H135A controls the redox activity of the Sco copper center. Kinetic and spectroscopic studies of the His135Ala variant of Bacillus subtilis Sco

Sco-like proteins contain copper bound by two cysteines and a histidine residue. Although their function is still incompletely understood, there is a clear involvement with the assembly of cytochrome oxidases that contain the Cu_A center in subunit 2, possibly mediating the transfer of copper into the Cu_A binuclear site. We are investigating the reaction chemistry of BSco, the homologue from Bacillus subtilis. Our studies have revealed that BSco behaves more like a redox protein than a metallochaperone. The essential H135 residue that coordinates copper plays a role in stabilizing the Cu(II) rather than the Cu(I) form.When H135 is mutated to alanine, the oxidation rate of both hydrogen peroxide and one-electron outer-sphere reductants increases by 3 orders of magnitude, suggestive of a redox switch mechanism between the His-on and His-off conformational states of the protein. Imidazole binds to the H135A protein, restoring the N superhyperfine coupling in the EPR, but is unable to rescue the redox properties of wild-type Sco. These findings reveal a unique role for H135 in Sco function. We propose a hypothesis that electron transfer from Sco to the maturing oxidase may be essential for proper maturation and/or protection from oxidative damage during the assembly process. The findings also suggest that interaction of Sco with its protein partner(s) may perturb the Cu(II)-H135 interaction and thus induce a sensitive redox activity to the protein.