The redox properties of the iron-sulphur cluster in hydrogenase from Chromatium vinosum, strain D.

The midpoint potentials of the changes in the electron spin resonance (ESR) spectra in the region of g = 2 in hydrogenase II from Chromatium vinosum were estimated by redox titrations. As the enzyme was progressively reduced, the g = 2.02 signal increased, while the satellite lines at g = 1.98 etc. decreased. At still lower potentials the signal at g = 2.02 decreased. The midpoint potentials of the two processes were estimated to be + 100 mV and - 20 mV, respectively, at pH 8.5. The first potential showed significant pH-dependence. The titration data fitted to n = 1 curves with reasonable reversibility. The enzyme activity showed no significant changes in this potential range. The results are discussed in relation to the interaction of the iron-sulphur cluster with nickel.