2 PK VALUES OF THE OXIDIZED RIESKE [2FE-2S] CLUSTER OBSERVED BY CD SPECTROSCOPY

The pH dependence of the CD spectrum of the oxidised 'Rieske' [2Fe-2S] cluster was measured over pH = 6.1 to 10.6. The spectral changes coul d be fitted with two pK values of 7.7 and 9.1. No pH dependent spectra l changes were observed on the reduced protein. The far UV CD spectrum of the protein was the same at pH = 6.0 and pH = 10.7; therefore, sec ondary structure changes are not responsible for the changes in the CD spectrum. It is concluded that the pK values belong to groups directl y associated with the [2Fe-2S] cluster, most likely to its histidine l igands.