A cluster exposed: Structure of the Rieske ferredoxin from biphenyl dioxygenase and the redox properties of Rieske Fe-S proteins

Background: Ring-hydroxylating dioxygenases are multicomponent systems that initiate biodegradation of aromatic compounds. Many dioxygenase systems in clude Rieske-type ferredoxins with amino acid sequences and redox propertie s remarkably different from the Rieske proteins of proton-translocating res piratory and photosynthetic complexes. In the latter, the [Fe2S2] clusters lie near the protein surface, operate at potentials above +300 mV at pH 7, and express pH- and ionic strength-dependent redox behavior. The reduction potentials of the dioxygenase ferredoxins are approximately -150 mV and are pH-independent. These distinctions were predicted to arise from difference s in the exposure of the cluster and/or interactions of the histidine ligan ds.