The effect of changes in the substituent at the 5'-position of pyridoxal 5'-phosphate on the coenzymatic activity of six analogues of this coenzyme was determined for three bacterial enzymes. Pyridoxal 5'-sulfate showed substantial coenzymatic activity for arginine decarboxylase and tryptophanase, but not for D-serine dehydratase; alpha(5)-pyridoxalmethylphosphonate showed substantial activity for D-serine dehydratase, but not for the other two enzymes. These results demonstrate that neither the dianionic phosphate group nor the ester oxygen of pyridoxal 5'-phosphate is a general requirement for coenzymatic activity. Minor variations in orientation and charge of the group at position 5 exert major effects on the capacity for complex formation between analogue and apoenzyme, and on the catalytic efficiency of the resulting complex, but have comparatively little effect on the substrate affinity of the analogue-apoenzyme complexes. These effects show no general pattern from enzyme to enzyme, and do not correlate with the affinity of analogue for apoenzyme under the conditions tested.

Apoproteins↗

PubMed1972

Reversibility of the tryptophanase reaction: synthesis of tryptophan from indole, pyruvate, and ammonia.

Degradation of tryptophan to indole, pyruvate, and ammonia by tryptophanase (EC 4....) from Escherichia coli, previously thought to be an irreversible reaction, is readily reversible at high concentrations of pyruvate and ammonia. Tryptophan and certain of its analogues, e.g., 5-hydroxytryptophan, can be synthesized by this reaction from pyruvate, ammonia, and indole or an appropriate derivative at maximum velocities approaching those of the degradative reactions. Concentrations of ammonia required for the synthetic reactions produce specific changes in the spectrum of tryptophanase that differ from those produced by K(+) and indicate that ammonia interacts with bound pyridoxal 5'-phosphate to form an imine. Kinetic results indicate that pyruvate is the second substrate bound, hence indole must be the third. These results favor a modified mechanism for the multitude of tryptophanase-catalyzed reactions in which alpha-aminoacrylate, which functions as a common enzyme-bound intermediate in both synthetic and degradative reactions, is not released into the medium during the latter reactions, but is degraded to pyruvate and ammonia by sequential reversible steps via enzyme-bound intermediates.

Acrylates↗

PubMed1972

Histidine decarboxylaseless mutants of Lactobacillus 30a: isolation and growth properties.

Mutants of Lactobacillus 30a deficient in their ability to form an inducible histidine decarboxylase (EC 4.1.1.22) were selected by plating nitrosoguanidine-treated cultures on a medium containing histidine and methyl red. Wild-type organisms produce histamine, thus raising the pH and forming yellow colonies; mutant colonies remain red. In the presence of added histidine, decarboxylase-producing cultures grow more heavily than mutant cultures when the initial pH of the growth medium is low or when the lactic acid produced lowers the pH to growth-limiting values. Addition of the decarboxylation products, histamine and carbon dioxide, did not favor growth in crude medium.

Azo Compounds↗

PubMed1972