This family of enzymes participates in a process that introduces a methyl branch into nascent polyketide products. The process begins with EC 4.1.1.124, malonyl-[acp] decarboxylase, which converts the common extender unit malonyl-[acp] to acetyl-[acp]. The enzyme is a mutated form of a ketosynthase enzyme, in which a Cys residue in the active site is modified to a Ser residue, leaving the decarboxylase function intact, but nulifying the ability of the enzyme to form a carbon-carbon bond. Next, EC 2.3.3.22, 3-carboxymethyl-3-hydroxy-acyl-[acp] synthase, utilizes the acetyl group to introduce the branch at the beta position of 3-oxoacyl intermediates attached to a polyketide synthase, forming a 3-hydroxy-3-carboxymethyl intermediate. This is followed by dehydration catalysed by EC 4.2.1.181, 3-carboxymethyl-3-hydroxy-acyl-[acp] dehydratase (often referred to as an ECH1 domain), leaving a 3-carboxymethyl group and forming a double bond between the alpha and beta carbons. The process concludes with decarboxylation catalysed by EC 4.1.1.125, 4-carboxy-3-alkylbut-2-enoyl-[acp] decarboxylase (often referred to as an ECH2 domain), leaving a methyl branch at the beta carbon. The enzymes are usually encoded by a cluster of genes referred to as an "HMGS cassette", based on the similarity of the key enzyme to EC 2.3.3.10, hydroxymethylglutaryl-CoA synthase. While the enzyme is similar to EC 2.3.3.10, it is specific for an [acyl-carrier protein] (acp)-bound donor and does not interact with a CoA substrate as donor.
