Functional characterization and structural modelling of Helianthus annuus (sunflower) ketoacyl-CoA synthases and their role in seed oil composition.

Main Conclusion: The enzymes HaKCS1 and HaKCS2 are expressed in sunflower seeds and contribute to elongation of C18 fatty acids, resulting in the C20-C24 fatty acids in sunflower oil. Most plant fatty acids are produced by plastidial soluble fatty acid synthases that produce fatty acids of up to 18 carbon atoms. However, further acyl chain elongations can take place in the endoplasmic reticulum, catalysed by membrane-bound synthases that act on acyl-CoAs. The condensing enzymes of these complexes are the ketoacyl-CoA synthase (KCSs), responsible for the synthesis of very long chain fatty acids (VLCFAs) and their derivatives in plants, these including waxes and cuticle hydrocarbons, as well as fatty aldehydes. Sunflower seeds accumulate oil that contains around 2-3% of VLCFAs and studies of the fatty acid elongase activity in developing sunflower embryos indicate that two different KCS isoforms drive the synthesis of these fatty acids. Here, two cDNAs encoding distinct KCSs were amplified from RNAs extracted from developing sunflower embryos and named HaKCS1 and HaKCS2. These genes are expressed in developing seeds during the period of oil accumulation and they are clear candidates to condition sunflower oil synthesis. These two KCS cDNAs complement a yeast elongase null mutant and when expressed in yeast, they alter the host's fatty acid profile, proving the encoded KCSs are functional. The structure of these enzymes was modelled and their contribution to the presence of VLCFAs in sunflower oil is discussed based on the results obtained.