Segregation of Unusual Fatty Acids into TAG Biosynthesis

The seeds of species that accumulate unusual fatty acids in their triacylglyc-erols possess additional microsomal acyltransferases, in particular, LPAT and DGAT. These enzymes exhibit a wide variation in substrate preference for both donor acyl groups and acceptor glycerolipids to facilitate channelling of unusual fatty acids into TAGs. In coconut, a seed-specific LPAT isoform is present that prefers medium-chain saturated fatty acids, allowing the synthesis of a trisaturate TAG (Knudson et al. 1995). Similarly, the synthesis of trierucin by Limnanthes douglasii requires a seed-specific 22 : 1-CoA-preferring LPAT isoform (Hanke et al. 1995). These tissue specific paralogs are absent from species that synthesise oils with common fatty acids at the sn-2 position of the triacylglycerol. The seeds of Cuphea lanceolata synthe-sise TAG that is rich in 10:0 mediated by the substrate selectivities of the acyltransferases (Bafor et al. 1990). The microsomal GPAT of C. lanceolata incorporates either medium- or long-chain fatty acids into position sn-1 of the Gro3P. When caprate is present in LPA, a LPAT isoform that prefers 10: 0-CoA catalyses acylation at the sn-2 position. The C. lanceolata DGAT exhibits a strong selectivity for DAG containing 10 : 0 and uses 10 : 0-CoA to acylate position sn-3 (Bafor et al. 1990). The DGATs of other species that accumulate unusual fatty acids also exhibit a selectivity for them (Wiberg et al. 1994). Since the DAG and PtdCho pools are in equilibrium, DGAT therefore plays an important role in the removal of these fatty acids from PtdCho by the channelling of the DAG moiety released by the action of CPT into TAG. As mentioned in Sect. 2.1, the CPT does not itself have strong substrate selectivity for DAG species (Vogel and Browse 1996) and it was concluded that CPT does not influence channelling to triacylglycerol.

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