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). Involved in Leu biosynthesis, but do not participate in the chain elongation of glucosinolates. 3-methyl-2-oxobutanoate + acetyl-CoA + H2O = (2S)-2-isopropylmalate + CoA + H(+) Feedback inhibition by Leu. kcat is 2.3 sec(-1) with 2-oxoisovalerate as substrate (PubMed:17189332). kcat is 1.9 sec(-1) with acetyl-CoA as substrate (PubMed:17189332). Optimum pH is 8.5. Amino-acid biosynthesis; L-leucine biosynthesis; L-leucine from 3-methyl-2-oxobutanoate: step 1/4. Homotetramer. Expressed in roots, stems, leaves, flowers and siliques. Plants do not show any changes in soluble amino acid content. Belongs to the alpha-IPM synthase/homocitrate synthase family. LeuA type 1 subfamily.