X-ray structures along the reaction pathway of cyclodextrin glycosyltransferase elucidate catalysis in the alpha-amylase family

Cyclodextrin glycosyltransferase (CGTase) is an enzyme of the alpha-amylase family, which uses a double displacement mechanism to process alpha-linked glucose polymers. We have determined two X-ray structures of CGTase comple xes, one with an intact substrate at 2.1 Angstrom resolution, and the other with a covalently bound reaction Intermediate at 1.8 Angstrom resolution. These structures give evidence for substrate distortion and the covalent ch aracter of the intermediate and for the first time show, in atomic detail, how catalysis in the alpha-amylase family proceeds by the concerted action of all active site residues.