Mimicking protein-protein electron transfer: voltammetry of Pseudomonas aeruginosa azurin and the Thermus thermophilus Cu_A 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) ω-derivatized alkanethiol/gold bead electrodes. Both azurin (P. aeruginosa) (pI = 5.6) and subunit II (CuA domain) of ba_3-type cytochrome c oxidase (T. thermophilus) (pI = 6.0) exhibit optimal voltammetric responses on 1:1 mixtures of [H_3C(CH_2)_nSH + HO(CH_2)_nSH] SAMs. The electron transfer (ET) rate vs distance behavior of azurin and CuA is independent of the ω-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 (H_3C− 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 CuA domain of ba3-type cytochrome c oxidase is coupled to the SAM headgroup (H_3C− and/or HO−) near the main chain carbonyl oxygen of Cys153 and that Phe88 (analogous to Trp143 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