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Leemhuis, Hans; Uitdehaag, Joost C M; Rozeboom, Henriette J; Dijkstra, Bauke W; Dijkhuizen, Lubbert
The Journal of biological chemistry, 01/2002, Letnik: 277, Številka: 2Journal Article
Cyclodextrin-glycosyltransferase (CGTase) catalyzes the formation of α-, β-, and γ-cyclodextrins (cyclic α-(1,4)-linked oligosaccharides of 6, 7, or 8 glucose residues, respectively) from starch. Nine substrate binding subsites were observed in an x-ray structure of the CGTase from Bacillus circulans strain 251 complexed with a maltononaose substrate. Subsite â6 is conserved in CGTases, suggesting its importance for the reactions catalyzed by the enzyme. To investigate this in detail, we made six mutant CGTases (Y167F, G179L, G180L, N193G, N193L, and G179L/G180L). All subsite â6 mutants had decreased k cat values for β-cyclodextrin formation, as well as for the disproportionation and coupling reactions, but not for hydrolysis. Especially G179L, G180L, and G179L/G180L affected the transglycosylation activities, most prominently for the coupling reactions. The results demonstrate that (i) subsite â6 is important for all three CGTase-catalyzed transglycosylation reactions, (ii) Gly-180 is conserved because of its importance for the circularization of the linear substrates, (iii) it is possible to independently change cyclization and coupling activities, and (iv) substrate interactions at subsite â6 activate the enzyme in catalysis via an induced-fit mechanism. This article provides for the first time definite biochemical evidence for such an induced-fit mechanism in the α-amylase family.
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