In enzymology, a malate synthase () is an enzyme that catalyzes the chemical reaction acetyl-CoA + H2O + glyoxylate (S)-malate + CoA The 3 substrates of this enzyme are acetyl-CoA, H2O, and glyoxylate, whereas its two products are (S)-malate and CoA. This enzyme participates in pyruvate metabolism and glyoxylate and dicarboxylate metabolism. Nomenclature This enzyme belongs to the family of transferases, specifically acyltransferases that convert acyl groups into alkyl groups on transfer. The systematic name of this enzyme class is acetyl-CoA:glyoxylate C-acetyltransferase (thioester-hydrolysing, carboxymethyl-forming). Other names in common use include L-malate glyoxylate-lyase (CoA-acetylating), glyoxylate transacetylase, glyoxylate transacetase, glyoxylic transacetase, malate condensing enzyme, malate synthetase, malic synthetase, and malic-condensing enzyme. Structure Malate synthases fall into two major families, isoforms A and G. Isoform G is monomeric with a size of about 80-kD and found exclusively in bacteria. Isoform A is about 65 kD per subunit and can form homomultimers in eukaryotes. This enzyme contains a central TIM barrel sandwiched between an N-terminal alpha-helical clasp and an alpha/beta domain stemming from two insertions into the TIM barrel sequence. The enzyme terminates with a C-terminal five-helix plug. The active site, where the acetyl-CoA and glyoxylate bind to the enzyme, lies between the TIM barrel and C-terminal plug. Upon binding, the acetyl-CoA molecule forms a J-shape inserted into the binding pocket, by an intramolecular hydrogen bond between N7 of the adenine ring and a hydroxyl group on the pantetheine tail. In addition, a critical magnesium ion within the active site coordinates with glyoxylate, glutamic acid 427, aspartic acid 455, and two water molecules. The amino acids interacting with acetyl CoA upon binding are highly conserved. Sequence identity is high within each class of isoforms, but between both classes sequence identity drops to about 15%. The alpha/beta domain, which has no apparent function, is not seen in isoform A. Mechanism The mechanism of malate synthase is an aldol reaction followed by thioester hydrolysis. Initially, aspartate 631 acts as a catalytic base, abstracting a proton from the alpha carbon of acetyl-CoA and creating an enolate that is stabilized by arginine 338. This is considered to be the rate-determining step of the mechanism. Then, the newly formed enolate acts as a nucleophile that attacks the aldehyde of glyoxylate, imparting a negative charge on the oxygen which is stabilized by arginine 338 and the coordinating magnesium cation. This malyl-CoA intermediate then undergoes hydrolysis at the acyl-CoA portion, generating