Involved in the catabolism of branched amino acids such as leucine. (3S)-hydroxy-3-methylglutaryl-CoA = acetoacetate + acetyl-CoA Metabolic intermediate metabolism; (S)-3-hydroxy-3-methylglutaryl-CoA degradation; acetoacetate from (S)-3-hydroxy-3-methylglutaryl-CoA: step 1/1. Homodimer and homotetramer. Belongs to the HMG-CoA lyase family.