Acetolactate Reductoisomerase

Figure 7.1. Overview of the biochemical routes leading to glycerol, pyruvate, and ethanol. Furthermore, valine biosynthesis and diacetyl formation are shown, which may be bypassed by introduction of a heterologous a-acetolactate decarboxylase that directly converts a-acetolactate to acetoin. GPDl and GPD2, glycerol dehydrogenases 1 and 2 ADHl, alcohol dehydrogenase 1 ILV2, acetolactate synthetase ID/5, acetolactate reductoisomerase [Refs in 502].

In the biosynthesis of valine 179 the first step is a benzoin condensation of 2 mol of pyruvate 95 to yield a-acetolactate 217. A single enzyme, a-acetohydroxyacid reductoisomerase (EC 1.1.1.86), then catalyzes the re-arrangement/reduction steps 217->218- 219 (Scheme 61). The same enzyme catalyzes a similar sequence implied in 222->223 in the biosynthesis of isoleucine 212. The substrates 217 and 222 for these reactions have been shown to have the (2S) configuration (189,190), the product 223 to have the (2R,3R) configuration (191-194), and the product 219 to have (IR) configuration (192,193). 

The presence of the second enzyme in the pathway in higher plants can be inferred from results obtained by Saytanrayana and Radhakrishnan (1965). NADPH-dependent conversion of the acetohydroxyacids to the oxo-analogues of isoleucine and valine indicated that partially purified extracts of Phaseolis radiatus contained the enzymes required to catalyze both the second and third reactions illustrated in Fig. 4. Reductoisomerase activity would require NADPH for synthesis of the dihydroxyacids which would, in turn, be dehydrated for synthesis of the oxoacids. The rates of NADPH oxidation differed when 2-acetolactate and 2-acetohydroxybutyrate were tested as alternate substrates, but it was not established whether more than one reductoisomerase was present in the preparations. 

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