One of the following is an example of phosphoprotein

The phosphate groups add negative charges to the polypeptides. The functional significance of this modification varies from one protein to the next. For example, the milk protein casein has many phosphoserine groups that bind Ca2+. Calcium, phosphate, and amino acids are all valuable to suckling young, so casein efficiently provides three essential nutrients. And as we have seen in numerous instances, phosphorylation dephosphorylation cycles regulate the activity of many enzymes and regulatory proteins. Ref: Lehninger's Principles of Biochemistry 7th Editoin - D L Nelson, Cox Lehninger - W H Freeman Pg no 2901 Ref: Lehninger's Principles of Biochemistry 4th Editoin - D L Nelson, Cox Lehninger - W H Freeman Pg no 1063.