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. Also able to use 2-ketopantoate, 2-ketoisovalerate, 2-ketovalerate, 2-ketobutyrate, 3-hydroxypyruvate, 3-hydroxy-2-ketobutyrate and pyruvate (PubMed:15654896). (2R)-2,3-dihydroxy-3-methylbutanoate + NADP(+) = (2S)-2-acetolactate + H(+) + NADPH (2R,3R)-2,3-dihydroxy-3-methylpentanoate + NADP(+) = (S)-2-ethyl-2-hydroxy-3-oxobutanoate + H(+) + NADPH Binds 2 magnesium ions per subunit. Inhibited by N-hydroxy-N-isopropyloxamate (IpOHA). kcat is 7.2 min(-1) for reductoisomerase activity with NADPH as substrate (PubMed:9015391). kcat is 3.1 min(-1) for reductoisomerase activity with NADPH as substrate (PubMed:9015391). kcat is 0.11 min(-1) for reductoisomerase activity with NADH as substrate (PubMed:9015391). kcat is 5.376 sec(-1) for reductoisomerase activity with 3-hydroxypyruvate as substrate (at pH 8 and 37 degrees Celsius) (PubMed:15654896). kcat is 3.6 sec(-1) for reductoisomerase activity with NADPH as substrate (PubMed:21515217). kcat is 3.511 sec(-1) for reductoisomerase activity with 3-hydroxy-3-methyl-2-ketobutyrate as substrate (at pH 8 and 37 degrees Celsius) (PubMed:15654896). kcat is 2.231 sec(-1) for reductoisomerase activity with 2-acetolactate as substrate (at pH 8 and 37 degrees Celsius) (PubMed:15654896). kcat is 0.594 sec(-1) for reductoisomerase activity with 3-hydroxy-2-ketobutyrate as substrate (at pH 8 and 37 degrees Celsius) (PubMed:15654896). kcat is 0.3 sec(-1) for reductoisomerase activity with NADH as substrate (PubMed:21515217). kcat is 0.194 sec(-1) for reductoisomerase activity with 2-ketopantoate as substrate (at pH 8 and 37 degrees Celsius) (PubMed:15654896). kcat is 0.182 sec(-1) for reductoisomerase activity with 2-ketoisovalerate as substrate (at pH 8 and 37 degrees Celsius) (PubMed:15654896). kcat is 0.167 sec(-1) for reductoisomerase activity with 2-ketobutyrate as substrate (at pH 8 and 37 degrees Celsius) (PubMed:15654896). kcat is 0.05 sec(-1) for reductoisomerase activity with 2-ketovalerate as substrate (at pH 8 and 37 degrees Celsius) (PubMed:15654896). kcat is 0.021 sec(-1) for reductoisomerase activity with pyruvate as substrate (at pH 8 and 37 degrees Celsius) (PubMed:15654896). Amino-acid biosynthesis; L-isoleucine biosynthesis; L-isoleucine from 2-oxobutanoate: step 2/4. Amino-acid biosynthesis; L-valine biosynthesis; L-valine from pyruvate: step 2/4. Homotetramer. In the presence of acetohydroxybutyrate and acetolactate and by the activator IlvY. Belongs to the ketol-acid reductoisomerase family.