Neutron crystallography for the elucidation of enzyme catalysis.

Hydrogen atoms and hydration water molecules in proteins are indispensable for many biochemical processes, especially enzymatic catalysis. The locations of hydrogen atoms in proteins are usually predicted based on X-ray structures, but it is still very difficult to know the ionization states of the catalytic residues, the hydration structure of the protein, and the characteristics of hydrogen-bonding interactions. Neutron crystallography allows the direct observation of hydrogen atoms that play crucial roles in molecular recognition and the catalytic reactions of enzymes. In this review, we present the current status of neutron crystallography in structural biology and recent neutron structural analyses of three enzymes: ascorbate peroxidase, the main protease of severe acute respiratory syndrome coronavirus 2, and copper-containing nitrite reductase. • Neutron crystallography can observe hydrogen atoms directly, which play essential roles in enzyme catalyzed reactions. • Ferric ascorbate peroxidase has a possible proton transfer pathway from ascorbate to the heme iron via arginines and waters. • Protonation states in the catalytic dyad of severe acute respiratory syndrome coronavirus 2 main protease are observed. • Electron transfer via a hydrogen-bond jump and a hydroxide ion ligation in copper-containing nitrite reductase are clarified. [ABSTRACT FROM AUTHOR]