Mimicking protein-protein electron transfer:: Voltammetry of Pseudomonas aeruginosa azurin and the Thermus thermophilus CuA domain at ω-derivatized self-assembled-monolayer gold electrodes

Well-defined voltammetric responses of redox proteins with acidic-to-neutral pI values have been obtained on pure alkanethiol as well as on mixed self-assembled-monolayer (SAM) omega-derivatized alkanethiol/gold bead electrodes. Both azurin (P. aeruginosa) (pI = 5.6) and subunit II (Cu-A domain) of ba(3)-type cytochrome c oxidase (T. thermophilus) (pI = 6.0) exhibit optimal voltammetric responses on 1:1 mixtures of [H3C(CH2)(n)SH + HO(CH2)(n)SH] SAMs. The electron transfer (ET) rate vs distance behavior of azurin and Cu-A is independent of the w-derivatized alkanethiol SAM headgroups. Strikingly, only wild-type azurin and mutants containing Trp48 give voltammetric responses: based on modeling, we suggest that electronic coupling with the SAM headgroup (H3C- and/or HO-) occurs at the Asn47 side chain carbonyl oxygen and that an Asn47-Cys112 hydrogen bond promotes intramolecular ET to the copper. Inspection of models also indicates that the Cu-A domain of ba3-type cytochrome c oxidase is coupled to the SAM headgroup (H3C- and/or HO-) near the main chain carbonyl oxygen of Cys153 and that Phe88 (analogous to Trpl 43 in subunit II of cytochrome c oxidase from R. sphaeroides) is not involved in the dominant tunneling pathway. Our work suggests that hydrogen bonds from hydroxyl or other proton-donor groups to carbonyl oxygens potentially can facilitate intermolecular ET between physiological redox partners.