Involved in the L-leucine, isovalerate and acyclic monoterpene catabolism. Catalyzes the cleavage of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to yield acetyl-CoA and acetoacetate. It can also catalyze the cleavage of 3-hydroxy-3-isohexenylglutaryl-CoA (HIHG_CoA) to yield 7-methyl-3-oxooct-6-enoyl-CoA and acetate. 3-hydroxy-3-(4-methylpent-3-en-1-yl)glutaryl-CoA = 7-methyl-3-oxooct-6-enoyl-CoA + acetate (3S)-hydroxy-3-methylglutaryl-CoA = acetoacetate + acetyl-CoA Divalent cations such as magnesium or manganese. Optimum pH is 7. Optimum temperature is 37 degrees Celsius. Metabolic intermediate metabolism; (S)-3-hydroxy-3-methylglutaryl-CoA degradation; acetoacetate from (S)-3-hydroxy-3-methylglutaryl-CoA: step 1/1. Homodimer. Cells lacking this gene are unable to utilize L-leucine, isovalerate or acyclic terpenes as carbon source. Belongs to the HMG-CoA lyase family.