ABSTRACT
One of the most interesting examples of limited proteolysis is the conversion of proinsulin to insulin. Biosynthesis of trypsinogen and chymotrypsinogen and their conversion to active enzymes are interesting illustrations not only of limited proteolysis but also of mechanisms that an organism uses to protect itself from digestion by proteolytic enzymes. Solution of a protein may contain at least two forms, a native form and a reversibly denatured form, and only the reversibly denatured form is subject to proteolysis. Irreversibly denatured protein is also subject to proteolysis. In most cases a protein molecule becomes more susceptible to further proteolysis after initial hydrolysis of one or a few peptide bonds. The blood clotting mechanism is the most complicated process of limited proteolysis known. Pepsin is produced only when the bond is hydrolyzed. Pepsin is quite resistant to further proteolysis in acid solution, but at pH 5 to 7 it is rapidly inactivated, in contrast to pepsinogen.
