A Competition Study on Copper-binding Affinity of SCO protein
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Cytochrome c oxidase catalyzes the reduction of molecular oxygen to water and contributes to the electrochemical gradient by translocating protons across the membrane. The SCO protein (for Synthesis of Cytochrome c Oxidase) is proposed to be an important assembly factor in biogenesis of the oxidase. Particularly, SCO has been demonstrated to function as a metallochaperone that receives copper ions from an upstream copper source in the cell and subsequently delivers them to the CuA centre in subunit II of cytochrome c oxidase. However, the SCO protein binds copper ions tightly and forms a stable complex in vitro that is extremely difficult to dissociate. Direct titration and differential scanning calorimetry, for example, have demonstrated a tight binding between SCO and Cu(II). Nonetheless, the reported dissociation constant KD falls in a wide range from 65 nM to 3.5 pM. In this study, binding affinities of Bacillus subtilis SCO (BsSCO) for both Cu(II) and Cu(I) ions were quantitatively estimated via competition with various copper ion ligands and chelators. Ethylenediamine tetraacetic acid (EDTA) was used as a competitor to BsSCO. In this case, BsSCO is able to compete with the chelator for copper-binding, which binds Cu(II) with KD ~ 3.1 x 10-16 M (i.e., 0.31 fM). Estimation of binding affinity via competition provides a different perspective of quantifying the interaction between the SCO protein and copper ions. The tight binding (i.e., in the fM range) between the Cu(II) and the SCO protein suggests that Cu(II) ion is unlikely to be the oxidation form during the transfer from SCO to the CuA site. Strong Cu(I) ligands, such as ferrozine (Fz), were introduced in quantification of the Cu(I) binding affinity of the SCO protein. Competition with Fz for Cu(I) binding supports a relatively weak interaction between BsSCO and Cu(I) ions, which characterized with KD ~10 μM. Therefore, we propose that a stable BsSCO-Cu(II) complex could be reduced to BsSCO-Cu(I) in order for the SCO protein to fulfill a copper chaperone role in vivo that delivers and releases its copper ion to the CuA site of cytochrome c oxidase.