BIOCHEMISTRY - L. Stryer - 1984

VOLUME 1

PART I. CONFORMATION AND DYNAMICS

CHAPTER 8. ZYMOGEN ACTIVATION: DIGESTIVE ENZYMES AND BLOOD CLOTTING FACTORS

Summary

Protein activation by proteolytic Cleavage of one or more peptide bonds is a widespread regulatory mechanism in biological systems. If the active protein is an enzyme, its inactive precursor is called a proenzyme (or zymogen). The most thoroughly studied zymogen activation process is The conversion of chymotrypsinogen to Chymotrypsin, which occurs as follows. Trypsin cleaves the peptide bond between residues 15 and 16 of chymotrypsinogen to yield chymotrypsin. The newly formed terminal amino group of isoleucine-16 tucks into the interior of the protein molecule, where it forms an electrostatic bond with aspartate-194. This interaction triggers a series of local conformational changes, ultimately resulting in The formation of a binding pocket for aromatic (or large nonpolar) substrate side chains. In addition, an oxyanion hole is formed, which stabilizes the Transition State.

A highly reactive Serine residue (serine-195) plays a critical role in the catalytic Mechanism of Chymotrypsin. In the first sta

ge of peptide substrate Hydrolysis, the carboxyl component of the substrate is esterified with the hydroxyl group of serine-195, yielding a covalently bound acyl-enzyme intermediate. This intermediate is hydrolyzed In the second step of catalysis. Histidine-57 also plays a key role in both the acylation and deacylation of the enzyme. In fact, serine-195 becomes a potent nucleophile through the formation of a hydrogen-bonded network: it is hydrogen-bonded to histidine-57, which in turn is hydrogen-bonded to aspartate-102 buried in the interior of the molecule. Consequently, these three residues form a charge-Relay system that accelerates catalysis about 10³-fold. A similar charge-relay system is present in trypsin, Elastase, and Thrombin. All of these Enzymes are similar to chymotrypsin in Amino Acid Sequence, conformation, and catalytic mechanism, but they differ in Specificity. These Variations on a theme are likely the result of common ancestry and subsequent divergent evolution. The other three Major Classes of Proteolytic Enzymes are the zinc, thiol, and carboxyl proteinases.

Zymogen activation also plays a leading role in the Regulation of Blood clotting. A striking feature of the clotting process is that it is organized as a cascade of zymogen activations, in which the active form of one clotting factor catalyzes the activation of the next. Blood clotting occurs through the interaction of two reaction pathways, known as the extrinsic and intrinsic pathways. Both pathways are essential for normal blood clotting. They merge into a common pathway that leads to the formation of a fibrin clot. Fibrin is formed from fibrinogen, a highly soluble plasma protein, by the hydrolysis of four peptide bonds between Arginine and Glycine residues. This reaction is catalyzed by thrombin, a trypsin-like enzyme. Hydrolysis releases two A Peptides and two B peptides from fibrinogen, accounting for about 3% of the fibrinogen molecule. The resulting fibrin monomer spontaneously polymerizes into long, insoluble fibrin threads. The fibrin clot is further stabilized by covalent cross-links formed by a transamidation reaction between the side chains of specific glutamine and Lysine residues. Vitamin K is required for the carboxylation of glutamate residues in prothrombin and several other clotting factors. The binding of Ca^{2 +} ions by γ-carboxyglutamate promotes the attachment of prothrombin to platelet membranes, which accelerates the activation of prothrombin by factors X_a and V.