Citrate synthases catalyze the first reaction in the citric acid cycle (the Krebs' cycle), namely the condensation of acetyl-coenzyme A with oxaloacetate to form citrate and coenzyme A. This reaction is important for energy generation and for carbon assimilation. The enzymes are found in two distinct structural types: type I enzymes (found in eukaryotes, Gram-positive bacteria and Archaea) forming homodimers and type II enzymes, which are found in Gram-negative bacteria and are hexameric in structure. The enzyme monomer is composed of two alpha-helical domains. The cleft between these domains forms the active site. Type II enzymes possess an extra N-terminal beta-sheet domain.