Pepsinogen, conformation

In contrast, however, we have demonstrated that the macromolecular conformation of the zymogen differs markedly from that of the enzyme (17). Thus, if pepsinogen is transferred from an aqueous solution to concentrated urea, the specific rotation, [a]m, decreases from —200° to —320° in the concentration range of 1.5 to 4.0M urea, and the rotatory dispersion constant, Xc, decreases from 236 to 216 m/z. As shown in Figure 2, which also includes the results obtained with pepsin, this change reflects a configurational transition, similar in sharpness to the transition from an a-... 

In view of the fact that the number of basic amino acid residues in pepsinogen exceeds that present in pepsin, and these residues may function as conformational determinants, the dependence of the specific optical rotation, [a ]366, on the pH of the solution had to be considered. If the pH of the solution is altered from 6.5 to 11.5, the levorotation increases markedly in the pH range of 9.2 to 10.8 with the mid-point at pH 10.0. This value approximates the apparent pK of the -amino group of the lysyl residues if present in peptide linkage. 

Turning now to a consideration of the activation of pepsinogen to pepsin and attempts to correlate the biological activity with the conformational characteristics, one sees immediately that only a rough correlation... 

The work carried out in our laboratory on the macromolecular conformation of pepsin and pepsinogen and its relation to the biological function would hardly have been possible without the active contribution of some of my collaborators. I acknowledge the help of S. Beychok in the dichroism experiments and my sincere thanks go to William F. Harrington and Aharon Katchalsky for many helpful discussions which made a vital contribution to this investigation. Most of all, I like to remember the friendship of the late K. Linderstrpm-Lang. 

Pepsinogen is secreted by the chief cells of the stomach. The gastric parietal cells secrete HCl. The acid in the stomach lumen alters the conformation of pepsinogen so that it can cleave itself, producing the active protease pepsin. Thus, the activation of pepsinogen is autocatalytic. 

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. 

In the absence of an inhibitor, the intermediate undergoes conformational change to bind the activation peptide portion of this same pepsinogen molecule in the active center to form an intramolecular enzyme-substrate complex (intermediate 0). This is followed by the intramolecular hydrolysis of a peptide bond which results in a base-denaturable pepsin species. Inteimediate 6 apparently does not activate another pepsinogen molecule via an intermolecular process. Neither does intermediate 6 hydrolyze globin substrate. 

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