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Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.

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Biochemistry. 5th edition.

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Section 10.5Many Enzymes Are Activated by Specific Proteolytic Cleavage

We turn now to a different mechanism of enzyme regulation. Many enzymes acquire full enzymatic activity as they spontaneously fold into their characteristic three-dimensional forms. In contrast, other enzymes are synthesized as inactive precursors that are subsequently activated by cleavage of one or a few specific peptide bonds. The inactive precursor is called a zymogen (or a proenzyme). A energy source (ATP) is not needed for cleavage. Therefore, in contrast with reversible regulation by phosphorylation, even proteins located outside cells can be activated by this means. Another noteworthy difference is that proteolytic activation, in contrast with allosteric control and reversible covalent modification, occurs just once in the life of an enzyme molecule.

Specific proteolysis is a common means of activating enzymes and other proteins in biological systems. For example:

1.

The digestive enzymes that hydrolyze proteins are synthesized as zymogens in the stomach and pancreas (Table 10.3).

2.

Blood clotting is mediated by a cascade of proteolytic activations that ensures a rapid and amplified response to trauma.

3.

Some protein hormones are synthesized as inactive precursors. For example, insulin is derived from proinsulin by proteolytic removal of a peptide.

4.

The fibrous protein collagen, the major constituent of skin and bone, is derived from procollagen, a soluble precursor.

5.

Many developmental processes are controlled by the activation of zymogens. For example, in the metamorphosis of a tadpole into a frog, large amounts of collagen are resorbed from the tail in the course of a few days. Likewise, much collagen is broken down in a mammalian uterus after delivery. The conversion of procollagenase into collagenase, the active protease, is precisely timed in these remodeling processes.

6.

Programmed cell death, or apoptosis, is mediated by proteolytic enzymes called caspases, which are synthesized in precursor form as procaspases. When activated by various signals, caspases function to cause cell death in most organisms, ranging from C. elegans (Section 2.4.3) to human beings. Apoptosis provides a means sculpting the shapes of body parts in the course of development and a means of eliminating cells producing anti-self antibodies or infected with pathogens as well as cells containing large amounts of damaged DNA.

Table 10.3. Gastric and pancreatic zymogens.

Table 10.3

Gastric and pancreatic zymogens.

We next examine the activation and control of zymogens, using as examples several digestive enzymes as well as blood-clot formation.

10.5.1. Chymotrypsinogen Is Activated by Specific Cleavage of a Single Peptide Bond

Chymotrypsin is a digestive enzyme that hydrolyzes proteins in the small intestine. Its mechanism of action was discussed in detail in Chapter 9. Its inactive precursor, chymotrypsinogen, is synthesized in the pancreas, as are several other zymogens and digestive enzymes. Indeed, the pancreas is one of the most active organs in synthesizing and secreting proteins. The enzymes and zymogens are synthesized in the acinar cells of the pancreas and stored inside membrane-bounded granules (Figure 10.31). The zymogen granules accumulate at the apex of the acinar cell; when the cell is stimulated by a hormonal signal or a nerve impulse, the contents of the granules are released into a duct leading into the duodenum.

Figure 10.31. Secretion of Zymogens by an Acinar Cell of the Pancreas.

Figure 10.31

Secretion of Zymogens by an Acinar Cell of the Pancreas.

Chymotrypsinogen, a single polypeptide chain consisting of 245 amino acid residues, is virtually devoid of enzymatic activity. It is converted into a fully active enzyme when the peptide bond joining arginine 15 and isoleucine 16 is cleaved by trypsin (Figure 10.32). The resulting active enzyme, called π-chymotrypsin, then acts on other π-chymotrypsin molecules. Two dipeptides are removed to yield α-chymotrypsin, the stable form of the enzyme. The three resulting chains in α-chymotrypsin remain linked to one another by two interchain disulfide bonds. The striking feature of this activation process is that cleavage of a single specific peptide bond transforms the protein from a catalytically inactive form into one that is fully active.

