Davies S G, Sim R B
Biosci Rep. 1981 Jun;1(6):461-8. doi: 10.1007/BF01121579.
The complement system proteins C3 and C4 and the plasma protease inhibitor alpha 2-macroglobulin, when activated by limited proteolysis, can bind covalently to other macromolecules. The three proteins also exhibit an unusual internal peptide-bond cleavage reaction when denatured. The covalent binding reaction is likely to occur by a transacylation mechanism involving an internal thiolester in the three proteins. However, the activated species of these proteins are much more reactive than simple thiolesters. Studies of molecular models of the thiolester region in C3 show that an intramolecular acid catalysis mechanism can both account for the exceptional reactivity of the activated form of these proteins and provide an explanation for the denaturation-induced peptide bond cleavage.
补体系统蛋白C3和C4以及血浆蛋白酶抑制剂α2-巨球蛋白在通过有限的蛋白水解作用被激活后，能够与其他大分子共价结合。这三种蛋白在变性时还会表现出一种不寻常的内部肽键裂解反应。共价结合反应可能是通过一种涉及这三种蛋白内部硫酯键的转酰基化机制发生的。然而，这些蛋白的活化形式比简单的硫酯键更具反应活性。对C3中硫酯键区域分子模型的研究表明，分子内酸催化机制既能解释这些蛋白活化形式的异常反应活性，又能为变性诱导的肽键裂解提供一种解释。
