Evolution of Fatty Acid Synthase complex during Seed Development in Sunflower

The de novo synthesis of fatty acids in plants occuring in the plastids, through the activity of the fatty acid synthase (FAS) complex, uses acetyl-CoA and malonyl-ACP as primary substrates. Plant FAS is a type II multiprotein complex formed by four individual enzymes. These are a condensing enzyme (ß-ketoacyl-ACP synthase, KAS), a first reductase (ß-ketoacyl-ACP reductase, KAR), a dehydrase (ß-hydroxyacyl-ACP dehydrase, DH) and a second reductase (enoyl-ACP reductase, ENR). By the reiterated action of the FAS complex 2-carbon units are added to the growing chain of an acyl-ACP, using malonyl-ACP as the donor substrate, and prepared for the next addition. In plants, three main condensing enzymes are found, called KAS I, II and III. The conventional model for FAS organization is largely based on the observations that the first condensation reaction is catalyzed by ß-ketoacyl-ACP synthase III (KAS III), which uses acetyl-CoA and malonyl-ACP substrates. The next six condensations are catalyzed by KAS I, whereas the final reaction between palmitoyl-ACP and malonyl-ACP is undertaken by KAS II. In this regard results, obtained from the characterization of all FAS components in sunflower developing seeds, pointed to a more complex scenario than expected with respect to KAS substrate specificities. We present a dynamic model accounting for our obervations.