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). 3-methyl-2-oxobutanoate + acetyl-CoA + H2O = (2S)-2-isopropylmalate + CoA + H(+) Inhibited by EDTA, mercuric salts and by leucine. Inhibition by leucine is non-competitive with respect to 3-methyl-2-oxobutanoate and competitive with respect to acetyl-CoA, and is more sensitive at pH 6.5 than pH 8.5. Optimum pH is 8.5. Amino-acid biosynthesis; L-leucine biosynthesis; L-leucine from 3-methyl-2-oxobutanoate: step 1/4. Homotetramer. Belongs to the alpha-IPM synthase/homocitrate synthase family. LeuA type 1 subfamily.