Methyl branching in short-chain lecithins: are both chains important for effective phospholipase A2 activity?

Several seven-carbon fatty acyl lecithins with varied acyl chain branching have been synthesized and characterized as potential phospholipase A2 substrates. Micellar bis(4,4-dimethylpentanoyl) phosphatidylcholine, bis(5-methylhexanoyl)phosphatidylcholine, bis(3-methylhexanoyl)phosphatidylcholine, and bis(2-methylhexanoyl)phosphatidylcholine are poor substrates for phospholipase A2 (Naja naja naja). These branched lecithins also inhibit the hydrolysis of diheptanoylphosphatidylcholine by the enzyme with Ki values comparable to or smaller than the apparent Km of the linear compound. The terminally branched lecithins are excellent substrates for another surface-active hydrolytic enzyme, phospholipase C from Bacillus cereus. When only one acyl chain bears a methyl group, the hybrid lecithins 1-heptanoyl-2-(2-methylhexanoyl)phosphatidylcholine and 1-(3-methylhexanoyl)-2-heptanoylphosphatidylcholine are substrates comparable to diheptanoylphosphatidylcholine. Analysis of micellar structure and dynamics by 1H and 13C NMR spectroscopy, quasi-elastic light scattering, and comparison of critical micellar concentrations indicates little significant difference in the conformation and dynamics of these seven-carbon fatty acyl lecithin micelles, even when the methyl groups are adjacent to the carbonyls. Phospholipase A2 UV difference spectra induced by phospholipid binding imply different enzyme conformations or aggregation states caused by linear-chain and asymmetric-chain lipids compared to bis(methylhexanoyl)phosphatidylcholines. The differences in hydrolytic activity of phospholipase A2 against the branched-chain micellar lecithins can then be attributed to an enzyme-lipid interaction at the active site. The species with both fatty acyl chains branched bind to phospholipase A2 but are not turned over rapidly. Since poor enzymatic activity only occurs for lecithins with both chains methylated, the interaction of both chains with the enzyme must be important for catalytic efficiency.