Studies on the mechanism of activation of a protease in the complement system, C3 convertase.

补体系统是由多种酶反应组成的体液宿主防御系统，C3转化酶是补体系统中的关键酶。它由C4b和C2a两个组分组装而成，这两个组分是C4和C2的活化产物。在本项目中，我们重点研究了与C3转换酶分子组装有关的C4b的功能结构域，并获得了以下结果：1)C4的链间二硫键模型，由三个多肽链(&lt；α&gt；，&lt；β&gt；，&lt；Gamma&gt；)组成。在&lt；α&>链和&lt；β&>链之间只有一个S-S键，而在&lt；α&>链和&lt；伽马&>链之间有两个S-S键。2)通过对C4b的温和还原，发现了C2a(C2的C端侧)和C2b(C2的N端侧)两个不同的结合部位。C2b的结合位点位于&lt；α&gt；-&lt；β&gt；链上，而C2a的结合位点在轻度还原后丢失。3)获得了3种功能不同的抗C4b的单抗。利用这些单抗，发现在&lt；beta&gt；链上存在一个新的功能位点，它使C2a与C4b结合。4)C2b与C3转换酶结合并导致C3转换酶解离，提示C2b可能对C3转换酶起调节作用。

Complement system is a humoral host defense system composed of multiple protease reactions and C3 convertase is a key enzyme in the complement system. It is assembled from two components, C4b and C2a, which are the activation products of C4 and C2. In this project, we have focused on the functional domains of C4b which are implicated in the molecular assembly of C3 convertase and obtained following results.1) A model of inter-chain disulfide bonds of C4 which is composed of three polypeptide chains ( <alpha> , <beta> , <gamma> ). There is a single S-S bond between <alpha> and <beta> chains, while two S-S bonds are linking between the <alpha> and <gamma> chains. 2) By mild reduction of C4b, two different binding sites for C2a (C-terminal side of C2) and C2b (N-terminal side of C2) were found. The site for C2b is located on the <alpha> - <beta> chains, whereas that for C2a is lost upon the mild reduction.3) Three functionally different monoclonal antibodies (MAb) against C4b were obtained. Using these MAb, it was found that a novel functional site is present on the <beta> chain which labilizes the binding of C2a to C4b.4) Binding of C2b to C3 convertase occurs and results in the dissociation of C3 convertase, suggesting that C2b may play as a regulator for C3 convertase.