β-ketoacyl-acyl carrier protein synthase IV:: a key enzyme for regulation of medium-chain fatty acid synthesis in Cuphea lanceolata seeds

With the aim of elucidating the mechanisms involved in the biosynthesis of medium-chain fatty acids in Cuphea lanceolata Ait., a crop accumulating up to 90% decanoic acid in seed triacylglycerols, cDNA clones of a beta-ketoacyl-acyl carrier protein (ACP) synthase IV (c1KAS IV, EC 2.3.1.41) were isolated from C. lanceolata seed embryos. The amino acid sequence deduced from c1KAS IV cDNA showed 80% identity to other plant KAS II-type enzymes, 55% identity towards plant KAS I and over 90% towards other Cuphea KAS IV-type sequences. Recombinant c1KAS IV was functionally overexpressed in Escherichia coli, and substrate specificity of purified enzyme showed strong preference for elongation of short-chain and medium-chain acyl-ACPs (C4- to C10-ACP) with nearly equal activity. Further elongation steps were catalysed with distinctly less activity. Moreover, short- and medium-chain acyl-ACPs exerted a chain-length-specific and concentration-dependent substrate inhibition of c1KAS IV. Based on these findings a regulatory mechanism for medium-chain fatty acid synthesis in C. lanceolata is presented.