CopA from the extreme thermophile Archaeoglobus fulgidus is a P-type ATPase that transports Cu+ and Ag+ and has individual metal-binding domains (MBDs) at both N- and C-termini. We expressed and purified full-length CopA as well as constructs with MBDs deleted either individually or collectively. Cu+ and Ag+-dependent ATPase assays showed that full-length CopA had submicromolar affinity for both ions, but was inhibited by concentrations above 1 mu M. Deletion of both MBDs had no effect on affinity but resulted in loss of this inhibition. Individual deletions implicated the N-terminal MBD in causing the inhibition at concentrations > 1 PM. Rates of phosphoenzyme decay indicated that neither the dephosphorylation step, nor the E1P-E2P equilibrium accounted for this inhibition, suggesting the involvement of a different catalytic step. Alternative hypotheses are discussed by which the N-terminal MBD could influence the catalytic activity of CopA. (c) 2006 Elsevier Inc. All rights reserved.