Studies on two types of phospholipase B from Penicillium notatum.
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The catalytic actions of the native type and the type modified by a protease attack of P. notatum phospholipase B were studied. The native and modified enzymes showed a significant difference in pH dependence of their catalytic activities. In the native enzyme, the optimum pH of phospholipase B activity (which catalyzes the complete deacylation of diacylglycerophospholipids) was pH 5.5 toward egg yolk phosphatidylcholine and its lysophospholipase activity (which catalyzes the deacylation of monoacylglycerophospholipids) showed a wide optimum pH range (pH 3.8-6.5). On the other hand, phospholipase B activity of the modified enzyme toward egg yolk phosphatidylcholine was strongly depressed between pH 3.0 and 7.0. The reduction in phospholipase B activity was due to the reduction of hydrolyzing activities for fatty acyl ester bonds at both positions 1 and 2 of diacylglycerophospholipids. However, toward 1, 2-dioctanoyl-sn-glycero-3-phosphocholine and egg yolk lysophosphatidylcholine, the catalytic activities of the modified enzyme were almost the same as those of the native enzyme below pH 4.0, although above pH 4.0 they were significantly lower. The binding activity to micellar substrates was investigated by following the fluorescence spectral changes of the enzymes on adding the enzymatically nondegradable substrate, 1-O-alkyl-sn-glycero-3-phosphocholine. At the optimum pH of the native enzyme (pH 5.0), the binding constant of the modified enzyme (0.46×105 M-1) was almost one order of magnitude lower than that of the native one (3.8×105 M-1), showing that the affinity to the substrate of the modified enzyme was decreased. In addition, at the same pH, 5.0, the negative ellipticities in the far-ultraviolet region of the circular dichroism spectra of the modified enzyme were increased compared with those of the native one. It was suggested that the conformational change in protein structure of phospholipase B resulted in the different pH dependence and low affinity to substrates in lipid-water interfaces.
- 社団法人 日本生化学会の論文
社団法人 日本生化学会 | 論文
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