Biochemistry - Chemical Reactions in the Living Cell Volume 2 - D. Metzler 1980

Types of reactions catalyzed by enzymes
Substitution reactions at carbonyl groups
Pepsin

Several different families of proteinases are known, and not all of them necessarily contain Serine in their active center. One family includes gastric pepsin and related Enzymes, such as rennin from the fourth Stomach (abomasum) of a calf. Rennin causes rapid milk coagulation and is widely used in cheesemaking. Certain intracellular cathepsins and proteinases from various Fungi also belong to this family. An unusual property of the pepsin family of proteinases is that they are most active in the pH range from 1 to 5. This property explains why serine and Histidine are not part of the active center of these enzymes. It is believed that in "acidic" proteinases, a carboxylate ion acts as the nucleophile in the double-displacement mechanism, while a second carboxyl group serves as the proton donor relative to the leaving group. Thus, The Mechanism of pepsin action is similar to that of Lysozyme.

Addendum 7-B

Animal and Plant Proteinase Inhibitors

Premature conversion of proenzymes such as trypsinogen into active proteinases in the Pancreas can have disastrous consequences. To prevent such premature activation, the pancreas must also produce specific inhibitors. The pancreatic Trypsin inhibitor is a small protein with a Molecular Weight of 6500, which specifically binds to the active center of trypsin (K_f = 10¹² M in an alkaline medium)^a. Determination of the crystal Structure of trypsin itself and its inhibitor has shown that these two molecules fit tightly against each other^b. The inhibitor binds as if it were a peptide substrate; one edge of the inhibitor molecule forms an antiparallel ß-Structure with the peptide chain of the enzyme. Lysine-15, which forms part of this ß-structure, enters the specific binding center for the basic amino acid of the substrate. Thus, the proteinase inhibitor is a modified substrate that can actually undergo attack at the active center. However, the fit of the two molecules is so tight that a Water molecule cannot participate in The final stage of the catalytic act, and the complex remains unreactive. (In the Small Intestine, The amount of inhibitor is not large enough to prevent the action of large quantities of trypsin formed from chymotrypsinogen.)

Proteinase inhibitors that block the action of trypsin have also been found in many plants. Usually, the highest antiproteinase activity is found in seeds and tubers, but the synthesis of proteinase inhibitors can also be induced in other parts of plants by surface damage. These inhibitors may protect plants from insects^c. The structure of soybean trypsin inhibitor and its complex with trypsin has been studied. This complex is similar to the complex containing the pancreatic trypsin inhibitor. However, the soybean trypsin inhibitor is slowly cleaved, and X-Ray Diffraction studies^d have shown that the complex exists as a tetrahedrally bound adduct, as shown in equation (7-13).

A completely different role is played by a₁-antitrypsin, found in the a-globulin fraction of Blood serum. Trypsin is absent in Tissues, but this inhibitor blocks the action of numerous Serine proteinases. The hereditary absence of a₁-antitrypsin frequently leads to The Development of severe pulmonary emphysema at an early age^d-g. Elastase and neutral proteinase are known to be released by lysosomal granules of blood granulocytes at sites of inflammation. In the absence of proteinase inhibitors, neutral proteinase and elastase (which causes local Collagen degradation) can operate in an uncontrolled manner. The hereditary defect may consist of the slow release of the proteinase inhibitor from its site of synthesis in Liver Cells^e. This situation is complicated by the fact that patients lacking a1-antitrypsin also lack the chemotactic factor inactivator (Addendum 5-J).

^а Huber R., Kukla D., Rühlmann A., Steigemann W. (1971). Cold Spring Harbor Symp. Quant. Biol., 36, 141—150.

^б Stroud R. M., Kay L. M., Dickerson R. E. (1971). Cold Spring Harbor Symp. Quant. Biol., 36, 125—140.

^в Green T. R., Ryan C. A. (1972). Science, 175, 776—777.

^г Blow D. M., Janin J., Sweet R. M. (1974). Nature (London), 249, 54—57.

^д Ward P. A., Talamo R. C. (1973). J. Clin. Invest., 52, 516—519.

^е Lieberman I., Mittman C., Gordon H. W. (1972). Science, 175, 63—65.

^ж Cohen A. B. (1973). J. Biol. Chem., 248, 7055—7059.