Figure 10.32. Proteolytic Activation of Chymotrypsinogen.

Figure 10.32

Proteolytic Activation of Chymotrypsinogen. The three chains of α-chymotrypsin are linked by two interchain disulfide bonds (A to B, and B to C).

10.5.2. Proteolytic Activation of Chymotrypsinogen Leads to the Formation of a Substrate-Binding Site

How does cleavage of a single peptide bond activate the zymogen? Key conformational changes, which were revealed by the elucidation of the three-dimensional structure of chymotrypsinogen, result from the cleavage of the peptide bond between amino acids 15 and 16.

1.

The newly formed amino-terminal group of isoleucine 16 turns inward and forms an ionic bond with aspartate 194 in the interior of the chymotrypsin molecule (Figure 10.33). Protonation of this amino group stabilizes the active form of chymotrypsin.

2.

This electrostatic interaction triggers a number of conformational changes. Methionine 192 moves from a deeply buried position in the zymogen to the surface of the active enzyme, and residues 187 and 193 become more extended. These changes result in the formation of the substrate-specificity site for aromatic and bulky nonpolar groups. One side of this site is made up of residues 189 through 192. This cavity for binding part of the substrate is not fully formed in the zymogen.

3.

The tetrahedral transition state in catalysis by chymotrypsin is stabilized by hydrogen bonds between the negatively charged carbonyl oxygen atom of the substrate and two NH groups of the main chain of the enzyme (Section 9.1.3). One of these NH groups is not appropriately located in chymotrypsinogen, and so the oxyanion hole is incomplete in the zymogen.

4.

The conformational changes elsewhere in the molecule are very small. Thus, the switching on of enzymatic activity in a protein can be accomplished by discrete, highly localized conformational changes that are triggered by the hydrolysis of a single peptide bond.

Figure 10.33. Conformations of Chymotrypsinogen (Red) and Chymotrypsin (Blue).

Figure 10.33

Conformations of Chymotrypsinogen (Red) and Chymotrypsin (Blue). Image mouse.jpg The electrostatic interaction between the carboxylate of aspartate 194 and the α-amino group of isoleucine 16, essential for the structure of active chymotrypsin, is possible only (more...)

10.5.3. The Generation of Trypsin from Trypsinogen Leads to the Activation of Other Zymogens

The structural changes accompanying the activation of trypsinogen, the precursor of the proteolytic enzyme trypsin, are somewhat different from those in the activation of chymotrypsinogen. X-ray analyses have shown that the conformation of four stretches of polypeptide, constituting about 15% of the molecule, changes markedly on activation. These regions, called the activation domain, are very flexible in the zymogen, whereas they have a well-defined conformation in trypsin. Furthermore, the oxyanion hole (Section 9.1.3) in trypsinogen is too far from histidine 57 to promote the formation of the tetrahedral transition state.

The digestion of proteins in the duodenum requires the concurrent action of several proteolytic enzymes, because each is specific for a limited number of side chains. Thus, the zymogens must be switched on at the same time. Coordinated control is achieved by the action of trypsin as the common activator of all the pancreatic zymogens—trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase, and prolipase, a lipid degrading enzyme. To produce active trypsin, the cells that line the duodenum secrete an enzyme, enteropeptidase, that hydrolyzes a unique lysine-isoleucine peptide bond in trypsinogen as the zymogen enters the duodenum from the pancreas. The small amount of trypsin produced in this way activates more trypsinogen and the other zymogens (Figure 10.34). Thus, the formation of trypsin by enteropeptidase is the master activation step.

Figure 10.34. Zymogen Activation by Proteolytic Cleavage.

Figure 10.34

Zymogen Activation by Proteolytic Cleavage. Enteropeptidase initiates the activation of the pancreatic zymogens by activating trypsin, which then activates other zymogens. Active enzymes are shown in yellow; zymogens are shown in orange.

