Hydrogenases are redox enzymes that catalyze the conversion of protons and molecular hydrogen (H2). Based on the composition of the active site cofactor, the monometallic [Fe]-hydrogenase is distinguished from the bimetallic [NiFe]- or [FeFe]-hydrogenase. The latter has been reported with particularly high turnover activities for both H2 release and H2 oxidation, notably at neutral pH, ambient temperatures, and negligible electric overpotential. Due to these properties, [FeFe]-hydrogenase represents the “gold standard” in enzymatic hydrogen turnover. Understanding hydrogenase chemistry is crucial for the design of transition metal complexes that serve as potentially sustainable proton reduction or H2 oxidation catalysts, e.g., in electrolytic devices or fuel cells. However, even 20 years after the crystal structures of [FeFe]-hydrogenase have been published, several aspects of biological hydrogen turnover are heatedly discussed. In this perspective, we give an overview on how the diversity of naturally occurring and artificially prepared, semisynthetic [FeFe]-hydrogenases deepens our understanding of hydrogenase chemistry. In parallel, we cover recent results from biophysical techniques that go beyond the scope of conventional X-ray diffraction, EPR, and FTIR spectroscopy. Taking into account both proton transfer and electron transfer as well as the notorious sensitivity of [FeFe]-hydrogenase toward carbon monoxide, the discussion further touches upon the molecular proceedings of biological hydrogen turnover.
