Reaction: acetyl-CoA + a malonyl-[acyl-carrier protein] = an acetoacetyl-[acyl-carrier protein] + CoA + CO2
Other name(s): 3-oxoacyl:ACP synthase III; 3-ketoacyl-acyl carrier protein synthase III; KASIII; KAS III; FabH; β-ketoacyl-acyl carrier protein synthase III; β-ketoacyl-ACP synthase III; β-ketoacyl (acyl carrier protein) synthase III
Systematic name: acetyl-CoA:malonyl-[acyl-carrier protein] C-acyltransferase
Comments: Involved in the dissociated (or type II) fatty-acid biosynthesis system that occurs in plants and bacteria. In contrast to EC 22.214.171.124 (β-ketoacyl-ACP synthase I) and EC 126.96.36.199 (β-ketoacyl-ACP synthase II), this enzyme specifically uses CoA thioesters rather than acyl-ACP as the primer . In addition to the above reaction, the enzyme can also catalyse the reaction of EC 188.8.131.52, [acyl-carrier-protein] S-acetyltransferase, but to a much lesser extent . The enzyme is responsible for initiating both straight- and branched-chain fatty-acid biosynthesis , with the substrate specificity in an organism reflecting the fatty-acid composition found in that organism [2,5]. For example, Streptococcus pneumoniae, a Gram-positive bacterium, is able to use both straight- and branched-chain (C4C6) acyl-CoA primers  whereas Escherichia coli, a Gram-negative organism, uses primarily short straight-chain acyl CoAs, with a preference for acetyl-CoA [4,5].
Links to other databases: BRENDA, EXPASY, KEGG, Metacyc, PDB, CAS registry number: 1048646-78-1
1. Tsay, J.T., Oh, W., Larson, T.J., Jackowski, S. and Rock, C.O. Isolation and characterization of the β-ketoacyl-acyl carrier protein synthase III gene (fabH) from Escherichia coli K-12. J. Biol. Chem. 267 (1992) 6807-6814. [PMID: 1551888]
2. Han, L., Lobo, S. and Reynolds, K.A. Characterization of β-ketoacyl-acyl carrier protein synthase III from Streptomyces glaucescens and its role in initiation of fatty acid biosynthesis. J. Bacteriol. 180 (1998) 4481-4486. [PMID: 9721286]
3. Khandekar, S.S., Gentry, D.R., Van Aller, G.S., Warren, P., Xiang, H., Silverman, C., Doyle, M.L., Chambers, P.A., Konstantinidis, A.K., Brandt, M., Daines, R.A. and Lonsdale, J.T. Identification, substrate specificity, and inhibition of the Streptococcus pneumoniae β-ketoacyl-acyl carrier protein synthase III (FabH). J. Biol. Chem. 276 (2001) 30024-30030. [PMID: 11375394]
4. Choi, K.H., Kremer, L., Besra, G.S. and Rock, C.O. Identification and substrate specificity of β-ketoacyl (acyl carrier protein) synthase III (mtFabH) from Mycobacterium tuberculosis. J. Biol. Chem. 275 (2000) 28201-28207. [PMID: 10840036]
5. Qiu, X., Choudhry, A.E., Janson, C.A., Grooms, M., Daines, R.A., Lonsdale, J.T. and Khandekar, S.S. Crystal structure and substrate specificity of the β-ketoacyl-acyl carrier protein synthase III (FabH) from Staphylococcus aureus. Protein Sci. 14 (2005) 2087-2094. [PMID: 15987898]
6. Li, Y., Florova, G. and Reynolds, K.A. Alteration of the fatty acid profile of Streptomyces coelicolor by replacement of the initiation enzyme 3-ketoacyl acyl carrier protein synthase III (FabH). J. Bacteriol. 187 (2005) 3795-3799. [PMID: 15901703]
7. Cronan, J.E., Jr. and Rock, C.O. Biosynthesis of membrane lipids. In: Neidhardt, F.C. (Ed.), Escherichia coli and Salmonella: Cellular and Molecular Biology, 2nd edn, vol. 1, ASM Press, Washington, DC, 1996, pp. 612-636.