10.5.4. Some Proteolytic Enzymes Have Specific Inhibitors

The conversion of a zymogen into a protease by cleavage of a single peptide bond is a precise means of switching on enzymatic activity. However, this activation step is irreversible, and so a different mechanism is needed to stop proteolysis. Specific protease inhibitors accomplish this task. For example, pancreatic trypsin inhibitor, a 6-kd protein, inhibits trypsin by binding very tightly to its active site. The dissociation constant of the complex is 0.1 pM, which corresponds to a standard free energy of binding of about -18 kcal mol-1 (-75 kJ mol-1). In contrast with nearly all known protein assemblies, this complex is not dissociated into its constituent chains by treatment with denaturing agents such as 8 M urea or 6 M guanidine hydrochloride.

The reason for the exceptional stability of the complex is that pancreatic trypsin inhibitor is a very effective substrate analog. X-ray analyses showed that the inhibitor lies in the active site of the enzyme, positioned such that the side chain of lysine 15 of this inhibitor interacts with the aspartate side chain in the specificity pocket of trypsin. In addition, there are many hydrogen bonds between the main chain of trypsin and that of its inhibitor. Furthermore, the carbonyl group of lysine 15 and the surrounding atoms of the inhibitor fit snugly in the active site of the enzyme. Comparison of the structure of the inhibitor bound to the enzyme with that of the free inhibitor reveals that the structure is essentially unchanged on binding to the enzyme (Figure 10.35). Thus, the inhibitor is preorganized into a structure that is highly complementary to the enzyme's active site. Indeed, the peptide bond between lysine 15 and alanine 16 in pancreatic trypsin inhibitor is cleaved but at a very slow rate: the half-life of the trypsin-inhibitor complex is several months. In essence, the inhibitor is a substrate, but its intrinsic structure is so nicely complementary to the enzyme's active site that it binds very tightly and is turned over slowly.

Figure 10.35. Interaction of Trypsin with Its Inhibitor.

Figure 10.35

Interaction of Trypsin with Its Inhibitor. Image mouse.jpg Structure of a complex of trypsin (yellow) and pancreatic trypsin inhibitor (red). Lysine 15 of the inhibitor penetrates into the active site of the enzyme and forms a salt bridge with aspartate 189 in the active (more...)

Image caduceus.jpg Why does trypsin inhibitor exist? Indeed, the amount of trypsin is much greater that that of the inhibitor. Under what circumstances is it beneficial to inhibit trypsin? Recall that trypsin activates other zymogens. Consequently, it is vital that even small amounts of trypsin be prevented from initiating the cascade prematurely. Trypsin molecules activated in the pancreas or pancreatic ducts could severely damage those tissues, leading to acute pancreatitis. Tissue necrosis may result from the activation of proteolytic enzymes (as well as prolipases) by trypsin, and hemorrhaging may result from its activation of elastase. We see here the physiological need for the tight binding of the inhibitor to trypsin.

Pancreatic trypsin inhibitor is not the only important protease inhibitor. α1-Antitrypsin (also called α1-antiproteinase), a 53-kd plasma protein, protects tissues from digestion by elastase, a secretory product of neutrophils (white blood cells that engulf bacteria). Antielastase would be a more accurate name for this inhibitor, because it blocks elastase much more effectively than it blocks trypsin. Like pancreatic trypsin inhibitor, α1-antitrypsin blocks the action of target enzymes by binding nearly irreversibly to their active sites. Genetic disorders leading to a deficiency of α1-antitrypsin show that this inhibitor is physiologically important. For example, the substitution of lysine for glutamate at residue 53 in the type Z mutant slows the secretion of this inhibitor from liver cells. Serum levels of the inhibitor are about 15% of normal in people homozygous for this defect. The consequence is that excess elastase destroys alveolar walls in the lungs by digesting elastic fibers and other connective-tissue proteins.

