Reaction: (1) 3-methylbutanoyl-CoA + a malonyl-[acyl-carrier protein] = a 5-methyl-3-oxohexanoyl-[acyl-carrier-protein] + CoA + CO2
(2) 2-methylpropanoyl-CoA + a malonyl-[acyl-carrier protein] = a 4-methyl-3-oxopentanoyl-[acyl-carrier-protein] + CoA + CO2
(3) (2S)-2-methylbutanoyl-CoA + a malonyl-[acyl-carrier protein] = a (4S)-4-methyl-3-oxohexanoyl-[acyl-carrier-protein] + CoA + CO2
Glossary: 3-methylbutanoyl-CoA = isovaleryl-CoA
2-methylpropanoyl-CoA = isobutanoyl-CoA = isobutyryl-CoA
Systematic name: 3-methylbutanoyl-CoA:malonyl-[acyl-carrier protein] C-acyltransferase
Comments: The enzyme is responsible for initiating branched-chain fatty acid biosynthesis by the dissociated (or type II) fatty-acid biosynthesis system (FAS-II) in some bacteria, using molecules derived from degradation of the branched-chain amino acids L-leucine, L-valine, and L-isoleucine to form the starting molecules for elongation by the FAS-II system. In some organisms the enzyme is also able to use acetyl-CoA, leading to production of a mix of branched-chain and straight-chain fatty acids [3] (cf. EC 2.3.1.180, β-ketoacyl-[acyl-carrier-protein] synthase III).
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, PDB, CAS registry number:
References:
1. 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]
2. Choi, K.H., Heath, R.J. and Rock, C.O. β-ketoacyl-acyl carrier protein synthase III (FabH) is a determining factor in branched-chain fatty acid biosynthesis. J. Bacteriol. 182 (2000) 365-370. [PMID: 10629181]
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. Singh, A.K., Zhang, Y.M., Zhu, K., Subramanian, C., Li, Z., Jayaswal, R.K., Gatto, C., Rock, C.O. and Wilkinson, B.J. FabH selectivity for anteiso branched-chain fatty acid precursors in low-temperature adaptation in Listeria monocytogenes. FEMS Microbiol. Lett. 301 (2009) 188-192. [PMID: 19863661]
5. Yu, Y.H., Hu, Z., Dong, H.J., Ma, J.C. and Wang, H.H. Xanthomonas campestris FabH is required for branched-chain fatty acid and DSF-family quorum sensing signal biosynthesis. Sci. Rep. 6 (2016) 32811. [PMID: 27595587]