3-hydroxy-3-methylglutaryl-CoA lyase, catalytic TIM barrel domain
3-hydroxy-3-methylglutaryl-CoA lyase (HMGL) catalyzes the cleavage of HMG-CoA to acetyl-CoA and acetoacetate, one of the terminal steps in ketone body generation and leucine degradation, and is a key enzyme in the pathway that supplies metabolic fuel to extrahepatic tissues. Mutations in HMGL cause a human autosomal recessive disorder called primary metabolic aciduria that affects ketogenesis and leucine catabolism and can be fatal due to an inability to tolerate hypoglycemia. HMGL has a TIM barrel domain with a catalytic center containing a divalent cation-binding site formed by a cluster of invariant residues that cap the core of the barrel. The cleavage of HMG-CoA requires the presence of a divalent cation like Mg2+ or Mn2+, and the reaction is thought to involve general acid/base catalysis. This family belongs to the DRE-TIM metallolyase superfamily. DRE-TIM metallolyases include 2-isopropylmalate synthase (IPMS), alpha-isopropylmalate synthase (LeuA), 3-hydroxy-3-methylglutaryl-CoA lyase, homocitrate synthase, citramalate synthase, 4-hydroxy-2-oxovalerate aldolase, re-citrate synthase, transcarboxylase 5S, pyruvate carboxylase, AksA, and FrbC. These members all share a conserved triose-phosphate isomerase (TIM) barrel domain consisting of a core beta(8)-alpha(8) motif with the eight parallel beta strands forming an enclosed barrel surrounded by eight alpha helices. The domain has a catalytic center containing a divalent cation-binding site formed by a cluster of invariant residues that cap the core of the barrel. In addition, the catalytic site includes three invariant residues - an aspartate (D), an arginine (R), and a glutamate (E) - which is the basis for the domain name "DRE-TIM".