Pepsinogen structure

Pepsin is secreted as the inactive pepsinogen, which is activated by H+ ions at a pH below 5. Determination of its crystal structure revealed that in the proenzyme the N-terminal 44-residue peptide segment lies across the active site, blocking it.384 At low pH the salt bridges that stabilize the proenzyme are disrupted and the active site is opened up to substrates. 

The inactive precursors are called trypsinogen, pepsinogen, chymotrypsino-gen, and procarboxypeptidase. These precursors are converted to the active enzymes by hydrolytic cleavage of a few specific peptide bonds under the influence of other enzymes (trypsin, for example, converts chymotrypsinogen to chymotrypsin). The digestive enzymes do not appear to self-destruct, probably because they are so constructed that it is sterically impossible to fit a part of one enzyme molecule into the active site of another. In this connection, it is significant that chymotrypsin attacks denatured proteins more rapidly than natural proteins with their compact structures of precisely folded chains. 

Figure 10-10 Structure of Pepsinogen and Its Conversion to Pepsin. Source From F.A. Bovey and S.S. Yanari, Pepsin, in The Enzymes, Vol. 4, P.D. Boyer et al., eds., 1960, Academic Press.

The first structure of human renin was obtained from prorenin produced by expression of its cDNA in transfected mammalian cells. Prorenin was cleaved in the laboratory to renin using the protease trypsin. Because the carbohydrates in renin are not required for bioactivity, oligosaccharides were removed enzymatically. This process facilitates crystallization in some cases and also removes the contribution of the heterogeneous sugar chains to the diffraction pattern. The structure was determined without the use of heavy-atom derivatives, by application of molecular replacement techniques based on the atomic coordinates of porcine pepsinogen as the model. The molecular dynamic method of refinement was used extensively to arrive at a 2.5 A resolution structure. However, some of the loop regions were not well resolved in this structure (Sielecki et al, 1989 Sail et al, 1990). 

V3. Van Vunakis, H., and Herriott, R., Structural changes associated with the conversion of pepsinogen to pepsin, I. The N-terminal amino acid residue and amino acid composition of the pepsin inhibitor. Biochim. Biophys. Acta 22, 537-543 (1956). 

Production of pepsin and other proteolytic digestive enz)unes must be carefully controlled because the active enzymes would digest and destroy the cell that produces them. Thus, the stomach lining cells that make pepsin actually synthesize and secrete an inactive form called pepsinogen. Pepsinogen has an additional forty-two amino acids in its primary structure. These are removed in the stomach to produce active pepsin. 

Figure 2-2. Roger Herriott s 1962 diagrammatic sketch of the pepsin and pepsinogen amino acid structure. (Reproduced from Herriott RM, Pepsinogen and pepsin. / Gen Physiol 45 57-76, 1962, by copyright permission of the Rockefeller University Press.)...

The effects of ethanol on the conformation of aj-antitrypsin, deoxyribonuclease, pepsinogen, soybean trypsin inhibitor, and unfolded ribonucleases have been studied by c.d. spectroscopy. In the presence of 50—75 volume % ethanol, the tertiary structure is perturbed and the polypeptide chain is reorganized into new conformations with higher contents of helix and /3-structure than those in the native state. 

Most proteins are synthesized in a neutral-pH environment thus their natural conformation state and functionality are adapted to this enviromnent. However, most aspartic proteinases are stable under acidic conditions and become irreversibly denatured at neutral pH conditions (Bohak, 1969 Fm-ton, 1971). In the case of pepsin, its zymogen, pepsinogen, is stable under neutral-pH conditions. Since pepsin and pepsinogen are almost identical in structure, minor differences may be responsible for pepsin s instability at neutral pHs. Why is pepsin so unstable, despite being similar in conformation to pepsinogen In a recent study, we undertook various mutations in attempts to stabilize the stmcture of pepsin. 

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