Glutamate “flick” enables proton tunneling during fast redox biocatalysis

Redox enzymes capable of carrying out multi-electron reactions serve as blueprints for the rational design of bio-inspired catalysts for future green technologies. During catalysis, enzymes transition through multiple ‘active’ intermediate states. Movement of both electrons and substrate to/from the active site is precisely controlled to achieve the equivalent of high Faradaic efficiencies, with minimal electrons wasted in detrimental side reactions. High turnover frequencies in metalloenzymes such as the nickel-iron (NiFe) hydrogenases require mechanisms for highly-choreographed movement of two quantum particles, protons and electrons. According to Marcus theory, structural rigidity is key to a low reorganisation energy barrier for rapid, outer-sphere electron transfer. However, this is at odds with the requirement for a degree of conformational flexibility to enable rapid proton tunnelling between residues separated by distances greater than ~2.7 Å. Here we exploit the specific redox poising possible with electrochemical control of protein crystals and characterise, structurally and spectroscopically, [NiFe]-hydrogenase in each of the key states of the catalytic cycle as well as its CO-bound and oxidised states. These structures confirm extraordinarily fixed metal coordination at the active site, conducive to fast multi-electron catalysis, and reveal a subtle carboxylate ‘flick’ that provides molecular detail of how glutamate acts as a proton shuttle.