Figure 5.

Proposed pathway for EPA biosynthesis and possible EPA import routes from the ER into the chloroplast of Phaeodactylum tricornutum.

Red text indicates increased pool size of metabolite or activity of enzyme in knockout cells. Blue text indicates reduced pool size or activity of enzyme. de novo synthesized 16:0‐ACP enters the prokaryotic pathway where it can be used in the plastid for the production of MGDG, DGDG, SQDG and PG via a canonical prokaryotic pathway or desaturated by PAD to 16:1n7‐ACP and further by soluble ACP desaturases. When exported to the cytosol through the long‐chain acyl‐CoA synthases pathway to enter the eukaryotic pathway, C16‐ACPs are hydrolyzed by unknown FAT and FFA are exported through the plastidial membrane to enter CoA pool. 16:0‐CoA in elongated to 18:0‐CoA with further desaturation by homolog of Arabidopsis ADS1 to produce 18:1Δ9‐CoA, which is at the base of PUFA biosynthesis. Newly produced EPA could be incorporated into acyl‐CoA and imported from ER into the chloroplast for galactolipid synthesis. From the ER, EPA could be imported into the chloroplast as incorporated into PA or DAG. EPA can be converted into TAG by an acyl‐independent PDAT after acyl‐editing or enter the Kennedy pathway and be converted into TAG by DGAT. ACP, acyl carrier protein; ADS1, Δ9‐acyl‐CoA desaturase; ATS1, plastidic glycerol‐3‐phosphate acyltransferase; ATS2, plastidic lysophosphatidic acid transferase; CoA, coenzyme A; DAG, diacylglycerols; DGAT, acyl‐CoA:diacylglycerol acyltransferase; DGDG, digalactosyldiacylglycerol; DGTA, diacylglycerylhydroxymethyltrimethyl‐β‐alanine; ER, endoplasmic reticulum; FAD, fatty acid desaturases; FASII, fatty acid synthase of type II; FAT, fatty acyl‐ACP thioesterases; FFA, free fatty acids; G3P, glycerol‐3‐phosphate; GPAT, glycerol‐3‐phosphate acyltransferase; LACS, long chain acyl‐coenzyme A synthetase; LC‐PUFA, long chain polyunsaturated fatty acids; LPA, lysophosphatidic acid; LPAAT, lysophosphatidic acid acyltransferase; LPC, lysophosphatidylcholine; LPCAT, lysophosphatidylcholine acyltransferase; MGDG, monogalactosyldiacylglycerol; PA, phosphatidic acid; PAD, palmitoyl‐ACP Δ9‐desaturase; PAP, phosphatidic acid phosphatase; PC, phosphatidylcholine; PDAT, phospholipid:diacylglycerol acyltransferase; PDCT, phosphatidycholine:diacylglycerol cholinephosphotransferase; PE, phosphatidylethanolamine; PG, phosphatidylglycerol; PGPS, phosphatidylglycerol phosphate synthase; PI, phosphatidylinositol; PS, phosphatidylserine; SQDG, sulfoquinovosyldiacylglycerol; TAG, triacylglycerols.