The first cleavage site in pepsinogen activation

Activation mechanism of monkey and porcine pepsinogens A. One-step and stepwise activation pathways and their relation to intramolecular and intermolecular reactions

The activation of Sepharose-bound monkey pepsinogen A under acidic conditions proceeded by cleavage of the Leu47-Ile48 bond, indicating the occurrence of the intramolecular one-step activation, although the rate of cleavage was very slow. On the other hand the activation of monkey pepsinogen A in solution was highly dependent on pepsinogen concentration and the addition of exogenous pepsin A accelerated the rate of activation, indicating the predominance of intermolecular reaction. The cleavage site, however, was also restricted to the Leu47-Ile48 bond. Thus, apparently exclusive one-step activation occurred in monkey pepsinogen. The activation of porcine pepsinogen A in solution was also dependent on pepsinogen concentration and the addition of exogenous pepsin A accelerated the rate of activation. The major cleavage site by the exogenously added pepsin was the Leu44-Ile45 bond. Therefore the site most susceptible to the intermolecular attacks was the bond connecting the activation segment and the pepsin moiety in both monkey and porcine pepsinogens. In porcine pepsinogen, however, a part of the zymogen was activated through the intermediate form, and an intramolecular reaction was suggested to be involved in the generation of this form. These results showed that in both pepsinogens A the intramolecular reaction occurred, first yielding pepsin A or the intermediate form, which then acted intermolecularly on the remaining pepsinogen or the intermediate form to complete the activation in a short time. A molecular mechanism for the activation reaction was proposed to explain consistently the experimental results.

Molecular structure of an aspartic proteinase zymogen, porcine pepsinogen, at 1.8 A resolution

The only well-understood mechanism of zymogen activation is that of the serine proteinases, in which proteolytic cleavage leads to conformational changes resulting in a functional active site. A different mechanism is now unveiled by the crystal structure of pepsinogen. Salt bridges that stabilize the positioning of the N-terminal proenzyme segment across the active site of pepsin are disrupted at low pH, releasing the amino-terminal segment and thereby exposing the catalytic apparatus and the substrate-binding sites.