Mechanism of Metal delivery and binding to transport sites of Cu+-transporting ATPases

CopA, a thermophilic membrane ATPase from Archaeoglobus fulgidus, drives the outward movement of Cu across cellular membranes. CopA contains at least two metal binding domains, a regulatory N-terminal Metal Binding Domain (N-MBD) and an occlusion/coordinating metal binding site in the 6, 7 and 8 transmembrane segments. Previous studies showed that the presence of millimolar concentration of Cys is essential for CopA activity. The high affinity of CopA for metal in the presence of millimolar concentration of Cys (consider Cu-Cys KD < 10 M) suggests a multifaceted interaction of the enzyme with Cys. To elucidate the role of Cys, we studied its effect on the partial reactions of the catalytic cycle of CopA. We observed that 2-50 mM Cys accelerates enzyme turnover with little effect on the Cu affinity of CopA. Cys accelerates enzyme phosphorylation, but has no effect on the dephosphorylation rates. Thus, Cys increases steady state phosphoenzyme levels. Besides, Cys has no significant effect on E1↔E2 equilibrium. Similar results were observed in truncated CopA lacking the N-MBD suggesting that enzyme activation by Cys is independent of the regulatory metal binding sites. These results and the kinetic analysis of activation curves suggest that while Cu is delivered to the transport site as a Cu-Cys complex, Cys in the mM range stimulates the ATPase acting as a non-essential activator.