Opening Image: Porcine pepsinogen, shown here in spacefill.

The N-terminal half of the propeptide is locked in an extended conformation that lies in a groove in the pepsin moiety. This groove leads to the much wider and deeper active site cleft. Here the C-terminal half of the propeptide becomes more globular, giving the propeptide a comma-like profile.

Initially some pepsinogen is activated slowly by H⁺. The pepsin formed can then quickly activate other pepsinogen molecules by cleaving the peptide bond between Leu-44p and Ile-1 (the N-terminal residue of pepsin).

The atoms are now colored by partial charge. At pH 7 pepsinogen bears 45 negative charges and 17 positive charges, assuming that the 3 histdine residues are fully ionized. In view of a net charge of -28, one might expect pepsinogen to be unstable at neutral pH. Why is pepsinogen stable at pH 7?

Color pepsin moiety resides by charge (acidic and basic).

The answer lies in the fact that propeptide contains 3 acidic and 13 basic residues out of the 44 in pepsinogen.

Color charged residues in the propeptide.

The 14 positive charges of the propeptide form an array of salt bridges with side chain carboxyls along the walls of the groove. This network of salt bridges is the electrostatic "glue" that holds the two lobes of the molecule together despite a net charge of -28. Some of the salt bridges lie at surface. These may be weaker than the salt bridges that are sandwiched below the surface between the propeptide and the pepsin moiety.

Isolate the propeptide. Display the backbone model with charged residues colored in CPK.

A good example of a strong electrostatic interaction is Lys-36p, which forms a salt bridge between the two active-site aspartate side chain carboxyls. Lys-36p extends toward the bottom of the active site cleft, where it forms a salt bridge with the two carboxyls of the catayltic dyad.

Lys-36p of the propeptide interacts strongly with the two catalytic aspartates. Toggle spacefill.

Let's return to the zymogen molecule. Acidic and basic residues are colored red and blue respectively. Can you find Lys 36p? Toggle cutaway view.

In summary, the propeptide forms a "belt" two thirds of the perimeter of the zymogen. Salt bridges between the basic residues of the propeptide and nearby side chain carboxyls of the pepsin moiety stabilize the zymogen despite a net charge of -28 at pH 7. Exposure of pepsinogen to low pH disrupts the salt bridges as the side chain carboxyls of the the pepsin moiety (that part of zymogen which becomes the active enzyme) are protonated. This leads to activation of the zymogen.