The resulting clinical condition is called emphysema (also known as destructive lung disease). People with emphysema must breathe much harder than normal people to exchange the same volume of air, because their alveoli are much less resilient than normal. Cigarette smoking markedly increases the likelihood that even a type Z heterozygote will develop emphysema. The reason is that smoke oxidizes methionine 358 of the inhibitor (Figure 10.36), a residue essential for binding elastase. Indeed, this methionine side chain is the bait that selectively traps elastase. The methionine sulfoxide oxidation product, in contrast, does not lure elastase, a striking consequence of the insertion of just one oxygen atom into a protein. We will consider another protease inhibitor, antithrombin III, when we examine the control of blood clotting.

Figure 10.36. Oxidation of Methionine to the Sulfoxide.

Figure 10.36

Oxidation of Methionine to the Sulfoxide.

10.5.5. Blood Clotting Is Accomplished by a Cascade of Zymogen Activations

Enzymatic cascades are often employed in biochemical systems to achieve a rapid response. In a cascade, an initial signal institutes a series of steps, each of which is catalyzed by an enzyme. At each step, the signal is amplified. For instance, if a signal molecule activates an enzyme that in turn activates 10 enzymes and each of the 10 enzymes in turn activates 10 additional enzymes, after four steps the original signal will have been amplified 10,000-fold. Blood clots are formed by a cascade of zymogen activations: the activated form of one clotting factor catalyzes the activation of the next (Figure 10.37). Thus, very small amounts of the initial factors suffice to trigger the cascade, ensuring a rapid response to trauma.

Figure 10.37. Blood-Clotting Cascade.

Figure 10.37

Blood-Clotting Cascade. A fibrin clot is formed by the interplay of the intrinsic, extrinsic, and final common pathways. The intrinsic pathway begins with the activation of factor XII (Hageman factor) by contact with abnormal surfaces produced by injury. (more...)

There are two means of initiating clotting: the intrinsic and extrinsic pathways. The intrinsic clotting pathway is activated by exposure of anionic surfaces on rupture of the endothelial lining of the blood vessels. These surfaces serve as binding sites for factors in the clotting cascade. Substances that are released from tissues as a consequence of trauma to them trigger the extrinsic clotting pathway. The extrinsic and intrinsic pathways converge on a common sequence of final steps to form a clot composed of the protein fibrin. The two pathways interact with each other in vivo. Indeed, both are needed for proper clotting, as evidenced by clotting disorders caused by a deficiency of a single protein in one of the pathways. Note that the active forms of the clotting factors are designated with a subscript “a.”

10.5.6. Fibrinogen Is Converted by Thrombin into a Fibrin Clot

The best-characterized part of the clotting process is the final step in the cascade: the conversion of fibrinogen into fibrin by thrombin, a proteolytic enzyme. Fibrinogen is made up of three globular units connected by two rods (Figure 10.38). This 340-kd protein consists of six chains: two each of Aα, Bβ, and γ. The rod regions are triple-stranded α-helical coiled coils, a recurring motif in proteins. Thrombin cleaves four arginine-glycine peptide bonds in the central globular region of fibrinogen. On cleavage, an A peptide of 18 residues from each of the two Aα chains and a B peptide of 20 residues from each of the two Bβ chains are released from the globular region. These A and B peptides are called fibrinopeptides. A fibrinogen molecule devoid of these fibrinopeptides is called a fibrin monomer and has the subunit structure (αβγ)2.

Figure 10.38. Structure of a Fibrinogen Molecule.

Figure 10.38

Structure of a Fibrinogen Molecule. Image mouse.jpg (A) A ribbon diagram. The two rod regions are α-helical coiled coils, connected to a globular region at each end. (B) A schematic representation showing the positions of the fibrinopeptides A and B.

