Alternative Allosteric Effects Exerted by End Products upon a Two-Substrate Enzyme in Rhodomicrobium vannielii

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Abstract The regulatory enzyme, 3-deoxy-d-arabino-heptulosonate 7-phosphate synthetase of Rhodomicrobium vannielii was studied. Both phenylalanine and tyrosine are competitive inhibitors with respect to the substrate, erythrose 4-phosphate. Tryptophan does not influence enzyme activity per se, but potentiates the effect of phenylalanine or tyrosine. Kinetic experiments led to the conclusion that alternative kinetic pathways to the ternary complex existed, and that the one forming (phosphoenolpyruvate-enzyme) as the initial binary complex was kinetically preferred. Accordingly, the ratio of the two substrates had a marked influence upon reaction velocity. The observed ability of phenylalanine and tyrosine to either activate or inhibit enzyme activity, depending upon the ratio of the two substrates was consistent with the postulated mechanism. An extremely wide range of enzyme activity exists, on the one hand, because of the stimulation of activity by one or two amino acid end products, and on the other hand, because of the potent concerted inhibition in the simultaneous presence of all three amino acids. The physiological consequences of this pattern of control for 3-deoxy-d-arabino-heptulosonate 7-phosphate synthetase are discussed.