Membrane lipid biosynthesis in Acholeplasma laidlawii B : Relationship between fatty acid structure and the positional distribution of esterified fatty acids in phospho- and glycolipids from growing cells

Abstract

The positional distributions of a wide variety of exogenous fatty acids in the monoglucosyl diglyceride, diglucosyl diglyceride, and phosphatidylglycerol fractions from Acholeplasma laidlawii B have been determined. The positional distribution of any given fatty acid is variable, depending upon the nature and quantity of other fatty acids present in the glyco- or phospholipid molecule. The fatty acid compositions of these membrane lipids were systematically controlled so that the positional distribution values determined here accurately reflect the relative positional affinities of the fatty acids studied. Fatty acid positional distribution depends markedly on chain length and chemical structure. Within any single chemical class of fatty acids, affinity for the 1-position generally decreases (and that for the 2-position increases) as fatty acid chain length decreases. At constant chain length, the affinity for the 1-position of the various chemical classes of fatty acids decreases as follows: straight-chain saturated > methyl-isobranched saturated > methyl-anteisobranched saturated > trans-monounsaturated > trans-cyclopropane > cis-monounsaturated > cis-cyclopropane > cis-diunsaturated > cis-triunsaturated. An excellent correlation is observed between the relative positional affinity and the physicochemical properties (melting temperatures) of most fatty acids tested, with the highest melting fatty acids exhibiting the greatest affinity for the 1-position and the lowest melting acids the greatest affinity for position 2. The positional distribution of all fatty acids examined is independent of the quantity of that fatty acid present in the glyco- or phospholipid fraction studied, provided that the fatty acid in question is not present in sufficient quantity to saturate the preferred position. The fatty acid positional distributions in the monoglucosyl diglyceride, diglucosyl diglyceride, and phosphatidylglycerol fractions are generally very similar.