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Hoffmann, R. A wiki for the life sciences where authorship matters. Nature Genetics (2008)

The location and nature of calcium-binding sites in salivary acidic proline-rich phosphoproteins.

The location of the calcium-binding sites in the human acidic proline-rich proteins, salivary proteins A and C, were determined by equilibrium dialysis of the tryptic peptides with buffers containing 45Ca. All the calcium-binding sites are located in the NH2-terminal tryptic peptide ( TX peptide). The nature of the calcium binding sites in the TX peptide and native salivary proteins A and C, as well as dephosphorylated proteins were compared. Two types of sites can be distinguished in peptide TX. Type I sites have an apparent dissociation constant (K) of 38 microM and are responsible for the binding of 2.6 mol of Ca/ mol of peptide. The corresponding figures for Type II sites are 780 microM and 5.3 mol of Ca/ mol of peptide. In the native proteins, the amount of calcium bound at the type II sites decreases to 3.9 mol of Ca/ mol of proteins A and C and K increases to 1100 microM. The amount of calcium bound at type I sites decreases to 1.5 mol/ mol of protein A and 0.6 mol/ mol of protein C, but there is no change in K. Dephosphorylation affects the calcium binding at both types of sites. The experiments indicate that the COOH-terminal parts of the native proteins affect the number and the nature of the protein calcium-binding sites. Proton and phosphorous NMR data demonstrate that beta-COOH in aspartic acid, as well as phosphoserine, are part of the calcium-binding sites. The difference in calcium binding to salivary proteins A and C may be due at least partially to differences in the environment of one or more aspartic acids.[1]


  1. The location and nature of calcium-binding sites in salivary acidic proline-rich phosphoproteins. Bennick, A., McLaughlin, A.C., Grey, A.A., Madapallimattam, G. J. Biol. Chem. (1981) [Pubmed]
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