Allosteric inhibition of [formula omitted] 7-phosphate synthetase by tyrosine, tryptophan and phenylpyruvate in Pseudomonas aeruginosa

Abstract

1. 1. In Pseudomonas aerginosa the allosteric control of 3-deoxy- d -arabino- heptulosonate 7-phosphate (DAHP) synthetase, EC 4.1.2.15, by tyrosine has been known for some time. However, a more detailed analysis of the enzyme in crude extracts revealed sensitivity to inhibition by tryptophan and phenylpyruvate in addition to tyrosine. It is concluded that a single enzyme exists bearing three inhibitor specificities. Testing of various inhibitor combinations revealed no synergistic or antagonistic effects of aromatic metabolites. Combinations of the three inhibitors produced cummulative or less-than-cumulative effects. 2. 2. Tyrosine and phenylpyruvate were both capable of producing more than 90% inhibition at saturating concentrations of substrates, but tyrosine was about 18 times more potent than phenylpyruvate on a molar basis. although inhibition by tryptophan was limited to a maximum of 25–30%, relatively low concentrations were effective. It is concluded that inhibition by either tyrosine or tryptophan is competitive with respect to phosphoenolpyruvate and noncompetitive with respect to erythrose-4- P. On the other hand, phenylpyruvate inhibition appears to be competitive with respect to erythrose-4- P and noncompetitive with respect to phosphoenolpyruvate. 3. 3. Hence, one metabolite of each terminal branch of the aromatic amino acid pathway exhibits allosteric influence upon the activity of DAHP synthetase, an observation in accord with expectations for balanced participation of multiple end products in the regulation of a branched pathway. The quantitative effects actually realized in vivo would depend in complex fashion upon the intracellular concentrations of tyrosine, tryptophan and phenylpyruvate and upon the intracellular concentrations of erythrose-4- P and phosphoenolpyruvate. 4. 4. A variety of physiological experiments with wild type and mutant cells led to the conclusion that the synthesis of DAPH synthetase is not controlled by repression in P. aeruginosa.