Copper binding traps the folded state of the SCO protein from Bacillus subtilis

► apo-BsSCO behaves as a two-state protein in kinetic and equilibrium studies. ► Cu(II) binding stabilizes reduced BsSCO making unfolding extremely slow. ►Cu(II) rapidly oxidizes the two cysteines of denature BsSCO. ► BsSCO folding with Cu(II) is a competition between oxidation and folding.