Tyrosine modification enhances metal-ion binding

Tyrosine sulfation is a common modification of many proteins, and the ability to phosphorylate tyrosine residues is an intrinsic property of many growth-factor receptors. In the present study, we have utilized the peptide hormone CCK(8) (cholecystokinin), which occurs naturally in both sulfated and unsulfated forms, as a model to investigate the effect of tyrosine modification on metal-ion binding. The changes in absorbance and fluorescence emission on Fe(3+) binding indicated that tyrosine sulfation or phosphorylation increased the stoichiometry from 1 to 2, without greatly affecting the affinity (0.6-2.8 microM at pH 6.5). Measurement of Ca(2+) binding with a Ca(2+)-selective electrode revealed that phosphorylated CCK(8) bound two Ca(2+) ions. CCK(8) and sulfated CCK(8) each bound only one Ca(2+) ion with lower affinity. Binding of Ca(2+), Zn(2+) or Bi(3+) to phosphorylated CCK(8) did not cause any change in absorbance, but substantially increased the change in absorbance on subsequent addition of Fe(3+). The results of the present study demonstrate that tyrosine modification may increase the affinity of metal-ion binding to peptides, and imply that metal ions may directly regulate many signalling pathways.