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Citrate synthase (CS), citryl-CoA lyase (CCL), the C-terminal portion of the single-subunit type ATP-citrate lyase (ACL) and the C-terminal portion of the large subunit of the two-subunit type ACL. CS catalyzes the condensation of acetyl coenzyme A (AcCoA) and oxalacetate (OAA) from citrate and coenzyme A (CoA), the first step in the oxidative citric acid cycle (TCA or Krebs cycle). Peroxisomal CS is involved in the glyoxylate cycle. Some CS proteins function as a 2-methylcitrate synthase (2MCS). 2MCS catalyzes the condensation of propionyl-CoA (PrCoA) and OAA to form 2-methylcitrate and CoA during propionate metabolism. CCL cleaves citryl-CoA (CiCoA) to AcCoA and OAA. ACLs catalyze an ATP- and a CoA- dependant cleavage of citrate to form AcCoA and OAA; they do this in a multistep reaction, the final step of which is likely to involve the cleavage of CiCoA to generate AcCoA and OAA. The overall CS reaction is thought to proceed through three partial reactions and involves both closed and open conformational forms of the enzyme: a) the carbanion or equivalent is generated from AcCoA by base abstraction of a proton, b) the nucleophilic attack of this carbanion on OAA to generate CiCoA, and c) the hydrolysis of CiCoA to produce citrate and CoA. This group contains proteins which functions exclusively as either a CS or a 2MCS, as well as those with relaxed specificity which have dual functions as both a CS and a 2MCS. There are two types of CSs: type I CS and type II CSs. Type I CSs are found in eukarya, gram-positive bacteria, archaea, and in some gram-negative bacteria and are homodimers with both subunits participating in the active site. Type II CSs are unique to gram-negative bacteria and are homohexamers of identical subunits (approximated as a trimer of dimers). Some type II CSs are strongly and specifically inhibited by NADH through an allosteric mechanism. In fungi, yeast, plants, and animals ACL is cytosolic and generates AcCoA for lipogenesis. In several groups of autotrophic prokaryotes and archaea, ACL carries out the citrate-cleavage reaction of the reductive tricarboxylic acid (rTCA) cycle. In the family Aquificaceae this latter reaction in the rTCA cycle is carried out via a two enzyme system the second enzyme of which is CCL.
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