Structural basis of a mutant Y195I alpha-cyclodextrin glycosyltransferase with switched product specificity from alpha-cyclodextrin to beta-/gamma-cyclodextrin

摘要

Cyclodextrin glycosyltransferase (EC 2.4.1.19) (CGTase) is an extracellular bacterial enzyme which has the unique capability of forming cyclodextrins from starch. Our previous investigation revealed that a mutant Y195I alpha-CGTase drastically altered the cyclodextrin specificity by switching toward the synthesis of both beta- and gamma-CDs (Xie et al., 2013a,b). In this study, we determined one X-ray structure of the mutant Y195I alpha-CGTase at 2.3 angstrom. The overall structure was similar to that of the typical beta-CGTase from Bacillus circulans 251, with minor difference in flexible domains since they showed about 70% homogeneity of amino acid sequences. The central site with isoleucine tended to be more flexible than tyrosine thus made the sugar chain, during the cyclization process, form a larger cyclodextrin like beta- and gamma-CDs surrounding the central site instead of alpha-CD. Superposition of the structure of Y195I alpha-CGTase with those of beta-CGTase and gamma-CGTase showed that residues Lys232, Lys89 and Arg177 at subsites +2, -3 and -7 could form smaller substrate binding cavity. In summary, the crystal structure revealed that moderate increase of mobility of the central site resulted in the switched product specificity from alpha-CD to beta- and gamma-CDs of the mutant Y195I alpha-CGTase. The space differences alongside the active domain may be another factor that impacts the product specificity of the CGTase