This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Fatty Acid and Lipid Biosynthesis|
Expected Taxonomic Range: Spermatophyta
Triacyglycerols (TAG) from vegetable oils are a major source of essential fatty acids in human diet, namely polyunsaturated fatty acids (PUFA) linoleate (18:2) and linolenate (18:3). In oilseeds, TAG is synthesized via the Kennedy pathway, where the precursor diacylglycerol (DAG) is de novo synthesized from glycerol-3-phosphate. In addition to the Kennedy pathway, there are several other paths supplying DAG for TAG biosynthesis during seed development in plants [Bates09]. Since further desaturation of 18:1 to form 18:2 and 18:3 occurs mainly in phosphatidylcholine (PC, see phospholipid desaturation), together, these alternative paths in converting PC to DAG can provide enrichment of the PUFA content in TAG.
1) reverse reaction of CDP-choline:1,2-diacylglycerol cholinephosphotransferase (CPT, 220.127.116.11)
Radio tracer studies using microsomes isolated from developing safflower cotyledons indicated that CPT catalyzes a reversible exchange of diacylglycerol moieties between pools of PC and DAG [Slack85]. The maximum activity of CPT coincides with the maximum rate of accumulation of PC and TAG. It is very plausible that the releasing of PUFA-contained DAG from PC, catalyzed by the reverse activity of CPT, may contribute to the enrichment of PUFA in TAG.
2) phosphatidylcholine:diacylglycerol cholinephosphotransferase
A novel enzyme was recently isolated from Arabidopsis [Lu09b]. The Arabidopsis phosphatidylcholine:diacylglycerol cholinephosphotransferase, encoded by ROD1, catalyzes the transfer of phosphocholine head group from PC to DAG. The interconversion of PC and DAG catalyzed by ROD1 provides a major route through which 18:1 (in DAG) enters PC for further desaturation, and in turn the PUFAs 18:2 and 18:3 (in PC) re-enter to the DAG pool for TAG biosynthesis. Arabidopsis ROD1 mutants had a 40% reduction of 18:2 and 18:3 in the seed TAG.
It was postulated that a phospholipase C (18.104.22.168) activity may also generate DAG for TAG synthesis. However no such activity has been reported in developing seeds [Bates09].
Bates09: Bates PD, Durrett TP, Ohlrogge JB, Pollard M (2009). "Analysis of acyl fluxes through multiple pathways of triacylglycerol synthesis in developing soybean embryos." Plant Physiol 150(1);55-72. PMID: 19329563
Lu09b: Lu C, Xin Z, Ren Z, Miquel M, Browse J (2009). "An enzyme regulating triacylglycerol composition is encoded by the ROD1 gene of Arabidopsis." Proc Natl Acad Sci U S A 106(44);18837-42. PMID: 19833868
Hjelmstad91: Hjelmstad RH, Bell RM (1991). "sn-1,2-diacylglycerol choline- and ethanolaminephosphotransferases in Saccharomyces cerevisiae. Mixed micellar analysis of the CPT1 and EPT1 gene products." J Biol Chem 266(7);4357-65. PMID: 1847919
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