Etoh Y, Shoun H, Arima K, Beppu T
J Biochem. 1982 Mar;91(3):747-53. doi: 10.1093/oxfordjournals.jbchem.a133761.
Mucor rennin, a milk-clotting acid protease produced by a fungus Mucor pusillus, was inactivated by photo-oxidation mediated by methylene blue according to first order kinetics. The pH profile of the inactivation rate showed that a dissociating group with a pK value of 7.6 was involved in the inactivation. Addition of pepstatin A, an inhibitor specific for acid proteases, caused a marked alkaline shift of the pK value. One of two histidyl residues in the enzyme was destroyed by the photo-oxidation, with complete loss of the enzyme activity. Analysis of inhibitor binding activity and chemical modification with diazoacetyl-DL-norleucine suggested that the photo-oxidized enzyme still retained its original conformation. These results indicated that one histidyl residue in addition to the two essential carboxyl groups is involved in the catalytic function of Mucor rennin.
毛霉凝乳酶是由微小毛霉产生的一种凝乳酸性蛋白酶，在亚甲蓝介导的光氧化作用下按照一级动力学失活。失活速率的pH曲线表明，pK值为7.6的一个解离基团参与了失活过程。添加酸性蛋白酶特异性抑制剂胃蛋白酶抑制剂A会导致pK值显著向碱性偏移。该酶中两个组氨酸残基之一被光氧化破坏，酶活性完全丧失。抑制剂结合活性分析以及用重氮乙酰-DL-正亮氨酸进行化学修饰表明，光氧化后的酶仍保留其原始构象。这些结果表明，除了两个必需的羧基外，一个组氨酸残基也参与了毛霉凝乳酶的催化功能。
