In enzymology, a β-ketoacyl-[acyl-carrier-protein] synthase III () is an enzyme that catalyzes the chemical reaction acetyl-CoA + malonyl-[acyl carrier protein] acetoacetyl-[acyl carrier protein] + CoA + CO2 Thus, the two substrates of this enzyme are acetyl-CoA and malonyl-[acyl-carrier-protein], whereas its 3 products are acetoacetyl-[acyl-carrier-protein], CoA, and CO2. This enzyme belongs to the family of transferases, to be specific those acyltransferases transferring groups other than aminoacyl groups. This enzyme participates in fatty acid biosynthesis. β-Ketoacyl-acyl-carrier-protein synthase III is involved in the dissociated (or type II) fatty-acid biosynthesis system that occurs in plants and bacteria. The role of FabH in fatty acid synthesis has been described in Streptomyces glaucescens, Streptococcus pneumoniae, and Streptomyces coelicolor. Nomenclature The systematic name of this enzyme class is acetyl-CoA:malonyl-[acyl-carrier-protein] C-acyltransferase. Other names in common use include: Role in tuberculosis Mycobacterium tuberculosis, the cause of tuberculosis, evades effective immune clearance through encapsulation, especially with mycolic acids that are particularly resistant to the normal degradative processes of macrophages. Furthermore, this capsule inhibits entry of antibiotics. The enzymes involved in mycolate biosynthesis are essential for survival and pathogenesis, and thus represent excellent drug targets. In M. tuberculosis, the beta-ketoacyl-[acyl-carrier-protein] synthase III enzyme is designated mtFabH and is a crucial link between the fatty acid synthase-I and fatty acid synthase-II pathways producing mycolic acids. FAS-I is involved in the synthesis of C16 and C26 fatty acids. The C16 acyl-CoA product acts as a substrate for the synthesis of meromycolic acid by FAS-II, whereas the C26 fatty acid constitutes the alpha branch of the final mycolic acid. MtFabH has been proposed to be the link between FAS-I and FAS-II by converting C14-CoA generated by FAS-I to C16-AcpM, which is channelled into the FAS-II cycle. According to in silico flux balance analyses, mtFabH is essential but not according to transposon site hybridization analysis. Unlike the enzymes in FAS-I, the enzymes of FAS-II, including mtFabH, and are not found in mammals, suggesting inhibitors of these enzymes are suitable choices for drug development. Structure and substrates Crystal structures of FabH have been reported from Mycobacterium tuberculosis,{ Staphylococcus aureus, Escherichia coli, and Thermus thermophilus. The catalytic activity and substrate specificity of mtFabH has been measured then further probed using crystallographic and directed mutagenesis methods Structures have been determined of ecFabH bound with substrates, (CoA, malonyl CoA, degraded CoA). Specific inhibitors developed using rational design have recently been reported. In 2005, the structure of a catalytically disabled