Second and Outer Coordination Sphere Effects in Nitrogenase, Hydrogenase, Formate Dehydrogenase, and CO Dehydrogenase

Gases like H-2, N-2, CO2, and CO are increasingly recognized as critical feedstock in green energy conversion and as sources of nitrogen and carbon for the agricultural and chemical sectors. However, the industrial transformation of N-2, CO2, and CO and the production of H-2 require significant energy input, which renders processes like steam reforming and the Haber-Bosch reaction economically and environmentally unviable. Nature, on the other hand, performs similar tasks efficiently at ambient temperature and pressure, exploiting gas-processing metalloenzymes (GPMs) that bind low-valent metal cofactors based on iron, nickel, molybdenum, tungsten, and sulfur. Such systems are studied to understand the biocatalytic principles of gas conversion including N-2 fixation by nitrogenase and H-2 production by hydrogenase as well as CO2 and CO conversion by formate dehydrogenase, carbon monoxide dehydrogenase, and nitrogenase. In this review, we emphasize the importance of the cofactor/protein interface, discussing how second and outer coordination sphere effects determine, modulate, and optimize the catalytic activity of GPMs. These may comprise ionic interactions in the second coordination sphere that shape the electron density distribution across the cofactor, hydrogen bonding changes, and allosteric effects. In the outer coordination sphere, proton transfer and electron transfer are discussed, alongside the role of hydrophobic substrate channels and protein structural changes. Combining the information gained from structural biology, enzyme kinetics, and various spectroscopic techniques, we aim toward a comprehensive understanding of catalysis beyond the first coordination sphere.

Automated summary

Gases like H-2, N-2, CO2, and CO are important feedstock for green energy conversion and as sources of nitrogen and carbon. However, their industrial transformation and production require significant energy input, whereas nature efficiently converts them at ambient conditions using gas-processing metalloenzymes (GPMs). In this review, the importance of the cofactor/protein interface in GPMs is emphasized, and the effects of second and outer coordination sphere on catalytic activity are discussed.