alpha-Factor-1 (WHWLQLKPGQPMY), a peptidic pheromone of Saccharomyces cerevisiae yeast, contains a XHX type copper(II) binding N-terminal site. Using a soluble analogue, WHWSKNR-amide, we demonstrated that the (WHW3)-H-1-W-2 site alone binds copper (II) with a K-d, value of 0.18 pM at pH 7.4 and also binds imidazole (Im) in a ternary complex (K-d of 1 mM at pH 7.4). This interaction boosts the ability of the peptide to sequester copper(II) depending on the Im concentration up to a subfemtomolar range, not available for any oligopeptidic system studied before. Therefore, alpha-factor-1 and other XHX-type peptides are likely copper(II) carriers in biological systems.