threonine binds noncooperatively. L-Serine, a threonine analog, is not capable, as the physiological effector, of inducing a complete R to T transition of the enzyme; the aspartokinase globules show a cooperative conformation change upon serine binding, but this conformation change is not found in the homoserine dehydrogenase globules.

Aspartokinase Homoserine Dehydrogenase↗
PubMed1976

The threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli K-12. Incubation of the enzyme in alkaline conditions: dissociation and disulfide-bridge formation.

Aspartokinase I - homoserine dehydrogenase I from Escherichia coli K-12, a homotetrameric enzyme, dissociates into dimers upon alkaline treatment. Both aspartokinase and homoserine dehydrogenase inactivation, as well as desensitazion towards L-threonine, occur in a multi-step process. Dithiothreitol stabilizes a dimeric form retaining full activity and sensitivity; L-homoserine stabilizing another dimeric form devoid of aspartokinase activity and retaining a substantial dehydrogenase activity insensitive toward L-threonine. A model is proposed showing that dissociation into dimers occurs in a first step, the resulting dimer losing both aspartokinase and homoserine dehydrogenase sensitivity in two subsequent steps involving the formation of intrachain disulfide bonds.

Alcohol Oxidoreductases↗
PubMed1976

Homoserine kinase from Escherichia coli K12.

Homoserine kinase was purified to apparent homogeneity from a derepressed strain of Escherichia coli K12, using standard fractionation techniques. It is a dimer (Mr = 60000) composed of apparently identical polypeptide chains (Mr = 29000). Its amino acid composition and N-terminal sequence have been determined. L-Threonine is a competitive inhibitor of the substrate L-homoserine; this inhibition is straighforward and shows no sign of co-operativity. Evidence is presented that homoserine and threonine bind to the same site of this non-allosteric enzyme. The binding of homoserine and threonine can also be studied by difference spectroscopy; the latter studies reveal an unexpected effect of magnesium ions, which might be the basis for the unusual high Mg2+ requirement for optimal enzyme reaction.

Amino Acid Sequence↗
PubMed1976

Immunological cross reactivity of four enzymes involved in the biosynthetic pathway of lysine, methionine and threonine in Escherichia coli K12.

In Escherichia coli K12 the biosynthetic pathway of lysine, methionine and threonine is characterized by three isofunctional aspartokinases and two homoserine dehydrogenases. A single polypeptide chain carries the threonine-sensitive aspartokinase and homoserine dehydrogenase (AK I-HDH I), and a different polypeptide chain carries the methionine-repressible aspartokinase and homoserine dehydrogenase (AK II-HDH II). Immuno-adsorbants prepared with rabbit antibodies against AK I-HDH I bind the lysine-sensitive aspartokinase (AK III), the AK II-HDH II, and the homoserine kinase (HSK), an enzyme of the threonine biosynthetic pathway. Saturation of the immunoadsorbant with AK I-HDH I results in a decreased binding capacity for the other enzymes. Displacement of bound AK III or HSK can be obtained with pure AK I-HDH I, showing that the affinity of the antibodies to homologous antigens is higher than to heterologous ones. Immunoadsorbants prepared with anti-HSK antibodies show the same type of recognition: binding of the three aspartkinases and a capacity to displace the heterologous antigens bound. Accordingly, the same antibodies, implicated in the binding of the homologous antigen, bind the other enzymes. None of the other enzymes of the pathway, or the other kinases tested are recognized by the two immunoadsorbants. It can be postulated that in E. coli K12, duplication of a common ancestor gene gave rise to the three aspartokinases and to the homoserine kinase; two of the genes coding for the aspartokinases fused with those coding for the homoserine dehydrogenases. Indicating that only few epitopes are shared by these enzymes, by conventional immuno-diffusion techniques no precipitation lines appeared with antibodies against AK I-HDH I and the other proteins.

Alcohol Oxidoreductases↗
PubMed1975

The threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli K12. Distribution and accessibility to antibodies of some epitopes of the bifunctional enzyme.

In the presence of l-threonine, the allosteric effector, most of the antigenic determinants situated in the aspartokinase region of the wild-type enzyme become unavailable to the antibodies raised against a fragment of the enzyme containing this region and devoid of homoserine dehydrogenase activity. The cross-reactivities of the antibodies raised against this fragment (extracted from a nonsense mutant) and a fragment endowed with homoserine dehydrogenase activity but devoid of aspartokinase activity (obtained by limited proteolysis) with the corresponding antigens were studied. The conclusion is drawn that the two fragments, which share an overlapping sequence of molecular weight about 17,000, share at least two antigenic determinants.

Alcohol Oxidoreductases↗
PubMed1975

Evolution of biosynthetic pathways: immunological approach.

Through the use of specific immunoadsorbent columns, it is shown that Escherichia coli aspartokinase I-homoserine dehydrogenase I, aspartokinase II-homoserine dehydrogenase II, aspartokinase III, and homoserine kinase, enzymes involved in the same complex biosynthetic pathway, share antigenic determinants. This raises the question of a common origin for the four cibtenoirart kinases. (Aspartate kinase or ATP:L aspartate 4-phosphotransferase, EC 2.7.2.4; homoserine dehydrogenase or Lhomoserine:NADP oxidoreductase, EC 1.1.1.3; homoserine kinase or ATP:L-homoserine O-phosphotransferase, EC 2.7.1.39.)

Alcohol Oxidoreductases↗