InterPro domain: IPR036969

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 and ATP citrate 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. This enzyme is induced during bacterial and fungal growth on propionate [ 3 , 4 ], while type II hexameric citrate synthase is constitutive [ 5 ]. ATP citrate synthase ( 2.3.3.8 ) (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 [ 6 ]. 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 [ 7 ]. This entry includes citrate synthase from Thermosulfidibacter takaii, which catalyses both citrate generation and citrate cleavage as it is part of a reversible tricarboxylic acid (TCA) cycle that can fix carbon dioxide autotrophically and may represent an ancestral mode of the conventional reductive TCA (rTCA) cycle [ 8 ].

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. Methylcitrate synthase from Aspergillus fumigatus is essential for manifestation of invasive aspergillosis. Cell. Microbiol. 10, 134-48
4. First Biochemical Characterization of a Methylcitric Acid Cycle from Bacillus subtilis Strain 168. Biochemistry 56, 5698-5711
5. Citrate synthase and 2-methylcitrate synthase: structural, functional and evolutionary relationships. Microbiology (Reading, Engl.) 144 ( Pt 4), 929-35
6. Both subunits of ATP-citrate lyase from Chlorobium tepidum contribute to catalytic activity. J. Bacteriol. 188, 6544-52
7. ATP citrate lyase is an important component of cell growth and transformation. Oncogene 24, 6314-22
8. A primordial and reversible TCA cycle in a facultatively chemolithoautotrophic thermophile. Science 359, 559-563