3-Methyl-2-oxobutanoate dehydrogenase

This enzyme complex [EC 1.2.4.4], also known as 3-methyl-2-oxobutanoate dehydrogenase (lipoamide) and 2-oxoisovalerate dehydrogenase, catalyzes the reaction of 3-methyl-2-oxobutanoate with lipoamide to produce S-(2-methylpropanoyl)dihydrolipoamide and carbon dioxide. Thiamin pyrophosphate is a required cofactor. The complex also can utilize (5)-3-methyl-2-oxopenta-noate and 4-methyl-2-oxopentanoate as substrates. The complex contains branched-cham a-keto acid decarboxylase, dihydrolipoyl acyltransferase, and dihydrolipoa-mide dehydrogenase [EC 1.8.1.4]. 

ATP -I- [3-methyl-2-oxobutanoate dehydrogenase (lipoamide)] = ADP -I- [3-methyl-2-oxobutanoate dehydrogenase (lipoamide)] phosphate... 

Glucose-grown cells of G. candidum SC 5469 have also catalyzed the stereoselective reduction of ethyl-, isopropyl-, and tertiary-butyl esters of 4-chloro-3-oxobutanoic acid and methyl and ethyl esters of 4-bromo-3-oxobutanoic acid. A reaction yield of >85% and e.e. of >94% were obtained. NAD+-depen-dent oxido-reductase responsible for the stereoselective reduction of P-keto esters of 4-chloro- and 4-bromo-3-oxobutanoic acid was purified 100-fold. The molecular weight of purified enzyme is 950,000. The purified oxido-reductase was immobilized on Eupergit C and used to catalyze the reduction of (39) to S-( - )-(40). The cofactor NAD+ required for the reduction reaction was regenerated by glucose dehydrogenase. 

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