| Type: | Domain | Name: | Citrate synthase-like, small alpha subdomain |
| Description: | Citrate synthase is a member of a small family of enzymes that can directly form a carbon-carbon bond without the presence of metal ion cofactors. It catalyses the first reaction in the Krebs' cycle, namely the conversion of oxaloacetate and acetyl-coenzyme A into citrate and coenzyme A. This reaction is important for energy generation and for carbon assimilation. The reaction proceeds via a non-covalently bound citryl-coenzyme A intermediate in a 2-step process (aldol-Claisen condensation followed by the hydrolysis of citryl-CoA). Citrate synthase enzymes are found in two distinct structural types: type I enzymes (found in eukaryotes, Gram-positive bacteria and archaea) form homodimers and have shorter sequences than type II enzymes, which are found in Gram-negative bacteria and are hexameric in structure. In both types, the monomer is composed of two domains: a large alpha-helical domain consisting of two structural repeats, where the second repeat is interrupted by a small alpha-helical domain. The cleft between these domains forms the active site, where both citrate and acetyl-coenzyme A bind. The enzyme undergoes a conformational change upon binding of the oxaloacetate ligand, whereby the active site cleft closes over in order to form the acetyl-CoA binding site []. The energy required for domain closure comes from the interaction of the enzyme with the substrate. Type II enzymes possess an extra N-terminal beta-sheet domain, and some type II enzymes are allosterically inhibited by NADH [].This entry represents the small alpha-helical domain from type I and II citrate synthase enzymes, as well as a homolgous domain found in the related enzyme ATP citrate synthase. ATP citrate synthase () (also known as ATP citrate lyase) catalyses the MgATP-dependent, CoA-dependent cleavage of citrate into oxaloacetate and acetyl-CoA, a key step in the reductive tricarboxylic acid pathway of CO2 assimilation used by a variety of autotrophic bacteria and archaea to fix carbon dioxide []. ATP citrate synthase is composed of two distinct subunits. In eukaryotes, ATP citrate synthase is a homotetramer of a single large polypeptide, and is used to produce cytosolic acetyl-CoA from mitochondrial-produced citrate []. | Short Name: | Citrate_synth-like_sm_a-sub |
