DeBose C D, Burns R A, Donovan J M, Roberts M F
Biochemistry. 1985 Mar 12;24(6):1298-306. doi: 10.1021/bi00327a005.
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.
几种具有不同酰基链分支的七碳脂肪酰卵磷脂已被合成并表征为潜在的磷脂酶A2底物。胶束双（4,4-二甲基戊酰基）磷脂酰胆碱、双（5-甲基己酰基）磷脂酰胆碱、双（3-甲基己酰基）磷脂酰胆碱和双（2-甲基己酰基）磷脂酰胆碱是磷脂酶A2（眼镜蛇）的不良底物。这些支链卵磷脂还抑制该酶对二庚酰磷脂酰胆碱的水解，其抑制常数（Ki）值与线性化合物的表观米氏常数（Km）相当或更小。末端支链卵磷脂是另一种表面活性水解酶——蜡样芽孢杆菌磷脂酶C的优良底物。当只有一个酰基链带有甲基时，杂合卵磷脂1-庚酰基-2-（2-甲基己酰基）磷脂酰胆碱和1-（3-甲基己酰基）-2-庚酰基磷脂酰胆碱是与二庚酰磷脂酰胆碱相当的底物。通过1H和13C核磁共振光谱、准弹性光散射对胶束结构和动力学进行分析，并比较临界胶束浓度，结果表明，即使甲基与羰基相邻，这些七碳脂肪酰卵磷脂胶束的构象和动力学也几乎没有显著差异。磷脂结合诱导的磷脂酶A2紫外差光谱表明，与双（甲基己酰基）磷脂酰胆碱相比，线性链和不对称链脂质会导致不同的酶构象或聚集状态。然后，磷脂酶A2对支链胶束卵磷脂水解活性的差异可归因于活性位点处的酶-脂质相互作用。两条脂肪酰链都有分支的物种与磷脂酶A2结合，但周转不迅速。由于只有两条链都甲基化的卵磷脂才表现出较差的酶活性，因此两条链与酶的相互作用对催化效率一定很重要。
