环糊精葡萄糖基转移酶的结构特征与催化机理

随着环糊精在食品、医药等领域的应用越来越广,生产环糊精所必需的环糊精葡萄糖基转移酶(CGT酶)已经成为当今研究的热点。特别是近二十年来,国外对该酶进行了比较深入的研究。首先介绍了CGT酶的功能特性与结构特征。CGT酶是一种多功能型酶,能催化三种转糖基反应(歧化、环化和耦合反应)和水解反应,其中,能将淀粉转化为环糊精的环化反应是特征反应;作为α-淀粉酶家族的成员,CGT酶除了具有与α-淀粉酶相同的A、B、C结构域外,还存在D和E结构域。另外,对CGT酶的催化机理包括底物结合方式、转糖苷反应机理以及环化机理等进行了详细的讨论。

Structural Characteristics and Catalytic Mechanisms of Cyclodextrin Glycosyltransferase

With the applications of cyclodextrins expanding in the industries related to food, pharmaceuticals, etc, cyclodextrin glycosyltransferase (CGTase), the essential enzyme for the production of cyclodextrins, has become the focus of scientific research nowadays. Especially in recent twenty years, CGTase has been studied extensively, leading to increasing knowledge of its structure and function. Firstly the function and structural characteristics of CGTase were focused on. CGTase is a multifunctional enzyme. It can catalyze three transglycosylation reactions (disproportionation, cyclization, and coupling), and a hydrolysis reaction. The specific CGTase reaction is the cyclization reaction, which can result in the formation of a cyclodextrin. CGTase is a member of the α-amylase family of glycosyl hydrolases. α-Amylases generally consist of three structural domains, A, B, and C, while CGTases show a similar domain organization with two additional domains, D and E. In addition, substrate binding of CGTase and mechanisms of transglycosylation and cyclization reactions catalyzed by CGTase were discussed in detail. Domain B contributes to substrate binding by providing several amino acid side chains alongside a substrate binding groove on the surface of the CGTase protein. Maltose binding site assists in guiding the linear starch chains into the substrate binding groove containing at least nine subsites. After the glycosidic bond cleavage reaction has taken place between subsites ＋1 and －1, the non-reducing end of the oligosaccharide at the groove is transferred to the reducing end of the same oligosaccharide chain, leading to cyclic products.