On the prosthetic groups of the NiFe sulfhydrogenase from Pyrococcus furiosus: topology, structure, and temperature-dependent redox chemistry

 The sulfhydrogenase complex of Pyrococcus furiosus is an αβγδ heterotetramer with both hydrogenase activity (borne by the αδ subunits) and sulfur reductase activity (carried by the βγ subunits). The β-subunit contains at least two 4Fe-4S] cubanes and the γ-subunit contains one 2Fe-2S] cluster and one FAD molecule. The δ-subunit contains three 4Fe-4S] cubanes and the α-subunit carries the NiFe dinuclear center. Only three Fe/S signals are observed in EPR-monitored reduction by dithionite, NADPH, or internal substrate upon heating. All other clusters presumably have reduction potentials well below that of the H⁺/H₂ couple. Heat-induced reduction by internal substrate allows, for the first time, EPR monitoring of the NiFe center in a hyperthermophilic hydrogenase, which passes through a number of states, some of which are similar to states previously defined for mesophilic hydrogenases. The complexity of the observed transitions reflects a combination of temperature-dependent activation and temperature-dependent reduction potentials. Received: 10 December 1998 / Accepted: 16 February 1999