The Catalytic Effect of Active Crystalline Papain on the Denaturation of Thyroglobulin*

It has been assumed that denaturation occurs preliminary to enzymatic hydrolytic fission in some proteins. The basis of this supposition is the generally accepted observation that these proteins arc less readily attacked by proteolytic enzymes in their native state than when they are denatured (l-3). In the case of such readily digestible proteins as gelatin, prolamin, or casein, Linderstrom-Lang states that there is hardly any doubt that fission of peptide bonds is the initial and sole reaction, but in the case of the slowly digestible “crystalline or globular proteins” he believes that a secondary structure in the native protein blocks the peptide bonds to direct enzyme action. Such a blocking is thought to have its origin in steric hindrances or in the presence of chemical structures into which peptide bonds have themselves entered. To account for the diierence between the action of proteolytic enzymes on the native and denatured forms of the slowly digestible proteins and in the absence of the proof of a direct action of enzymes on the native proteins Linderstriim-Lang proposes that the process involves reversible denaturation. He assumes that native proteins as such are not attacked by the enzyme, but the change occurs as a result of an equilibrium between the native, N, and denatured, D, forms. Then, if the proteolytic enzyme promotes the removal of the denatured form by hydrolysis, the native form shifts to D to maintain the equilibrium as follows: