Regulation of Chorismate mutase-prephenate dehydratase and prephenate dehydrogenase from alcaligenes eutrophus.

摘要

Highly purified enzymes from Alcaligenes eutrophus H 16 were used for kinetic studies. Chorismate mutase was feedback inhibited by phenylalanine. In the absence of the inhibitor, the double-reciprocal plot was linear, yielding a Km for chorismate of 0.2 mM. When phenylalanine was present, a pronounced deviation from the Michaelis-Menten hyperbola occurred. The Hill coefficient (n) was 1.7, and Hill plots of velocity versus inhibitor concentrations resulted in a value of n' = 2.3, indicating positive cooperativity. Chorismate mutase was also inhibited by prephenate, which caused downward double-reciprocal plots and a Hill coefficient of n = 0.7, evidence for negative cooperativity. The pH optimum of chorismate mutase ranged from 7.8 to 8.2; its temperature optimum was 47 C. Prephenate dehydratase was competitively inhibited by phenylalanine and activated by tyrosine. Tyrosine stimulated its activity up to 10-fold and decreased the Km for prephenate, which was 0.67 mM without effectors. Tryptophan inhibited the enzyme competitively. Its inhibition constant (Ki = 23 muM) was almost 10-fold higher than that determined for phenylalanine (Ki = 2.6 muM). The pH optimum of prephenate dehydratase was pH 5.7; the temperature optimum was 48 C. Prephenate dehydrogenase was feedback inhibited by tyrosine. Inhibition was competitive with prephenate (Ki = 0.06 mM) and noncompetitive with nicotinamide adenine dinucleotide. The enzyme was further subject to product inhibition by p-hydroxyphenylpyruvate (Ki = 0.13 mM). Its Km for prephenate was 0.045 mM, and that for nicotinamide adenine dinucleotide was 0.14 mM. The pH optimum ranged between 7.0 and 7.6; the temperature optimum was 38 C. It is shown how the sensitive regulation of the entire enzyme system leads to a well-balanced amino acid production.