CRYSTALLOGRAPHIC STUDIES OF THE INTERACTION OF CYCLODEXTRIN GLYCOSYLTRANSFERASE FROM BACILLUS-CIRCULANS STRAIN-251 WITH NATURAL SUBSTRATES AND PRODUCTS

Asp-229, Glu-257, and Asp-328 constitute the catalytic residues in cyclodextrin glycosyl transferase from Bacillus circulans strain 251. Via site-directed mutagenesis constructed D229N, E257Q, and D328N mutant proteins showed a 4,000-60,000 fold reduction of cyclization activity. A D229N/E257Q double mutant showed a 700,000-fold reduction and was crystallized for use in soaking experiments with alpha-cyclodextrin. Crystal structures were determined of wild type CGTase soaked at elevated pH with alpha cyclodextrin (resolution, 2.1 Angstrom) and maltoheptaose (2.4 Angstrom). In addition, structures at cryogenic temperature were solved of the unliganded enzyme (2.2 Angstrom) and of the D229N/E257Q mutant after soaking with alpha-cyclodextrin (2.6 Angstrom). In the crystals soaked in alpha-cyclodextrin and maltoheptaose, a maltotetraose molecule is observed to bind in the active site. Residue 229 is at hydrogen bonding distance from the C-6 hydroxyl group of the sugar, which after cleavage will contain the new reducing end. In the D229N/E257Q double mutant structure, two alpha-cyclodextrins are observed to replace two maltoses at the E-domain, thus providing structural information on product inhibition via binding to the enzyme's ram starch binding domain.