Phosphoprotein are complex proteins, containing phosphoric acid, connected with the polypeptide chain. Phosphoric acid can easily be chipped off under the action of alkali and much more difficult - in the presence of acids. To phosphoprotein include casein, the whites of eggs (ovalbumin, vitellin, Tsviten) and some tissue enzymes (fosfaurilaza And, glucokinase and others), with which the transfer of a phosphate group. Reversible phosphorylation (see) of a number of proteins plays an important biological role in muscle contraction, regulation of gene activity and other body functions.