Crystal structure of alkalophilic asparagine 233-replaced cyclodextrin glucanotransferase complexed with an inhibitor, acarbose, at 2.0 Å resolution

The product specificity of cyclodextrin glucanotransferase (CGTase) from alkalophilic Bacillus sp, #1011 is improved to near-uniformity by mutation of histidine-233 to asparagine, Asparagine 233-replaced CGTase (H233N-CGTase) no longer produces ol-cyclodextrin, while the wild-type CGTase from the same bacterium produces a mixture of predominantly alpha-, beta-, and gamma-cyclodextrins, catalyzing the conversion of starch into cyclic or linear alpha-1,4-linked glucopyranosyl chains. In order to better understand the protein engineering of H233N-CGTase, the crystal structure of the mutant enzyme complexed with a maltotetraose analog, acarbose, was determined at 2.0 Angstrom resolution with a final crystallographic R value of 0.163 for all data. Taking a close look at the active site cleft in which the acarbose molecule is bound, the most probable reason for the improved specificity of H233N-CGTase is the removal of interactions needed to form a compact ring like alpha-cyclodextrin.