Catalyzes the isomerization between 2-isopropylmalate and 3-isopropylmalate, via the formation of 2-isopropylmaleate. (2R,3S)-3-isopropylmalate = (2S)-2-isopropylmalate Amino-acid biosynthesis; L-leucine biosynthesis; L-leucine from 3-methyl-2-oxobutanoate: step 2/4. Heterodimer of LeuC and LeuD. Belongs to the LeuD family. LeuD type 1 subfamily.