InterPro domain: IPR024176

Citrate synthase 2.3.3.1 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 [ 1 ]. 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 [ 2 ].

This entry represents types I and II citrate synthase enzymes, as well as the related enzymes 2-methylcitrate synthase. 2-methylcitrate ( 2.3.3.5 ) synthase catalyses the conversion of oxaloacetate and propanoyl-CoA into (2R,3S)-2-hydroxybutane-1,2,3-tricarboxylate and coenzyme A (CoA). This enzyme is induced during bacterial growth on propionate, while type II hexameric citrate synthase is constitutive [ 3 ]. This entry contains citrate synthases from bacteria and some eukaryotic (peroxisomal), and also an enzyme from Bacillus subtilis (citrate/2-methylcitrate synthase) that acts as both a citrate synthase and a methylcitrate synthase [ 4 , 5 ].

1. Investigating the accessibility of the closed domain conformation of citrate synthase using essential dynamics sampling. J. Mol. Biol. 339, 515-25
2. Structure of a NADH-insensitive hexameric citrate synthase that resists acid inactivation. Biochemistry 45, 13487-99
3. Citrate synthase and 2-methylcitrate synthase: structural, functional and evolutionary relationships. Microbiology (Reading, Engl.) 144 ( Pt 4), 929-35
4. A sigma E dependent operon subject to catabolite repression during sporulation in Bacillus subtilis. J. Bacteriol. 178, 4778-86
5. First Biochemical Characterization of a Methylcitric Acid Cycle from Bacillus subtilis Strain 168. Biochemistry 56, 5698-5711