Fibrin monomers spontaneously assemble into ordered fibrous arrays called fibrin. Electron micrographs and low-angle x-ray patterns show that fibrin has a periodic structure that repeats every 23 nm (Figure 10.39). Higher-resolution images reveal the detailed structure of the fibrin monomer, how the fibrin monomers come together, and how this assembly is facilitated by removal of the fibrinopeptides. The homologous β and γ chains have globular domains at the carboxyl-terminal ends (Figure 10.40). These domains have binding “holes” that interact with peptides. The β domain is specific for sequences of the form H3N+-Gly-His-Arg-, whereas the γ domain binds H3N+-Gly-Pro-Arg-. Exactly these sequences (sometimes called “knobs”) are exposed at the amino-terminal ends of the β and α chains, respectively, on thrombin cleavage. The knobs of the α subunits fit into the holes on the γ subunits of another monomer to form a protofibril. This protofibril is extended when the knobs of the β subunits fit into holes of β subunits of other protofibrils. Thus, analogous to the activation of chymotrypsinogen, peptide-bond cleavage exposes new amino termini that can participate in specific interactions. The newly formed clot is stabilized by the formation of amide bonds between the side chains of lysine and glutamine residues in different monomers.

Image ch10fu10.jpg

Figure 10.39. Electron Micrograph of Fibrin.

Figure 10.39

Electron Micrograph of Fibrin. The 23-nm period along the fiber axis is half the length of a fibrinogen molecule. [Courtesy of Dr. Henry Slayter.]

Figure 10.40. Formation of a Fibrin Clot.

Figure 10.40

Formation of a Fibrin Clot. (1) Thrombin cleaves fibrinopeptides A and B from the central globule of fibrinogen. (2) Globular domains at the carboxyl-terminal ends of the β and γ chains interact with “knobs” exposed at (more...)

This cross-linking reaction is catalyzed by transglutaminase (factor XIIIa), which itself is activated from the protransglutaminase form by thrombin.

10.5.7. Prothrombin Is Readied for Activation by a Vitamin K-Dependent Modification

Thrombin is synthesized as a zymogen called prothrombin, which comprises four major domains, with the serine protease domain at its carboxyl terminus. The first domain is called a gla domain, whereas domains 2 and 3 are called kringle domains (Figure 10.41). These domains work in concert to keep prothrombin in an inactive form and to target it to appropriate sites for its activation by factor Xa (a serine protease) and factor Va (a stimulatory protein). Activation is accomplished by proteolytic cleavage of the bond between arginine 274 and threonine 275 to release a fragment containing the first three domains and by cleavage of the bond between arginine 323 and isoleucine 324 (analogous to the key bond in chymotrypsinogen) to yield active thrombin.

Figure 10.41. Modular Structure of Prothrombin.

Figure 10.41

Modular Structure of Prothrombin. Cleavage of two peptide bonds yields thrombin. All the γ-carboxyglutamate residues are in the gla domain.

Image caduceus.jpg Vitamin K (Section 8.6.2 and Figure 10.42) has been known for many years to be essential for the synthesis of prothrombin and several other clotting factors. The results of studies of the abnormal prothrombin synthesized in the absence of vitamin K or in the presence of vitamin K antagonists, such as dicoumarol, revealed the mode of action of this vitamin.Dicoumarolis found in spoiled sweet clover and causes a fatal hemorrhagic disease in cattle fed on this hay. This coumarin derivative is used clinically as an anticoagulantto prevent thromboses in patients prone to clot formation. Dicoumarol and such related vitamin K antagonists as warfarinalso serve as effective rat poisons. Cows fed dicoumarol synthesize an abnormal prothrombin that does not bind Ca2+, in contrast with normal prothrombin. This difference was puzzling for some time because abnormal prothrombin has the same number of amino acid residues and gives the same amino acid analysis after acid hydrolysis as does normal prothrombin.

Figure 10.42. Structures of Vitamin K and Two Antagonists, Dicoumarol and Warfarin.

