The cyclization mechanism of cyclodextrin glycosyltransferase (CGTase) as revealed by a gamma-cyclodextrin-CGTase complex at 1.8-A resolution |
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Authors: | Uitdehaag J C Kalk K H van Der Veen B A Dijkhuizen L Dijkstra B W |
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Institution: | BIOSON Research Institute, Laboratory of Biophysical Chemistry, Groningen Biomolecular Sciences, University of Groningen, Nijenborgh 4, 9747AG Groningen, the Netherlands. |
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Abstract: | The enzyme cyclodextrin glycosyltransferase is closely related to alpha-amylases but has the unique ability to produce cyclodextrins (circular alpha(1-->4)-linked glucoses) from starch. To characterize this specificity we determined a 1.8-A structure of an E257Q/D229N mutant cyclodextrin glycosyltransferase in complex with its product gamma-cyclodextrin, which reveals for the first time how cyclodextrin is competently bound. Across subsites -2, -1, and +1, the cyclodextrin ring binds in a twisted mode similar to linear sugars, giving rise to deformation of its circular symmetry. At subsites -3 and +2, the cyclodextrin binds in a manner different from linear sugars. Sequence comparisons and site-directed mutagenesis experiments support the conclusion that subsites -3 and +2 confer the cyclization activity in addition to subsite -6 and Tyr-195. On this basis, a role of the individual residues during the cyclization reaction cycle is proposed. |
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