Abstact

The crystal structure of asparagine 233-replaced cyclodextrin glucanotransferase from alkalophilic Bacillus sp. 1011 was determined at 1.9 A resolution. While the wild-type CGTase from the same bacterium produces a mixture of mainly alpha-, beta- and gamma-cyclodextrins, catalyzing the conversion of starch into cyclic or linear alpha-1,4-linked glucopyranosyl chains, site-directed mutation of histidine-233 to asparagine changed the nature of the enzyme such that it no longer produced alpha-cyclodextrin. This is a promising step towards an industrial requirement, i.e. unification of the products from the enzyme. Two independent molecules were found in an asymmetric unit, related by pseudo two-fold symmetry. The backbone structure of the mutant enzyme was very similar to that of the wild-type CGTase except that the position of the side chain of residue 233 was such that it is not likely to participate in the catalytic function. The active site cleft was filled with several water molecules, forming a hydrogen bond network with various polar side chains of the enzyme, but not with asparagine-233. The differences in hydrogen bonds in the neighborhood of asparagine-233, maintaining the architecture of the active site cleft, seem to be responsible for the change in molecular recognition of both substrate and product of the mutant CGTase.