Desaturase

Fig. 3.9 Biosynthesis of unsaturated fatty acids.

functional 4'-phosphopantotheine group as coenzyme A and fulfils a similar role. It is assumed that the yeast enzyme complex also used the same acyl carrier protein. Chain elongation proceeds via sequential condensation reactions between the product of the fatty acid synthetase and additional malonyl groups. Thus, palmitic acid results from seven cycles of fatty acid synthetase activity. Odd numbered fatty acids are synthe-sised in the manner described above, but in this case the initial activation reaction is between acyl carrier protein and propionyl-CoA. After the fatty acid has been released from the synthetase complex, desaturation may proceed, providing molecular oxygen is available.

The roles of sterols and unsaturated fatty acids in conferring specific structural and functional properties on membranes is described in Section 4.1.2.2. In some respects, the effects of these two classes of lipid are complementary and synergistic. Using model membrane systems, Hapala et al. (1990) concluded that membrane phospholipids were influential in regulating intramembrane sterol transfer. In particular the nature of the phospholipid was important. Butke et al. (1988) demonstrated that starving a sterol biosynthetic mutant of unsaturated fatty acids resulted in accumulation of squalene. Addition of exogenous unsaturated fatty acids restored the activity of squalene epoxide and allowed sterol synthesis to proceed. The most effective fatty acids were those which reduced the medium chain saturated fatty acid content of phospholipids. The effect was complicated in that de novo protein synthesis was essential for the activity of squalene expoxidase to be restored in cells deprived of unsaturated fatty acids.

Unsaturated fatty acids have roles in yeast cells other than contributing to membrane fluidity. Unsaturated fatty acids exert effects on the formation of esters (see Section 3.7.3). Thus, it has been reported that addition of linoleic acid resulted in reduced ester synthesis and this was due to inhibition of the alcohol acetyl transferase involved in ester formation (Thurston et al., 1982). More recently, Fuji et al. (1997) duplicated this result but concluded that the effects of unsaturated fatty acids were more complex and were exerted at the gene level. These authors concluded that aeration or supplementation with unsaturated fatty acid repressed expression of the ATF1 gene, which codes for alcohol acetyl transferase.

O'Connor Cox et al. (1993) concluded that synthesis of adequate levels of unsaturated fatty acids was of critical importance in many aspects of fermentation performance. The major effect was ascribed to the role of these molecules in mitochondrial development and function. Thus, these authors duplicated the earlier findings of Gbelska et al. (1983) that cells treated with the inhibitor of the mitochondrial ATP/ADP translocase, bongkrekic acid, exhibited slow growth compared to unsupplemented controls. Addition of unsaturated fatty acids and ergosterol was partially effective at restoring normal growth patterns, indicating that these metabolites were in some way able in part to overcome the block in supply of energy from cytosol to mitochondria. Further research is required to clarify further the roles of unsaturated lipids in yeast metabolism.

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