Neutron Crystallography Visualizes How Nature’s Most Efficient Enzyme Works
Enzymes play a critical role in all aspects of life by speeding up specific chemical reactions in living cells. The glycoside hydrolases (GHs) are a group of enzymes that catalyze the breakdown of large quantities of organic matter in nature, specifically cellulose and hemicellulose, and that are being applied industrially to the conversion of biomass to useful products. GHs speed up the cleavage of an otherwise very stable chemical bond through a complex process that is not well understood. New research led by scientists at Oak Ridge National Laboratory (ORNL) on the key steps in the action of xylanase, a GH that cuts xylan chains in hemicellulose (a major component of biomass) into smaller units, has shown how this enzyme coordinates the movement of hydrogen ions to speed up the breakdown process. The scientists combined information from several neutron and X-ray crystallography experiments to visualize the exact atomic structure of the xylanase during the initial steps of the reaction. They found that a side chain of the enzyme amino acid residue that is key to its activity moves between two orientations to first accept a hydrogen ion and then deliver it to the place where the xylan is to be cut. In the former orientation, the side chain is more basic and thus is able to grab a hydrogen ion from water, whereas in the latter it becomes more acidic and ready to initiate the catalytic process. This publication is the first from the new Macromolecular Neutron Diffractometer (MaNDi) at ORNL’s Spallation Neutron Source. Scientists at Los Alamos National Laboratory, Argonne National Laboratory, the University of Toledo, and universities and user facilities in the People’s Republic of China, Sweden, and Germany collaborated in the research.
Wan, Q., et al. 2015. “Direct Determination of Protonation States and Visualization of Hydrogen Bonding in a Glycoside Hydrolase with Neutron Crystallography,” Proceedings of the National Academy of Sciences (USA) 112(40), 12,384–389. DOI:10.1073/pnas.1504986112.