The two 3-deoxy-d-arabino-heptulosonate-7-phosphate synthase isoenzymes from Saccharomyces cerevisiae show different kinetic modes of inhibition

Abstract

Activity of the tyrosine-inhibitable 3-deoxy-d-arabino-heptulosonate-7-phosphate synthase (EC 4.1.2.15) from Saccharomyces cerevisiae that was encoded by the ARO4 gene cloned on a high-copy-number plasmid was enhanced 64-fold as compared to the wild-type. The enzyme was purified to apparent homogeneity from the strain that harbored this recombinant plasmid. The estimated molecular weight of 42,000 of the enzyme corresponded to the calculated molecular mass of 40 kDa deduced from the DNA sequence. The enzyme could be inactivated by EDTA in a reaction that was reversed by several bivalent metal ions; presumably a metal cofactor is required for enzymatic catalysis. The Michaelis constant of the enzyme was 125 μM for phosphoenolpyruvate and 500 μM for erythrose 4-phosphate. The rate constant was calculated as 6 s^–1, and kinetic data indicated a sequential mechanism of the enzymatic reaction. Tyrosine was a competitive inhibitor with phosphoenolpyruvate as substrate of the enzyme (K _i of 0.9 μM) and a noncompetitive inhibitor with erythrose 4-phosphate as substrate. This is in contrast to the ARO3-encoded isoenzyme, where phenylalanine is a competitive inhibitor with erythrose 4-phosphate as a substrate of the enzyme and a noncompetitive inhibitor with phosphoenolpyruvate as substrate.