Biosynthesis of Valine and Isoleucine
نویسندگان
چکیده
Evidence published in recent years has shown that the biosynthesis of valine and isoleucine proceeds by a similar sequence of reactions (24). Fig. 1 depicts the reactions considered to be involved. In this report only Reactions II, III, and IV will be considered. Reaction II, the isomerization of the Lu-hydroxy$keto acid (ac-acetolactate for valine biosynthesis and a-aceto-a-hydroxybutyrate for isoleucine) was originally suggested by Meister (5). The reaction was included in the pathway proposed by Wagner et al. (4) inasmuch as their study showed that extracts of Neurospora crassa possess an enzyme that catalyzes the reduction of the a-keto-&hydroxy acids (proposed products of Reaction II) to the corresponding a! ,&dihydroxy acids (Reaction III). This enzyme was referred to as the a-keto-P-hydroxy acid reductase (1). Further evidence showed that in addition to the reductase there is also present in certain microorganisms an enzyme that catalyzes a single-step conversion of the oL-hydroxy-/3-keto acids to the corresponding a!,/?-dihydroxy acids (Reaction IV). This enzyme was referred to as the cr-hydroxy-B-keto acid reductoisomerase (1) .I Radhakrishnan et al. (l), in studies on N. crassa and Escherichia co&, succeeded in separating the reductase (Reaction III) from the reductoisomerase (Reaction IV). Strassman et al. (7) demonstrated a similar reductase and reductoisomerase in cell-free extracts of Saccharomyces cerevisiae. The possibility of a two-step reaction that is catalyzed by a single enzyme (the reductoisomerase) created an uncertainty as to whether the biosynthesis of valine and isoleucine occurs via Reactions II and III or via Reaction IV. This question has been thoroughly discussed (1,6). In connection with the above studies, Wagner and Bergquist (8) have demonstrated the presence of both the reductase and reductoisomerase in cell-free extracts of Salmonella typhimurium. Because of the interesting nature of cY-hydroxy-@-keto reductoisomerase, purification of this enzyme was attempted. The work presented in this report concerns the purification (lOO-fold) and properties of the reductoisomerase of wild type S. typhimurium. Evidence is also presented, from data on E. co& and X. typhimurium, which indicates a relationship between the reductase and the reductoisomerase.
منابع مشابه
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