Redox properties of an engineered purple CuA azurin

Purple CuA centers are a class of binuclear, mixed-valence copper complexes found in cytochrome c oxidase and nitrous oxide reductase. An engineered CuA protein was formed by replacing a portion of the amino acid sequence that contains three of the ligands to the native type I copper center of Pseudomonas aeruginosa azurin with the corresponding portion of sequence from the CuA center of cytochrome c oxidase from Paracoccus denitrificans [Proc. Natl. Acad. Sci. USA 93 (1996) 461]. Oxidation–reduction midpoint potential (Em) values of the CuA azurin of +399±10 and +380±2 mV, respectively, were determined by cyclic voltammetry and spectrochemical titration. An n value of one was obtained, indicating that the redox reaction is cycling between the mixed valence and the fully reduced states. Whereas the Em value of native azurin is pH dependent, the Em value of CuA azurin is not, as expected for the CuA center. Similarities and differences in the redox properties are discussed in terms of the known crystal structures of CuA centers in cytochrome c oxidase and CuA azurin.