Casein enzymatic dephosphorylation

Figure 1.50 uses P-casein as an example to show that the solubihty of a phosphoprotein in the presence of calcium ions is greatly improved by partial enzymatic dephosphorylation. 

In experiments in which a- and (3-casein are remixed in different proportions, it is noticed that if the relative concentration of the (3-casein in the mixture exceeds 20% the presence of this protein inhibits the enzymatic action, the degree of inhibition being proportional to the concentration of the (3-protein. These results may be taken as an explanation for the failure of previous investigators to dephosphorylate unfractionated casein without preceding transformation to phosphopeptones. 

Phosphoamino acids that are part of proteins known to bind metal ions are posttranslational modifications introduced by specific protein kinases (Meggio et al, 1981 Vogel and Biidger, 1982c). The bovine milk protein casein and the hen egg-white protein ovalbumin, as well as possibly the human saliva acidic proline-iich proteins share sequence homology of their phosphorylated sites. Dephosphorylation of such sites by enzymatic phosphatase treatment usually reduces the affinity of such proteins for metal-ion binding (Bennick et al., 1981). Hence it is likely that dianionic phosphoryl moieties are directly involved in the complexation of metal ions. This seems particularly important for the two polyelectrolyte proteins that contain large amounts of phosphoserine residues, phosvitin purified from egg yolk (Ta-borsky, 1974), and the phosphoprotein purified from dentine (Linde et al, 1980). 

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