One of the major energy sources on earth is methane, which also is one of the two most significant greenhouse gases in our atmosphere. Methanogenic archaea are responsible for the production of 1 billion tons of methane per year and account for nearly all methane found on earth. Methyl Coenzyme M Reductase (MCR) is the key enzyme in methanogenesis (Equation 1). MCR containing a nickel tetrapyrrole (F430) at its active site which rests at the bottom of a 50 Å channel that also accommodates the two substrates and products of the reaction. The active state of MCR contains Ni(I), which is highly oxygen- and redox-sensitive Ni(I). Spectroscopic and kinetic studies reveal a methyl radical at the transition state for methane synthesis. We have developed anaerobic methods for growth of crystals and in crystallo validation of their redox state (UV-visible, XES, unit cell analyses) during structural measurements. These advances have allowed us to determine 1.5 Å structures of active Ni(I)-MCR by X-ray diffraction (XRD) and X-ray Free Electron Laser (XFEL) methods. Our results reveal inactive Ni(II)-MCR is in a “locked-in” state, while active N(I)-MCR is highly dynamic. In the Ni(I) state, structural changes at the Ni center and the F430 tetrapyrrole promote conformational changes that radiate through the entire protein structure to promote substrate binding and catalysis.
Equation 1. CH3-SCoM + CoBSH → CH4 + CoB-SS-CoM ΔG0’ = - 30 kJ/mol