Saccharomyces cerevisiae homocitrate synthase and related proteins, catalytic TIM barrel domainHomocitrate synthase (HCS) catalyzes the condensation of acetyl-CoA and alpha-ketoglutarate to form homocitrate, the first step in the lysine biosynthesis pathway. This family includes the Yarrowia lipolytica LYS1 protein as well as the Saccharomyces cerevisiae LYS20 and LYS21 proteins. 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".