Figure 10.42

Structures of Vitamin K and Two Antagonists, Dicoumarol and Warfarin.

Nuclear magnetic resonance studies revealed that normal prothrombin contains γ-carboxyglutamate, a previously unknown residue that evaded detection because its second carboxyl group is lost on acid hydrolysis. The abnormal prothrombin formed subsequent to the administration of anticoagulants lacks this modified amino acid. In fact, the first 10 glutamate residues in the amino-terminal region of prothrombin are carboxylated to γ-carboxyglutamate by a vitamin K-dependent enzyme system (Figure 10.43). The vitamin K-dependent carboxylation reaction converts glutamate, a weak chelator of Ca2+, into γ-carboxyglutamate, a much stronger chelator. Prothrombin is thus able to bind Ca2+, but what is the effect of this binding? The binding of Ca2+ by prothrombin anchors the zymogen to phospholipid membranes derived from blood platelets after injury. The binding of prothrombin to phospholipid surfaces is crucial because it brings prothrombin into close proximity to two clotting proteins that catalyze its conversion into thrombin. The proteolytic activation of prothrombin removes the calcium-binding domain and frees thrombin from the membrane so that it can cleave fibrinogen and other targets.

Image ch10fu11.jpg

Figure 10.43. The Calcium-Binding Region of Prothrombin.

Figure 10.43

The Calcium-Binding Region of Prothrombin. Prothrombin binds calcium ions with the modified amino acid γ-carboxyglutamate (red).

10.5.8. Hemophilia Revealed an Early Step in Clotting

Image caduceus.jpg Some important breakthroughs in the elucidation of clotting path ways have come from studies of patients with bleeding disorders. Classic hemophilia, or hemophilia A, the best-known clotting defect, is genetically transmitted as a sex-linked recessive characteristic. In classic hemophilia, factor VIII (antihemophilic factor) of the intrinsic pathway is missing or has markedly reduced activity. Although factor VIII is not itself a protease, it markedly stimulates the activation of factor X, the final protease of the intrinsic pathway, by factor IXa, a serine protease (Figure 10.44). Thus, activation of the intrinsic pathway is severely impaired in hemophilia.

Figure 10.44. Action of Antihemophilic Factor.

Figure 10.44

Action of Antihemophilic Factor. Antihemophilic factor (VIII) stimulates the activation of factor X by factor IXa. It is interesting to note that the activity of factor VIII is markedly increased by limited proteolysis by thrombin and factor Xa. This (more...)

In the past, hemophiliacs were treated with transfusions of a concentrated plasma fraction containing factor VIII. This therapy carried the risk of infection. Indeed, many hemophiliacs contracted hepatitis and AIDS. A safer preparation of factor VIII was urgently needed. With the use of biochemical purification and recombinant DNA techniques, the gene for factor VIII was isolated and expressed in cells grown in culture. Recombinant factor VIII purified from these cells has largely replaced plasma concentrates in treating hemophilia.

An account of a hemorrhagic disposition existing in certain families-

“About seventy or eighty years ago, a woman by the name of Smith settled in the vicinity of Plymouth, New Hampshire, and transmitted the following idiosyncrasy to her descendants. It is one, she observed, to which her family is unfortunately subject and has been the source not only of great solicitude, but frequently the cause of death. If the least scratch is made on the skin of some of them, as mortal a hemorrhage will eventually ensue as if the largest wound is inflicted…. It is a surprising circumstance that the males only are subject to this strange affection, and that all of them are not liable to it…. Although the females are exempt, they are still capable of transmitting it to their male children.”

