环糊精葡萄糖基转移酶CGTase.ppt

Cyclodextrin  Glycosyltransferase Contents Introduction 1 Construction 2 Catalytic mechanism 3 Application 4 Introduction Cyclodextrin glycosyltransferase(EC 9) (CGTase) is a member of the α-amylase family of glycoside hydrolases(family 13). CGTase has been identified in bacteria belonging to the genera Bacillus ,Clostridium , Klebsiella, Micrococcus ,Thermoanaerobacter , and Thermoanaerobacterium , and in a single archaeon,Thermococcus. Its main products when acting on starch are cyclodextrins and highly branched high-molecular-weight dextrins (CGTase limit dextrins). CGTase have been characterized from both mesophilic and thermophilic sources. The optimum temperature for activity is about 90℃.The optimum pH for activity is generally around pH 5–6. Introduction Cyclodextrins are torus-shaped molecules of cyclic α (1-4)-linked oligosaccharides mainly consisting of six , seven, or eight glucose residues (α- ， β- ，or γ cyclodextrin, respectively). Cyclodextrin Cyclodextrin Construction of CGTase Fig . 2 Stereo-view of the structure of Bacillus circulans strain 251 cyclodextrin glycosyltransferase Comparison Comparison Catalyzed reactions Fig Schematic representation of the CGTase -catalyzed reactions Catalyzed reactions The different reactions catalyzed by CGTase Cyclization Proposed modelof substrate binding in cyclization reaction ofCGTase Cyclization Scheme of the cyclization reaction of CGTase Application Application in food additives and modification of functional ingredients Food field Application in baking bread a、Modification of Vc Vc is very unstable in aqueous solution, and easily oxidized by oxygen in the air and other oxidizing agents. VC will quickly degrade when exposed to neutral pH, heat, light and heavy metal. b、Improvement of sweeteners Stevioside is a compound with a bitter taste and a low solubility. Glycosylation reactions can reduce the bitterness of stevioside