title: Tyrosine modification enhances metal-ion binding.
abstract: 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.
authors.affiliation: The University of Melbourne Department of Surgery, Austin Health, Heidelberg, Victoria 3084, Australia. grahamsb@unimelb.edu.au
authors.forename: Graham S
authors.initials: GS
authors.lastname: Baldwin
authors.forename: Michael F
authors.initials: MF
authors.lastname: Bailey
authors.forename: B Philip
authors.initials: BP
authors.lastname: Shehan
authors.forename: Ioulia
authors.initials: I
authors.lastname: Sims
authors.forename: Raymond S
authors.initials: RS
authors.lastname: Norton
mesh_terms: D000818:Animals; D001729:Bismuth / Q000378:metabolism; D019556:COS Cells; D002118:Calcium / Q000378:metabolism*; D002522:Chlorocebus aethiops; D002766:Cholecystokinin / Q000737:chemistry* / Q000378:metabolism; D005290:Ferric Compounds / Q000378:metabolism*; D006801:Humans; D010446:Peptide Fragments / Q000737:chemistry* / Q000378:metabolism; D010766:Phosphorylation; D011949:Receptors, Cholecystokinin / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000737:chemistry* / Q000378:metabolism*; D015032:Zinc / Q000378:metabolism
chemical_list: D005290:Ferric Compounds; D010446:Peptide Fragments; D011949:Receptors, Cholecystokinin; C410885:cholecystokinin 8; C005662:tyrosine O-sulfate; D014443:Tyrosine; D002766:Cholecystokinin; D015032:Zinc; D002118:Calcium; D001729:Bismuth