Characterization of temperature dependent and substrate-binding cleft movements in Bacillus circulans family 11...

Xylanases (EC 3.2.1.8) hydrolyze xylan, one of the most abundant plant polysaccharides found in nature, and have many potential applications in biotechnology.

Molecular dynamics simulations were used to investigate the effects of temperature between 298 to 338 K and xylobiose binding on residues located in the substrate-binding cleft of the family 11 xylanase from Bacillus circulans (BcX).

In the absence of xylobiose the BcX exhibits temperature dependent movement of the thumb region which adopts an open conformation exposing the active site at the optimum catalytic temperature (328 K). In the presence of substrate, the thumb region restricts access to the active site at all temperatures, and this conformation is maintained by substrate/protein hydrogen bonds involving active site residues, including hydrogen bonds between Tyr69 and the 2' hydroxyl group of the substrate. Substrate access to the active site is regulated by temperature dependent motions that are restricted to the thumb region, and the BcX/substrate complex is stabilized by extensive intermolecular hydrogen bonding with residues in the active site.

These results call for a revision of both the "hinge-bending" model for the activity of group 11 xylanases, and the role of Tyr69 in the catalytic mechanism.