Ketol-acid reductoisomerase (EC ) is involved in the biosynthesis of the branched-chain amino acids. It is a bifunctional enzyme that cata-lyzes two quite different reactions at a common active site; an isomeriza-tion consisting of an alkyl migration, followed by an NADPH-dependent reduction of a 2-ketoacid. The 2-ketoacid formed by the alkyl migration is not released. Using the pure recombinant Escherichia colienzyme, we show that the isomerization reaction has a highly unfavourable equili-brium constant
