Crystallographic and activity-based evidence for thumb flexibility and its relevance in glycoside hydrolase family 11 xylanases.

Enzyme intramolecular mobility and conformational changes of loops in particular play a significant role in biocatalysis. In this respect, the highly conserved thumb loop of glycoside hydrolase family (GH) 11 xylanases is an intriguing and characteristic structural element, of which the true dynamic nature and function in catalysis is still unknown. Crystallographic analysis of the structure of a Bacillus subtilis xylanase A mutant, found as a dimer in an asymmetric unit, revealed that the thumb region can adopt an extended conformation, which is stabilized in the crystal lattice through intermolecular contacts. In contrast to the closed thumb conformation of GH11 xylanases and the previously observed small conformational changes upon substrate binding, a relocation of the tip of the thumb of more than 15 A was observed. Site-directed mutagenesis of five thumb residues, including putative hinge point residues, and enzyme kinetics assays showed that Arg112, Asn114, and Thr126 play a role in the open-close thumb movement. Replacement of Arg112 by glycine or proline caused a strong decrease of turnover numbers and elevated Michaelis constants on xylan. Mutant N114P hindered thumb movement, provoking a fourfold decrease of turnover numbers and a sharp rise in Michaelis constants, whereas the proline mutant of Thr126 displayed an increase in specific activity. The observation that extensive thumb opening is possible combined with the kinetic data suggests that the thumb plays a crucial role in both binding of substrate and release of product from the active site.