The Nonphysiological Reductant Sodium Dithionite and [FeFe] Hydrogenase: Influence on the Enzyme Mechanism

[FeFe] hydrogenases are highly active enzymes for interconverting protons and electrons with hydrogen (H-2). Their active site H-cluster is formed of a canonical [4Fe-4S] cluster ([4Fe-4S](H)) covalently attached to a unique [2Fe] subcluster ([2Fe](H)), where both sites are redox active. Heterolytic splitting and formation of H-2 takes place at [2Fe](H), while [4Fe-4S](H) stores electrons. The detailed catalytic mechanism of these enzymes is under intense investigation, with two dominant models existing in the literature. In one model, an alternative form of the active oxidized state H-ox, named HoxH, which forms at low pH in the presence of the nonphysiological reductant sodium dithionite (NaDT), is believed to play a crucial role. HoxH was previously suggested to have a protonated [4Fe-4S](H). Here, we show that HoxH forms by simple addition of sodium sulfite (Na2SO3, the dominant oxidation product of NaDT) at low pH. The low pH requirement indicates that sulfur dioxide (SO2) is the species involved. Spectroscopy supports binding at or near [4Fe-4S](H), causing its redox potential to increase by similar to 60 mV. This potential shift detunes the redox potentials of the subclusters of the H-cluster, lowering activity, as shown in protein film electrochemistry (PFE). Together, these results indicate that HoxH and its one-electron reduced counterpart H-red'H are artifacts of using a nonphysiological reductant, and not crucial catalytic intermediates. We propose renaming these states as the dithionite (DT) inhibited states H-ox-DTi and H-red-DTi. The broader potential implications of using a nonphysiological reductant in spectroscopic and mechanistic studies of enzymes are highlighted.

Automated summary

FeFe hydrogenases are highly active enzymes that catalyze proton and electron interconversion with hydrogen. Their active site consists of a [4Fe-4S] cluster and a [2Fe] subcluster, with the former storing electrons and the latter responsible for hydrogen splitting and formation. Investigating the catalytic mechanism reveals the presence of various models, with one suggesting the involvement of an alternative oxidized state, HoxH, under certain conditions.