EARLY CATALYTIC STEPS OF EUGLENA-GRACILIS CHLOROPLAST TYPE-II FATTY-ACID SYNTHASE

Euglena gracilis is a very ancient eukaryote whose chloroplast acquisi tion and evolution has been independent of higher plants. The organism is unique in possessing two de novo fatty acid synthases, a true mult ienzyme complex of great size in the cytosol and a plastid-localized t ype II fatty acid synthase composed of discrete enzymes and acyl carri er protein (ACP). The enzymology of the early steps of fatty acid bios ynthesis differed in the Euglena type 11 fatty acid synthase compared to those of Escherichia coli and plants. The enzymes of Euglena partic ipating in both priming and elongation reactions to form a new carbon- carbon bond were acetyl-CoA-ACP transacylase, malonyl-CoA-ACP transacy lase, and beta-ketoacyl-ACP synthase I. The effects of inhibitors on t he three different enzymes were noted. All carbon-carbon bond formatio n was inhibited by cerulenin. Although neither fatty acid biosynthesis nor any of the isolated enzymes were sensitive to diisopropylphosphof luoridate, the three Euglena enzymes studied were sensitive to differe nt sulfhydryl-alkylating agents. Acetyl-ACP supported fatty acid biosy nthesis as effectively as did comparable amounts of ACPSH and acetyl-C oA. There was no evidence for a beta-ketoacyl-ACP synthase III for pri ming such as has been reported in type II fatty acid synthase of highe r plants and bacteria. The roles of the acetyl-CoA-ACP transacylase an d beta-ketoacyl-ACP synthase I appear to be unique in the type II fatt y acid synthase of Euglena. Acetyl-CoA-ACP transacylase, malonyl-CoA-A CP transacylase, and beta-ketoacyl-ACP synthase I were separated from one another and shown to have different molecular weights.