Escherichia coli (Ec) citrate synthase (CS) gltA and Arabidopsis thaliana (Ath) peroxisomal (Per) CS_like. CS catalyzes the condensation of acetyl coenzyme A (AcCoA) and oxalacetate (OAA) to form citrate and coenzyme A (CoA), the first step in the citric acid cycle (TCA or Krebs cycle). 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 citryl-CoA, and c) the hydrolysis of citryl-CoA to produce citrate and CoA. 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, including EcCS, are strongly and specifically inhibited by NADH through an allosteric mechanism. Included in this group is an NADH-insensitive type II Acetobacter acetii CS which has retained many of the residues used by EcCS for NADH binding. C. aurantiacus is a gram-negative thermophilic green gliding bacterium; its CS belonging to this group may be a type I CS. It is not inhibited by NADH or 2-oxoglutarate and is inhibited by ATP. Both gram-positive and gram-negative bacteria are found in this group. This group also contains three Arabidopsis peroxisomal CS proteins, CYS-1, -2, and -3 which participate in the glyoxylate cycle. AthCYS1, in addition to a peroxisomal targeting sequence, has a predicted secretory signal peptide; it may be targeted to both the secretory pathway and the peroxisomes and perhaps is located in the extracellular matrix. AthCSY1 is expressed only in siliques and specifically in developing seeds. AthCSY2 and 3 are active during seed germination and seedling development and are thought to participate in the beta-oxidation of fatty acids.