Analysis of the electron paramagnetic resonance properties of the [2Fe-2S]1+ centers in molybdenum enzymes of the xanthine oxidase family: assignment of signals I and II.

Molybdoenzymes of the xanthine oxidase family contain two [2Fe-2S]1+,2+ clusters that are bound to the protein by very different cysteine motifs. In the X-ray crystal structure of Desulfovibrio gigas aldehyde oxidoreductase, the cluster ligated by a ferredoxin-type motif is close to the protein surface, whereas that ligated by an unusual cysteine motif is in contact with the molybdopterin [Romao, M. J., Archer, M., Moura, I., Moura, J. J. G., LeGall, J., Engh, R., Schneider, M., Hof, P., and Huber, R. (1995) Science 270, 1170−1176]. These two clusters display distinct electron paramagnetic resonance (EPR) signals:  the less anisotropic one, called signal I, is generally similar to the gav ≈ 1.96-type signals given by ferredoxins, whereas signal II often exhibits anomalous properties such as very large g values, broad lines, and very fast relaxation properties. A detailed comparison of the temperature dependence of the spin−lattice relaxation time and of the intensity of these signals in D. gigas aldehyde ...