Further characterization of the spin coupling observed in oxidized hydrogenase from Chromatium vinosum. A Mössbauer and multifrequency EPR study.

Hydrogenase from Chromatium vinosum contains 1 Ni, 11-12 Fe, and ca. 9 sulfides. EPR and Mössbauer studies of the enzyme prepared in four different oxidation states show that the enzyme contains two Fe4S4 and one Fe3S4 cluster. In the oxidized (2+) state, the Mössbauer parameters of the two Fe4S4 clusters are typical for this cluster type. Upon reduction, however, these clusters do not exhibit the familiar g = 1.94 signal. The unusual nature of the reduced clusters is also borne out by the Mössbauer spectra which exhibit fairly small magnetic hyperfine interactions similar to those of centers I and II of the Desulfovibrio gigas enzyme. The Mössbauer spectra of the Fe3S4 cluster in the oxidized (1+) and reduced states are typical for this cluster type. The C. vinosum hydrogenase undergoes a reversible redox reaction at Em = +150 mV (vs NHE). Above +150 mV the EPR spectra exhibit signals (previously called signals 2 and 4) that reflect a weak interaction between Ni(III) and an Fe-containing moiety. By clamping the Ni in the diamagnetic Ni(II).CO form, we have discovered that signal 2 (X-band resonances at g = 2.01, 1.974, and 1.963) involves the Fe3S4 cluster and an as yet unidentified paramagnetic moiety. The "coupled" system exhibits magnetic hyperfine interactions quite different from those of the uncoupled [Fe3S4]1+ cluster. We have not yet been able to assign a spin to the coupled state but some of the features of the state are reminiscent of an S = 1 system. The Mössbauer data suggest, but do not prove, that an extra Fe site may be present that shuttles between low-spin Fe(III) and low-spin Fe(II) with Em = +150 mV. The Fe(III) may be located between the Ni(III) and the Fe3S4 cluster enabling it to mediate the interaction between the cluster and the Ni site. In this picture, the Fe(III) site is part of the coupled state that gives rise to signal 2. Other possibilities for signal 2 involve a ligand-based oxidation of the [Fe3S4]1+ cluster or generation of a nearby radical.