On the conformational, physical properties and functions of polyunsaturated acyl chains.

The conformational properties of the acyls of biological membranes--hydrocarbon chains with isolated cis double bonds--were studied by computer simulation. The Monte Carlo method was used, with continuous variation of bond rotation angles within the (0, 360 degree) range considered. It has been shown, that if all double bonds of molecules are separated only by one methylene group, and their number in the chain is maximum, the molecule is characterized by the highest equilibrium flexibility (at temperatures only encountered by biological systems) as compared to any similar molecules. It is such a structure which is inherent to docosahexaenoic acid. The above molecule coefficient that characterizes the temperature sensitivity of the molecule sizes is 10-times lower than that of a saturated chain. The polyunsaturated chain segment with high probability assumes the extended (in perfect crystal structures the 'angle iron-shaped') conformation when all the molecules are efficiently packed below the phase-transition temperatures. The annular lipid layer of embedded enzymes is assumed to be enriched with polyunsaturated fatty acid acyls. The above physical properties of polyunsaturated chains are bound to favour the maintenance of the proper conformational mobility of biomembrane enzymes, to relax the negative influence of environmental temperature changes on their activity. When freezing biological membranes they are bound to provide the molecule packing which is free of high tensions.