The internal residues in proteins can be modified by attachment of a variety of chemical groups to their side chains. Glycosylation, the attachment of carbohydrate groups to the side chains of asparagine, serine, or theronine is important in the synthesis of many secreted and cell-surface glycoproteins. Another common modification is phosphorylation, the substitution of a phosphate group for a hydroxyl group in serine, threonine, and tyrosine residues. In addition to chemical modification of residues, processing of proteins alters their activity. Proteolytic cleavage is a common mechanism of activation or inactivation, especially of enzymes involve in blood coagulation digestion.
Proteolysis also generates active peptide hormones from larger precursor polypeptides.
Three sucessive enzyme-catalyzed cleavages produce inulin from preproinsulin. The first cleavage occurs immediately after synthesis of preproinsulin, a single chain of 108 amino acids. This cleavage removes the 24- amino acids which is a signal sequence from the amino end of the molecule. The remaining 84 amino acids constitute proinsulin, a molecule in which all the correct disulfide bridges are present. While the hormone is being packaged for secretion, 33-residue C segment is removed via two proteolytic cleavages, yielding A and B chains of insulin.