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 <i>Desulfovibrio gigas</i> to gain further insights into the copper-cysteine redox chemistry. EPR spectroscopy results are consistent with Cu-Rd harboring a Cu^II center in a sulfur-rich coordination, in a distorted tetrahedral structure (<i>g</i>_∥,⊥ = 2.183 and 2.032 and <i>A</i>_∥,⊥ = 76.4 × 10^–4 and 12 × 10^–4 cm^–1). In Cu-Rd, two oxidation states at Cu-center (Cu^II and Cu^I) 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^I(S–Cys)₂(Cys–S–S–Cys)]/[Cu^II(S–Cys)₄] redox couple was found to be −0.15 V vs NHE showing a large separation of cathodic and anodic peaks potential (Δ<i>E</i>_p = 0.575 V). Interestingly, sulfur-rich Cu^II-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 O₂, Cu^II plays the role of the electron carrier from one Cys to O₂ followed by internal geometry rearrangement at the Cu site, which facilitates reduction at Cu-center to yield Cu^I(S–Cys)₂(Cys–S–S–Cys). Photoinduced (irradiated at λ_ex = 280 nm) reduction of the Cu^II 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