Desulfovibrio Gigas Hydrogenase: Redox Properties of the Nickel and Iron‐Sulfur Centers

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Abstract
Below 30 K, oxidized Desulfovibrio gigas hydrogenase presents an intense electron paramagnetic resonance (EPR) signal centered at g= 2.02, typical of an iron-sulfur center. In addition a rhombic EPR signal, attributed to Ni(III) species, is also observed [LeGall, J., Ljungdahl, P., Moura, I., Peck, H. D., Jr, Xavier, A. V., Moura, J. J. G., Teixeira, M., Huynh, B. H., and DerVartanian, D. V. (1982) Biochem. Biophys. Res. Commun. 106, 610–616; and Cammack, R., Patil, D., Aguirre, R., and Hatchikian, E. C., (1982) FEBS Lett. 142, 289–292]. At higher temperatures (77 K) the iron-sulfur EPR signal is broader and all the EPR features of the rhombic nickel signal can easily be observed. We have now obtained additional information concerning the redox properties of these EPR active centers, using an EPR redox titration method in the presence of dye mediators at pH = 8.5. The mid-point potential was determined to be - 70 mV for the Fe, S cluster and - 220 mV for the Ni center. Intermediate oxidation states were obtained upon partial reduction with either dithionite or hydrogen. Although upon dithionite reduction the centers are reduced in the order of decreasing mid-point reduction potentials, under a hydrogen atmosphere the nickel center reduces preferentially. This suggests a catalytic involvement of the nickel redox center in the binding of hydrogen. Preliminary Mössbauer studies on Desulfovibrio gigas hydrogenase reveal the presence of a paramagnetic 3 Fe center and two 4 Fe centers. The 3 Fe center is responsible for the g= 2.02 EPR signal but the two 4 Fe centers have been so far undetectable by EPR.