The cyclization mechanism of cyclodextrin glycosyltransferase (CGTase) as revealed by a gamma-cyclodextrin-CGTase complex at 1.8-angstrom resolution

The enzyme cyclodextrin glycosyltransferase is closely related to alpha-amy lases but has the unique ability to produce cyclodextrins (circular alpha(1 -->4)-linked glucoses) from starch. To characterize this specificity we det ermined a 1.8-Angstrom structure of an E257Q/D229N mutant cyclodextrin glyc osyltransferase in complex with its product gamma-cyclodextrin, which revea ls for the first time how cyclodextrin is competently bound. Across subsite s -2, -1, and fl, the cyclodextrin ring binds in a twisted mode similar to linear sugars, giving rise to deformation of its circular symmetry. At subs ites -3 and +2, the cyclodextrin binds in a manner different from linear su gars. Sequence comparisons and site-directed mutagenesis experiments suppor t 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 individua l residues during the cyclization reaction cycle is proposed.