(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. 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. Belongs to the ketol-acid reductoisomerase family.