- 學位論文
米根黴菌葡萄糖澱粉酶之澱粉結合區與異麥芽寡糖複合物晶體結構之研究
Crystal Structures of Starch Binding Domain of Glucoamylase from Rhizopus oryzae in Complex with Isomaltooligosaccharide
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摘要
Glucoamylase hydrolyzes starch and polysaccharides to β-D-glucose. Rhizopus oryzae glucoamylase (RoGA) consists of two functional domains, an N-terminal starch binding domain (SBD) and a C-terminal catalytic domain and these two domains are connected by an O-glycosylated linker. RoSBD belongs to carbohydrate-binding module 21 (CBM21). In previous study, two crystal structures of RoSBD complexes with linear maltoheptaose (RoSBD-G7) and β-cyclodextrin (RoSBD-βCD) with α-(1,4) glycosidic linkage have been solved in our lab. The overall structures belong to an immunoglobulin-like folding and two carbohydrate-binding sites were observed. Site I is created by several conserved aromatic residues, Trp47, Tyr83, and Tyr94, forming a broad, flat, and firm hydrophobic binding surface. Site II is built up by Tyr32 and Phe58, producing a protruded and narrow binding environment. In this study, two crystal structures of the RoSBD complexes with α-(1,6) gluco-oligosaccharide, isomaltotriose (RoSBD-isoG3) and isomaltotetraose (RoSBD-isoG4), were determined at 1.2 and 1.3A respectively. Two unique polyN loops play an important role in distinguish difference between α-(1,4) and α-(1,6) gluco-oligosaccharide upon protein-carbohydrate interaction. Although there are two carbohydrate-binding sites available and exist in RoSBD, only one binding site, site II, was observed in the RoSBD-isoG3 complex. Site II (Tyr32) might act as the initial and recognition binding site and bind to carbohydrate first. Site I (Trp47) could not bind to carbohydrate independently; it participated and supported the binding after that Site II (Tyr32) has been bound with carbohydrate. When the carbohydrate length is longer than three glucosyl units the second binding site, site I would be activated and promoted the carbohydrate binding. Two binding sites would assist each other to reinforce the binding ability as well as stabilize the binding environment when bind to the long gluco-oligosaccharide as well as starch.
參考文獻
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延伸閱讀
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- 溫怡翔（2010）。Crystal Structure of the Dehydroquinate Synthase Substrate Complex from Mycobacterium tuberculosis〔碩士論文，國立清華大學〕。華藝線上圖書館。https://www.airitilibrary.com/Article/Detail?DocID=U0016-1901201111393866