Elucidation of the biosynthesis of eicosapentaenoic acid in the microalga Porphyridium cruentum: II. Studies with radiolabeled precursors

Abstract

In the course of the study of the biosynthesis of the fatty acid eicosapentaenoic acid (EPA) in the microalga Porphyridium cruentum, cells were pulse-labeled with various radiolabeled fatty acid precursors. Our data show that the major end products of the biosynthesis are EPA-containing galactolipids of a eukaryotic and prokaryotic nature. The prokaryotic molecular species contain EPA and arachidonic acid at the sn-1 position and C16 fatty acids, mainly 16:0, at the sn-2 positions, whereas in the eukaryotic species both positions are occupied by EPA or arachidonic acid. However, we suggest that both the eukaryotic and prokaryotic molecular species are formed in two pathways, ω6 and ω3, which involve cytoplasmic and chloroplastic lipids. In the ω6 pathway, cytoplasmic 18:2-phosphatidylcholine (PC) is converted to 20:4ω6-PC by a sequence that includes a Δ6 desaturase, an elongation step, and a Δ5 desaturase. In the minor ω3 pathway, 18:2-PC is presumably desaturated to 18:3ω3, which is sequentially converted by the enzymatic sequence of the ω6 pathway to 20:5ω3-PC. The products of both pathways are exported, as their diacylglycerol moieties, to the chloroplast to be galactosylated into their respective monogalactosyldiacylglycerol molecular species. The 20:4ω6 in both eukaryotic and prokaryotic monogalactosyldiacylglycerol can be further desaturated to EPA by a chloroplastic Δ17 (ω3) desaturase.