Stability of oxidized, reduced and copper bound forms of Bacillus subtilis Sco

Sco is an accessory protein required for assembly of the CuA center of cytochrome c oxidase. Functions proposed for Sco include as a copper chaperone and as a thiol-disulfide exchange protein. Differential scanning calorimetry (DSC) is used here to assess the interaction between the Bacillis subtilis version of Sco (BsSco) and Cu(II). When BsSco binds Cu(II) its melting temperature increases by 23 °C, which corresponds to an equilibrium dissociation constant of 3.50 pM. In contrast BsSco exhibits a much weaker affinity for Cu(I) (KD = 10 μM). BsSco-Cu(II) is stable over days indicating an extremely slow dissociation for BsSco-Cu(II). However, at high ionic strength in the presence of excess copper, BsSco-Cu(II) returns to its oxidized, disulfide-bonded state and loses its copper binding capacity with a half time of 100 s. DSC of BsSco at high ionic strength indicates an increase in stability of metal free, reduced BsSco combined with a small destabilization of BsSco-Cu(II). It is proposed that BsSco undergoes an ionic strength induced conformational change that promotes electron transfer from the thiol groups on BsSco to Cu(II) to effect copper release. Such a redox transformation could be an important aspect of the copper transfer role proposed for BsSco in CuA assembly.