Ketol-acid reductoisomerase; Involved in the biosynthesis of branched-chain amino acids (BCAA). Catalyzes an alkyl-migration followed by a ketol-acid reduction of (S)-2-acetolactate (S2AL) to yield (R)-2,3-dihydroxy-isovalerate. In the isomerase reaction, S2AL is rearranged via a Mg-dependent methyl migration to produce 3-hydroxy-3-methyl-2-ketobutyrate (HMKB). In the reductase reaction, this 2-ketoacid undergoes a metal-dependent reduction by NADPH to yield (R)-2,3-dihydroxy-isovalerate.

Threonine dehydratase; Catalyzes the anaerobic formation of alpha-ketobutyrate and ammonia from threonine in a two-step reaction. The first step involved a dehydration of threonine and a production of enamine intermediates (aminocrotonate), which tautomerizes to its imine form (iminobutyrate). Both intermediates are unstable and short-lived. The second step is the nonenzymatic hydrolysis of the enamine/imine intermediates to form 2- ketobutyrate and free ammonia. In the low water environment of the cell, the second step is accelerated by RidA.

3-isopropylmalate dehydrogenase; Catalyzes the oxidation of 3-carboxy-2-hydroxy-4- methylpentanoate (3-isopropylmalate) to 3-carboxy-4-methyl-2- oxopentanoate. The product decarboxylates to 4-methyl-2 oxopentanoate. Belongs to the isocitrate and isopropylmalate dehydrogenases family. LeuB type 1 subfamily.

Acetolactate synthase, large subunit, biosynthetic type; Identified by match to protein family HMM PF00205; match to protein family HMM PF02775; match to protein family HMM PF02776; match to protein family HMM TIGR00118.

Formate acetyltransferase; Identified by match to protein family HMM PF01228; match to protein family HMM PF02901; match to protein family HMM TIGR01255.

Malate dehydrogenase (oxaloacetate-decarboxylating); Identified by similarity to SP:P16468; match to protein family HMM PF00390; match to protein family HMM PF01842; match to protein family HMM PF03949.

Pyruvate kinase; Identified by match to protein family HMM PF00224; match to protein family HMM PF00391; match to protein family HMM PF02887; match to protein family HMM TIGR01064; Belongs to the pyruvate kinase family.

2-isopropylmalate synthase/homocitrate synthase family protein; Identified by match to protein family HMM PF00682; match to protein family HMM TIGR00977; Belongs to the alpha-IPM synthase/homocitrate synthase family.

2-isopropylmalate synthase; Catalyzes the condensation of the acetyl group of acetyl-CoA with 3-methyl-2-oxobutanoate (2-oxoisovalerate) to form 3-carboxy-3- hydroxy-4-methylpentanoate (2-isopropylmalate); Belongs to the alpha-IPM synthase/homocitrate synthase family. LeuA type 1 subfamily.