Function and Tertiary-and Quaternary-structure of Cyclodextrin-hydrolyzing Enzymes(CDase),a Group of Multisubstrate Specific Enzymes Belonging to the alpha-Amylase Family

摘要

Cyclodextrin-hydrolyzing enzymes(CDases)such as cyclomaltodextrinase(CDase),neopullulanase(NPase),Thermoactinomyces vulgaris amylase II(TVA II),and maltogenic amylase(MAase)are multisub-strate enzymes,belonging to a subfamily of the Glycoside Hydrolase family 13,and act on cyclodextrins,various maltooligosaccharides,pullulan and starch.In terms of quaternary structure,many CDases are unique since they act not only as monomers,but also as oligomers by forming dimers,tetramers or even higher oli-gomers.The N-terminal domain of approximately 130 residues absent in ordinary alpha-amylases contributes to the formation of the oligomeric state in this group of enzymes.Dimerization and oligomerization can provide enzymes with a number of functional advantages such as high stability and efficacy in accessibility and specificity of active sites.CDase from Thermits sp.exists as a 3D domain-swapped dimer which exhibits different binding preferences for various substrates due to increased specificity via dimerization.Three-dimensional domain swapping is a basic unit of the oligomer.CDase from alkalophilic Bacillus sp.1-5 exists as a dodecamer by forming an assembly of six 3D domain-swapped dimeric subunits.Oligomerization of the CDase also affects the catalytic activity of transglycosylation,thereby preferentially forming an alpha-1,6-glycosidic linkage in the transfer product.We demonstrated that Glu 332 at the interdomain interface played an important role in the binding of the acceptor molecules.The association/dissociation process of CDase examined in various oligomeric states is of great interest to identify the mechanism and forces that contribute to the supramolecu-lar assembly and function of the enzyme.In this paper we discuss the physiochemical properties of CDase in light of the consequences of oligomerization:1)three-dimensional structure,2)multisubstrate specificity/catalytic efficiency,3)transglycosylation activity at the interface of the dimer,4)dissociation/association of supra-molecular assembly and 5)a possible physiological role in microorganisms.