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 could be fitted with two pK values of 7.7 and 9.1. No pH dependent spectral 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, secondary structure changes are not responsible for the changes in the CD spectrum. It is concluded that the pK values belong to groups directly associated with the [2Fe-2S] cluster, most likely to its histidine ligands.