Current status of neutron crystallography in structural biology.

Hydrogen atoms and hydration water molecules in proteins are essential for many biochemical processes, especially enzyme catalysis. Neutron crystallography enables direct observation of hydrogen atoms, and reveals molecular recognition through hydrogen bonding and catalytic reactions involving proton-coupled electron transfer. The use of neutron crystallography is still limited for proteins, but its popularity is increasing owing to an increase in the number of diffractometers for structural biology at neutron facilities and advances in sample preparation. According to the characteristics of the neutrons, monochromatic or quasi-Laue methods and the time-of-flight method are used in nuclear reactors and pulsed spallation sources, respectively, to collect diffraction data. Growing large crystals is an inevitable problem in neutron crystallography for structural biology, but sample deuteration, especially protein perdeuteration, is effective in reducing background levels, which shortens data collection time and decreases the crystal size required. This review also introduces our recent neutron structure analyses of copper amine oxidase and copper-containing nitrite reductase. The neutron structure of copper amine oxidase gives detailed information on the protonation state of dissociable groups, such as the quinone cofactor, which are critical for catalytic reactions. Electron transfer via a hydrogen-bond jump and a hydroxide ion ligation in copper-containing nitrite reductase are clarified, and these observations are consistent with the results from the quantum chemical calculations. This review article is an extended version of the Japanese article, Elucidation of Enzymatic Reaction Mechanism by Neutron Crystallography, published in SEIBUTSU-BUTSURI Vol. 61, p.216-222 (2021).