John Otto (1803)

10.5.9. The Clotting Process Must Be Precisely Regulated

Image caduceus.jpg There is a fine line between hemorrhage and thrombosis. Clots must form rapidly yet remain confined to the area of injury. What are the mechanisms that normally limit clot formation to the site of injury? The lability of clotting factors contributes significantly to the control of clotting. Activated factors are short-lived because they are diluted by blood flow, removed by the liver, and degraded by proteases. For example, the stimulatory proteins factors Va and VIIIa are digested by protein C, a protease that is switched on by the action of thrombin. Thus, thrombin has a dual function: it catalyzes the formation of fibrin and it initiates the deactivation of the clotting cascade.

Specific inhibitors of clotting factors are also critical in the termination of clotting. The most important one is antithrombin III, a plasma protein that inactivates thrombin by forming an irreversible complex with it. Antithrombin III resembles α1-antitrypsin except that it inhibits thrombin much more strongly than it inhibits elastase. Antithrombin III also blocks other serine proteases in the clotting cascade—namely, factors XIIa, XIa, IXa, and Xa. The inhibitory action of antithrombin III is enhanced by heparin, a negatively charged polysaccharide found in mast cells near the walls of blood vessels and on the surfaces of endothelial cells (Figure 10.45). Heparin acts as an anticoagulant by increasing the rate of formation of irreversible complexes between antithrombin III and the serine protease clotting factors. Antitrypsin and antithrombin are serpins, a family of serine protease inhibitors.

Figure 10.45. Electron Micrograph of a Mast Cell.

Figure 10.45

Electron Micrograph of a Mast Cell. Heparin and other molecules in the dense granules are released into the extracellular space when the cell is triggered to secrete. [Courtesy of Lynne Mercer.]

The importance of the ratio of thrombin to antithrombin is illustrated in the case in which a 14-year-old boy died of a bleeding disorder because of a mutation in his α1-antitrypsin, which normally inhibits elastase (Section 10.5.4). Methionine 358 in α1-antitrypsin's binding pocket for elastase was replaced by arginine, resulting in a change in specificity from an elastase inhibitor to a thrombin inhibitor. α1-Antitrypsin activity normally increases markedly after injury to counteract excess elastase arising from stimulated neutrophils. The mutant α1-antitrypsin caused the patient's thrombin activity to drop to such a low level that hemorrhage ensued. We see here a striking example of how a change of a single residue in a protein can dramatically alter specificity and an example of the critical importance of having the right amount of a protease inhibitor.

Antithrombin limits the extent of clot formation, but what happens to the clots themselves? Clots are not permanent structures but are designed to be dissolved when the structural integrity of damaged areas is restored. Fibrin is split by plasmin, a serine protease that hydrolyzes peptide bonds in the coiled-coil regions. Plasmin molecules can diffuse through aqueous channels in the porous fibrin clot to cut the accessible connector rods. Plasmin is formed by proteolytic activation of plasminogen, an inactive precursor that has a high affinity for the fibrin clots. This conversion is carried out by tissue-type plasminogen activator (TPA), a 72-kd protein that has a domain structure closely related to that of prothrombin (Figure 10.46).

Figure 10.46. Modular Structure of Tissue-Type Plasminogen Activator (TPA).

Figure 10.46

Modular Structure of Tissue-Type Plasminogen Activator (TPA).

However, a domain that targets TPA to fibrin clots replaces the membrane-targeting gla domain of prothrombin. The TPA bound to fibrin clots swiftly activates adhering plasminogen. In contrast, TPA activates free plasminogen very slowly. The gene for TPA has been cloned and expressed in cultured mammalian cells. The results of clinical studies have shown that TPA administered intravenously within an hour of the formation of a blood clot in a coronary artery markedly increases the likelihood of surviving a heart attack (Figure 10.47).

Figure 10.47. The Effect of Tissue-Type Plasminogen Factor.

Figure 10.47

The Effect of Tissue-Type Plasminogen Factor. TPA leads to the dissolution of blood clots, as shown by x-ray images of blood vessels in the heart (A) before and (B) 3 hours after the administration of TPA. The position of the clot is marked by the arrow (more...)

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