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Hydrogenases catalyze the reversible oxidation of H2 into protons and electrons and are usually readily inactivated by O2. However, a subgroup of the [NiFe] hydrogenases, including the membrane-bound [NiFe] hydrogenase from Ralstonia eutropha, has evolved remarkable tolerance toward O2 that enables their host organisms to utilize H2 as an energy source at high O2. This feature is crucially based on a unique six cysteine-coordinated [4Fe-3S] cluster located close to the catalytic center, whose properties were investigated in this study using a multidisciplinary approach. The [4Fe-3S] cluster undergoes redox-dependent reversible transformations, namely iron swapping between a sulfide and a peptide amide N. Moreover, our investigations unraveled the redox-dependent and reversible occurence of an oxygen ligand located at a different iron. This ligand is hydrogen bonded to a conserved histidine that is essential for H2 oxidation at high O2. We propose that these transformations, reminiscent of those of the P-cluster of nitrogenase, enable the consecutive transfer of two electrons within a physiological potential range. [ABSTRACT FROM AUTHOR]