[Two stages in the digestion of proteins by pepsin]

Studies on a monosubstrate system (pepsin - substrate) yielded values which characterize pepsin digestion as a multi-stage process. During the first stage, pepsin hydrolyzes the most accessible parts of the protein molecule, i.e., those portions of the peptide chains which are situated on the surface of the globule. During the stage of apparent cessation of the reaction, very few peptide bonds are hydrolyzed which are essential for the formation of the tertiary structure. After hydrolysis of these bonds, the inner parts of the globule become accessible, the hydrolysis of which proceeds more slowly.