Mets motifs, which refer to methionine-rich sequences found in the high-affinity copper transporter Ctr1, also appear in other proteins involved in copper trafficking and homeostasis, including other Ctrs as well as Pco and Cop proteins isolated from copper-resistant bacteria, To understand the coordination chemistry utilized by these proteins, we studied the copper binding properties of a peptide labeled Mets7-PcoC with the sequence Met-Thr-Gly-Met-Lys-Gly-Met-Ser. By comparing this sequence to a series of mutants containing noncoordinating norleucine in place of methionine, we confirm that all three methionine residues are involved in a thioether-only binding site that is selective for Cu(I). Two independent methods, one based on mass spectrometry and one based on rate differences for the copper-catalyzed oxidation of ascorbic acid, provide an effective K-D of similar to 2.5 mu M at pH 4.5 for the 1:1 complex of Mets7-PcoC with Cu(I). These results establish that a relatively simple peptide containing an MX2-MX2M motif is sufficient to bind Cu(I) with an affinity that corresponds well with its proposed biological function of extracellular copper acquisition.