Unusual Reduction Mechanism of Copper in Cysteine-Rich Environment.

Copper-cysteine interactions play an important role in Biology and herein we used the copper-substituted rubredoxin (Cu-Rd) from Desulfovibrio gigas to gain further insights into the copper-cysteine redox chemistry. EPR spectroscopy results are consistent with Cu-Rd harboring a Cu center in a sulfur-rich coordination, in a distorted tetrahedral structure ( g = 2.183 and 2.032 and A = 76.4 × 10 and 12 × 10 cm). In Cu-Rd, two oxidation states at Cu-center (Cu and Cu) are associated with Cys oxidation-reduction, alternating in the redox cycle, as pointed by electrochemical studies that suggest internal geometry rearrangements associated with the electron transfer processes. The midpoint potential of [Cu(S-Cys)(Cys-S-S-Cys)]/[Cu(S-Cys)] redox couple was found to be -0.15 V vs NHE showing a large separation of cathodic and anodic peaks potential (Δ E = 0.575 V). Interestingly, sulfur-rich Cu-Rd is highly stable under argon in dark conditions, which is thermodynamically unfavorable to Cu-thiol autoreduction. The reduction of copper and concomitant oxidation of Cys can both undergo two possible pathways: oxidative as well as photochemical. Under O, Cu plays the role of the electron carrier from one Cys to O followed by internal geometry rearrangement at the Cu site, which facilitates reduction at Cu-center to yield Cu(S-Cys)(Cys-S-S-Cys). Photoinduced (irradiated at λ = 280 nm) reduction of the Cu center is observed by UV-visible photolysis (above 300 nm all bands disappeared) and tryptophan fluorescence (∼335 nm peak enhanced) experiments. In both pathways, geometry reorganization plays an important role in copper reduction yielding an energetically compatible donor-acceptor system. This model system provides unusual stability and redox chemistry rather than the universal Cu-thiol auto redox chemistry in cysteine-rich copper complexes.