Structural studies of two electron transfer proteins by cooperative use of X-ray and neutron diffraction

Information about hydrogen atoms and valence electrons is important to understand functions of electron transfer proteins, because they are directly connected with the reactivity of electron transfer reactions. Therefore, high resolution crystal structures are required to detect small structural changes due to the electron transfer reactions. We have determined high-resolution X-ray and neutron crystal structures of two electron transfer proteins, high-potential iron-sulfur protein (HiPIP) and NADH cytochrome b5 reductase (b5R). HiPIP and b5R possess Fe4S4 cluster and FAD/NADH cofactors, respectively, and their electron transfer reactions occur via these cofactors. Structures were refined at 0.48 Å resolution for HiPIP (Hirano et al., Nature, 2016) and at 0.78 Å resolution for b5R (Takaba et al., Sci. Rep., 2017) by using diffraction data collected at the BL41XU beamline of SPring-8. They clearly visualized valence electron densities of the cofactors in HiPIP and b5R. Valence electron densities provide detailed information about interactions between proteins and cofactors. Neutron diffraction data were collected at the BL03 beamline of J-PARC/MLF to 1.1 Å resolution for HiPIP and to 1.4 Å resolution for b5R. High resolution neutron structures display protonation states of charged amino acid residues and hydrogen atoms of solvent molecules on the surface of the proteins.1st QST Internationl Symposium "Quantum Life Science