Physiological implications of the substrate specificities of acetohydroxy acid synthases from varied organisms

N. Gollop, B. Damri, D. M. Chipman, Z. Barak

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56 Scopus citations


Acetohydroxy acid synthase (AHAS; EC catalyzes the following two parallel, physiologically important reactions: condensation of two molecules of pyruvate to form acetolactate (AL), in the pathway to valine and leucine, and condensation of pyruvate plus 2-ketobutyrate to form acetohydroxybutyrate (AHB), in the pathway to isoleucine. We have determined the specificity ratio R with regard to these two reactions (where V(AHB) and V(AL) are rates of formation of the respective products) as follows: V(AHB)/V(AL) = R [2-ketobutyrate]/[pyruvate] for 14 enzymes from 10 procaryotic and eucaryotic organisms. Each organism considered has at least one AHAs of R > 20, and some appear to contain but a single biosynthetic AHAS. The implications of this for the design of the pathway are discussed. The selective pressure for high specificity for 2-ketobutyrate versus pyruvate implies that the 2-ketobutyrate concentration is much lower than the pyruvate concentration in all these organisms. It seems important for 2-ketobutyrate levels to be relatively low to avoid a variety of metabolic interferences. These results also reinforce the conclusion that biosynthetic AHAS isozymes of low R (1 to 2) are a special adaptation for heterotrophic growth on certain poor carbon sources. Two catabolic 'pH 6 AL-synthesizing enzymes' are shown to be highly specific for AL formation only (R < 0.1).

Original languageEnglish
Pages (from-to)3444-3449
Number of pages6
JournalJournal of Bacteriology
Issue number6
StatePublished - 1 Jan 1990

ASJC Scopus subject areas

  • Microbiology
  • Molecular Biology


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