Axial methionine has much less influence on reduction potentials in a Cu_A center than in a blue copper center

The role of the highly conserved axial methionine of the purple Cu_A center in an engineered Cu_A azurin on modulating the reduction potentials of the copper center was investigated by a systematic replacement of the methionine with glutamate, aspartate, and leucine. In contrast to the same substitutions in the structurally related blue copper azurin, much smaller changes in reduction potential were observed in the Cu_A azurin upon replacing the methionine ligand with negatively charged Glu (-8 mV) and Asp (-5 mV) and more hydrophobic Leu (+16 mV). These findings are important in understanding the different roles of the two cupredoxins. The diamond core Cu₂S₂(Cys) structure of the CuA is much more resistant to variations of axial ligand interactions than the distorted tetrahedral structure of the blue copper protein. This difference may translate into a much wider range of reduction potentials (>1000 mV) for blue copper proteins that transfer electrons to a variety of partners in many different biological systems and a much narrower range of reduction potentials (<40 mV) for Cu_A proteins where a small difference in reduction potentials between the Cu_A and its redox partners is required.