 Substrate access to the active site of the HMGCL enzyme involves substrate binding across a cavity located at the C-terminal end of a beta barrel structure.In addition, the lysine 48 residue which is mutated in patients with 3-hydroxy-3-methylglutaryl-CoA lyase deficiency is also found to be necessary for substrate binding.The HMGCL protein plays an essential role in breaking down dietary proteins and fats for energy.