Iron-sulphur clusters in fumarate reductase from Vibrio succinogenes

(1) The fumarate reductase complex from Vibrio succinogenes contains one FAD molecule, one [4Fe-4S]^3+(3+,2+) and one [2Fe-2S]^2+(2+,1+) cluster per enzyme molecule. Both clusters can be partly reduced by succinate. In the presence of excess Na₂S₂O₄ and fumarate, the [2Fe-2S] cluster is completely oxidized, whereas the other cluster is largely reduced. (2) The [2Fe-2S] cluster is localized in the M_r 31 000 subunit. The EPR spectrum of the reduced cluster in the isolated subunit differs slightly in line width, but not in g-value, from the spectrum of the reduced, intact enzyme complex. This demonstrates that the immediate environment of the cluster is little perturbed by dissociating this subunit from the FAD-containing M_r 79 000 subunit. The temperature dependence of the power-saturation behaviour has, however, greatly decreased in the isolated subunit, the saturation at 11 K of the paramagnetic cluster being much less than in the enzyme complex. Moreover, the temperature dependence of the power-saturation behaviour of this cluster in the enzyme is greater with succinate as reducing agent, than with dithionite. (3) The [4Fe-4S] cluster is located on the M_r 79 000 subunit. This cluster is unstable in air when the subunit has been dissociated from the enzyme complex.