The cyclin-dependent kinase CDK2 is inactivated by phosphorylation on either of the two neighbouring residues Thr14 or Tyr15. The effect of phosphorylation oil metal ion binding has been investigated with peptides incorporating residues 6-20 of CDK2. The stoichiometry of Ca2+ binding increased from I in the un- and singly-phosphorylated peptides to 2 in the doubly phosphorylated peptide, without large changes in the affinity (75-250 mu M). In contrast although binding of ferric ions to the un-phosphorylated Peptide was not detected, both Singly- and (doubly-phosphorylated peptides bound two Fe3+ ions. Binding of Ca2+ or Zn 21 ions to the doubly phosphorylated CDK2 peptide did not cause any change in absorbance, but increased the affinity of the peptide for Fe3+ ions. These results demonstrate that double phosphorylation of CDK2 peptides increases the stoichiometry of metal ion binding, and hence May contribute to the previously observed regulation of CDK2 activity by metal ions. (C) 2008 Elsevier Inc. All rights reserved