De novo synthesised oleate may be modified by desaturation reactions catalysed by FAD2 and FAD3 or other modifying enzymes depending on whether the species accumulates unusual fatty acids (Sect. 3.2). The 18 : 2 and 18 : 3, or otherwise modified fatty acids may then be transferred from PtdCho and the acyl-CoA pool (Fig. 1, step 7) by a freely reversible exchange reaction catalysed by acyl-CoA:lyso-PtdCho acyltransferase (LPC-AT); the basic biochemistry is reviewed in (Frentzen 1993). The released polyunsaturated or modified acyl-CoA may be incorporated into triacylglycerol by the acyltrans-ferases of the Kennedy pathway. As yet, a gene encoding acyl-CoA:LPC-AT has not been identified from any organism. The development of LPC photo-affinity probes gave hope that these proteins might be identified directly (Tumaney and Rajasekharan 1999) but this approach has not yet borne fruit.
It seems most likely that acyl-CoA:LPC-AT genes will be identified through detailed analysis of the LPAT homologs in Arabidopsis described in Sect. 1.1.
The important role that LPC-AT plays in linking pools of acyl substrates has become evident from attempts to manipulate polyunsaturated fatty-acid biosynthesis in yeast and plants. In yeast that was engineered to produce arachidonic acid (20 : 4), an accumulation of highly unsaturated fatty acids in PtdCho indicated that synthesis of acylCoA thioesters was limiting the production of docosahexanoic acid (22 : 6). These studies suggested that the LP-CAT, controlling the release of delta-6 desaturated 18 : 3 from sn-2 of PtdCho and its transfer to the acyl-CoA pool to allow elongation, is rate-limiting (Domergue et al. 2003). In yeast, whereas the transfer from acyl-CoA to PtdCho was very efficient, the rate of the reverse reaction was slow, suggesting the existence either of different isoenzymes for each reaction or a strong and different substrate preference for the forward and reverse reactions (Domergue et al. 2003). Similarly, analysis of seed lipids from transgenic plants expressing the appropriate elongases and desaturases indicated that the yield of polyunsaturated fatty acids in TAG was also limited by the transfer of delta-6 saturated fatty acids from PtdCho to the acyl-CoA pool (Abbadi et al. 2004). The delta-6 unsaturated 18-carbon polyunsaturated fatty acids were directly and rapidly incorporated into TAG, preventing transfer to the acyl-CoA pool, and permitting elongation for the production of C20 polyunsaturated fatty acids (Abbadi et al. 2004).
PtdCho can also contribute modified fatty acids to TAG synthesis by the donation of its DAG moiety catalysed by the reverse reaction catalysed by CPT described in Sect. 2.1. This permits the accumulation of DAG containing polyunsaturated fatty acids, to provide a precursor for TAG synthesis (Stymne and Stobart 1987). The relative importance of the CPT and LPC-AT routes in providing modified fatty acids for TAG is unclear and may vary according to the plant species.
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