Enzymatic circularization of a malto-octaose linear chain studied by stochastic reaction path calculations on cyclodextrin glycosyltransferase.

Cyclodextrin glycosyltransferase (CGTase) is an enzyme belonging to the alpha-amylase family that forms cyclodextrins (circularly linked oligosaccharides) from starch. X-ray work has indicated that this cyclization reaction of CGTase involves a 23-A movement of the nonreducing end of a linear malto-oligosaccharide from a remote binding position into the enzyme acceptor site. We have studied the dynamics of this sugar chain circularization through reaction path calculations. We used the new method of the stochastic path, which is based on path integral theory, to compute an approximate molecular dynamics trajectory of the large (75-kDa) CGTase from Bacillus circulans strain 251 on a millisecond time scale. The result was checked for consistency with site-directed mutagenesis data. The combined data show how aromatic residues and a hydrophobic cavity at the surface of CGTase actively catalyze the sugar chain movement. Therefore, by using approximate trajectories, reaction path calculations can give a unique insight into the dynamics of complex enzyme reactions.