Discriminations among unsaturated fatty acids

Several different systems in which unsaturated fatty acids selectively serve an important role provide evidence that the enzymes that select the acyl chains can discriminate among small structural features that were not commonly regarded to be an important feature in explaining how the "good" fatty acids serve their roles. Acyltransferases placing low-melting unsaturated acids in phospholipids do not seem to select the acids for that property. Similarly, the high melting saturated acids seem to be selected for esterification by discrimination of features other than a lack of unsaturation or a high melting point. Finally, the fatty acid essential for optimal growth of young mammals, arachidonate (20:4n-6), is often converted to eicosanoids more rapidly than is compatible with a good quality of life for the adults. In contrast, the n-3 analog, eicosapentaenoate (20:5n-3), does not support such rapid growth and has been classified as non-essential, yet it may moderate the formation and function of n-6 eicosanoids in ways that permit a better chance of survival from eicosanoid-mediated disorders. As a result of attention to presumed functions, we have classified certain fatty acids by using features not actually regulating the enzymatic discriminations by which Nature functions. Such dissonance seems to assure that stories we have been providing about the "appropriateness" of certain membrane lipid structures will be altered by future researchers evaluating the action of lipids in the kinetics of the life and death of a cell.