Covalent modifications cause conformational changes in the enzymes. Enzymes can be covalently modified to attain their active form by phosphorylation and proteolysis. In phosphorylation, conformational changes are brought about by covalent bonding of a phosphate group to the polypeptide chain, for example, the conversion of glycogen phosphorylase. Glycogen phosphorylase occurs in two forms: phosphorylase a (active form) and phosphorylase b (relatively inactive form). Both forms are inter-convert-able. Phosphorylase b is converted into active phosphorylase or by covalent binding of phosphate groups.
Phosphorylase kinase enzyme involve in this conversion. The second way, in which enzymes can be covalently modified to attain their active form, is proteolysis. In proteolysis, conformational changes are brought by removal of a small peptide chain from the inactive enzyme. Converts it into active form, for example, a number of digestive enzymes including pepsin, trypsin, chemotropism belong to this group. These enzymes are synthesized in their inactive forms. known as pro-enzymes or zymogen. These pro-enzymes are converted into active forms by the removal of a small peptide chain. Trypsinogen (pro-enzyme) in converted to active trypsin by the removal of six amino acids from the n—terminus of the inactive enzyme. Enzyme enterokinase takes part in this reaction.