Catalyzes the isomerization between 2-isopropylmalate and 3-isopropylmalate, via the formation of 2-isopropylmaleate (PubMed:19150812, PubMed:20663849). Functions in both the biosynthesis of leucine and in the methionine chain elongation pathway of aliphatic glucosinolate formation (PubMed:19597944). (2R,3S)-3-isopropylmalate = (2S)-2-isopropylmalate a 2-(omega-methylsulfanyl)alkylmalate = a 2-(omega-methylsulfanyl)alkylmaleate + H2O 2-(3-methylsulfanyl)propylmalate = 2-(2-methylsulfanyl)propylmaleate + H2O a 3-(omega-methylsulfanyl)alkylmalate = a 2-(omega-methylsulfanyl)alkylmaleate + H2O 2-(2-methylsulfanyl)ethylmalate = 2-(2-methylsulfanyl)ethylmaleate + H2O 3-(2-methylsulfanyl)ethylmalate = 2-(2-methylsulfanyl)ethylmaleate + H2O 3-(3-methylsulfanyl)propylmalate = 2-(2-methylsulfanyl)propylmaleate + H2O Binds 1 [4Fe-4S] cluster per subunit. Amino-acid biosynthesis; L-leucine biosynthesis; L-leucine from 3-methyl-2-oxobutanoate: step 2/4. Heterodimer of the large LEUC/IIL1 subunit and the small LEUD (SSU1, SSU2 or SSU3) subunits. Expressed in roots, leaves, stems and flowers. Expressed at low levels in siliques. The environment dependent reduction in IIL1 activity and severely impaired growth of cv. Bur-0 are caused by a triplet repeat expansion in the third intron of the gene (PubMed:19150812). Belongs to the aconitase/IPM isomerase family.