Conformational control over proton-coupled electron transfer in metalloenzymes

From the reduction of dinitrogen to the oxidation of water, the chemical transformations catalysed by metalloenzymes underlie global geochemical and biochemical cycles. These reactions represent some of the most kinetically and thermodynamically challenging processes known and require the complex choreography of the fundamental building blocks of nature, electrons and protons, to be carried out with utmost precision and accuracy. The rate-determining step of catalysis in many metalloenzymes consists of a protein structural rearrangement, suggesting that nature has evolved to leverage macroscopic changes in protein molecular structure to control subatomic changes in metallocofactor electronic structure. The proton-coupled electron transfer mechanisms operative in nitrogenase, photosystem II and ribonucleotide reductase exemplify this interplay between molecular and electronic structural control. We present the culmination of decades of study on each of these systems and clarify what is known regarding the interplay between structural changes and functional outcomes in these metalloenzyme linchpins. Rate-limiting conformational changes often gate the formation of catalytically active metalloenzyme states. We review examples of the interplay between macroscopic changes in protein molecular structure and subatomic changes in metallocofactor electronic structure that together enable precision control over nature's redox machines.