A-acetohydroxybutyrate

Of the pyruvic acid formed during glycolysis, a proportion is used for biosynthetic reactions (see Fig. 17.8). Pyruvate is converted to a-acetolactate by the enzyme acetohydroxy acid synthetase. The substrates for this reaction are pyruvate and hydroxyethyl thiamine pyrophosphate. In a similar reaction involving hydroxyethyl TPP and a-oxobutyrate, a-acetohydroxybutyrate is formed (Fig. 17.13). Both acetohydroxy acids are excreted by yeast and are non-enzymically converted, in the medium, to vicinal diketones. 

Inhibitors of Acetolactate Synthase (ALS/AHAS) The enzyme acetolactate synthase (ALS) plays in plants an essential role in branched-chain amino acid biosynthesis. In the pathway leading to valine and leucine, ALS catalyzes the formation of 2-acetolactate from two pyruvate molecules, and in the pathway to isoleucine the formation of 2-acetohydroxybutyrate from 2-ketobutyrate and pyruvate. Due to this double function the enzyme is also called with a more general term aceto-hydroxyacid synthase. ALS is inhibited by several groups of herbicides, mainly the sulfonylureas (SUs), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinylthiobenzoates(PTBs) and sulfonylaminocarbonyltriazolinone (SCTs) (see Chapter 2.1, M. E. Thompson). 

The HEThDP intermediate reacts with either a second molecule of pyruvate or with 2-ketobutyrate to give acetolactate (AL) or acetohydroxybutyrate (AHB), respectively. In-depth studies of the kinetics of this reaction have been conducted on E. coli AHAS III [9]. Comparison of these results with earlier data for AHAS I and II indicate that all three bacterial isozymes catalyze similar mechanisms. However, AHAS II and III show a much more pronounced preference for reac-... 

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