Active site geometry in the high oxido-reduction potential rusticyanin from Thiobacillus ferrooxidans

Rusticyanin is a blue copper protein involved in the oxidation of iron catalyzed by Thiobacillus ferrooxidans. This protein is characterized by a high oxido-reduction potential and a high stability at low pH. The three dimensional structure of this protein is still unknown and in order to investigate the geometric properties of the copper center which could be correlated to the high oxido-reduction potential, we have studied rusticyanin by UV-Visible, EPR and NMR spectroscopies, at different pH values. Our results suggest that rusticyanin is stable between pH 2 and pH 9 and that the copper center does not undergo significant geometric modifications in this pH range. Moreover, the copper atom could be buried more deeply in the protein than in other type I copper proteins and the atomic distance Cu-S(Met), one of the four bonds involved in copper coordination, is probably shorter in rusticyanin than in other cupredoxins. These two properties of the copper site are expected to be responsible, in part, for the high oxido-reduction potential observed in rusticyanin.