Fig. 8.

Schematic diagram of the main lipid classes and biochemical pathways involved in the channelling of fatty acids into TAG in developing seeds. The three primary routes for the incorporation of non-native LC-PUFAs such as EPA into TAG are shown (mechanisms A, B, and C). For mechanism A, EPA esterified to PC (i.e. site of synthesis) is under a constant dynamic exchange with the acyl-CoA pool in a process described as acyl-editing. Removal of EPA from PC can proceed by the reverse action of acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT) or the combined action of phospholipase A2 PLA₂ and long-chain acyl-CoA synthetase, LACS. Once in the acyl-CoA pool, EPA-CoA and glycerol-3-phosphate (G3P) can be converted into TAG by the consecutive action of acyl-CoA:glycerol 3-phosphate acyltransferase (GPAT), acyl-CoA:lysophosphatidic acid acyltransferase (LPAT), phosphatidic acid phosphatase (PAP), and acyl-CoA:diacylglycerol acyltransferase (DGAT) in a series of reactions known as the Kennedy pathway. For mechanism B, the PC head group can be removed, producing a DAG molecule containing EPA at the sn-2 position. This reaction can proceed by four enzymatic mechanisms: phospholipase C, phospholipase D along with PAP, the reverse action of CDP-choline: diacylglycerol cholinephosphotransferase (CPT), or the recently identified phosphatidylcholine:diacyglycerol cholinephosphotransferase, (PDCT). The DAG produced by these mechanisms can then be utilized to produce TAG. For mechanism C, direct transfer of the sn-2 EPA of PC to the sn-3 of DAG produces TAG via a phospholipid:diacylglycerol acyltransferase (PDAT).