Term
Study Guide and Objectives
(1) List the three mechanisms by which hemostasis is accomplished |
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Definition
Hemostasis, the cessation of bleeding, is accomplished by three mechanisms:
1. rapid constriction of the injured vessel to reduce flow through the vessel
2. clumping of platelets to form a plug on the injured surface of the blood
vessel
3. clot formation |
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Term
Study Guide and Objectives
2) Describe the first event in platelet aggregation. Describe how adhering platelets
induce further aggregation. |
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Definition
The Platelet Plug
Platelets are disk-shaped cells derived from megakarocytes in the bone marrow. They lack
nuclei and contain distinctive granules or dense bodies. As blood flows out of a severed blood
vessel, its platelets come into contact with the vessel wall and adhere tightly to collagen fibers.
Platelet adhesion is mediated by von Willebrand factor (vWF), a plasma protein. vWF binds
to a receptor on the platelet membrane and to collagen fibers in the tissue. Changes in the
shape and structure of the platelets follow - they become spiny spheres - and the contents of
the platelets' granules are secreted. The substances released include: ADP, a stimulant for
platelet aggregation; serotonin and epinephrine, vasoactive amines; and thromboxane A2, a
metabolite of the prostaglandins, which stimulates secretion of ADP from the dense granules.
Thus platelet aggregation is enhanced. Other passing platelets adhere, building a platelet plug.
The platelet plug acts as a nucleus around which the fibrin clot is formed. |
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Term
Study Guide and Objectives
3) List the physical characteristics of fibrinogen. |
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Definition
Characteristics of Fibrinogen
Fibrinogen is a protein with 45 nm length, 9 nm maximum diameter, and a molecular weight
of 340,000. It contains three nodules (globular regions) connected by two rods (α-helical
regions). It is comprised of six polypeptide chains. Two sets of three unlike chains, are linked head to head and side to side by disulfide bonds near their amino terminal regions. |
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Term
Study Guide and Objectives
4) Describe the role of thrombin in the conversion of fibrinogen to fibrin monomers. |
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Definition
Conversion of Fibrinogen to Fibrin
Conversion of fibrinogen to fibrin is catalyzed by
thrombin, a serine endopeptidase. Thrombin cleaves
four Arg-Gly peptide bonds in fibrinogen to yield an A peptide from each of two α-chains and a B peptide
from each of two β-chains. The A and B peptides are
called fibrinopeptides. The remaining molecule after
cleavage has 97% of the amino acid residues of
fibrinogen and is called a fibrin monomer.
Fibrin monomers have lower solubility than the parent
fibrinogen and spontaneously associate to form fibrin.
Why? The fibrinopeptides have a large net negative
charge due to an abundance of aspartate, glutamate,
and tyrosine-O-sulfate. These negatively charged
groups probably keep the fibrinogen molecules apart
when the fibrinopeptides are still part of fibrinogen.
Fibrin has a periodic structure that repeats every
23 nm. Since fibrinogen is 45 nm long, the fibrin
monomers probably come together to form a half
staggered array. |
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Term
Study Guide and Objectives
5) State why fibrin monomers polymerize. |
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Definition
Fibrin monomers have lower solubility than the parent
fibrinogen and spontaneously associate to form fibrin. Why? The fibrinopeptides have a large net negative charge due to an abundance of aspartate, glutamate, and tyrosine-O-sulfate. These negatively charged groups probably keep the fibrinogen molecules apart when the fibrinopeptides are still part of fibrinogen. |
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Term
Study Guide and Objectives
6) State the function of transglutaminase and write the reaction that is catalyzes. |
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Definition
Stabilization of the Fibrin Clot
The fibrin clot is stabilized by formation of covalent crosslinks between the side chains of
glutamine and lysine in the fibrin monomer. The reaction is catalyzed by transglutaminase
(Factor XIIIa).
The covalent cross-links form end to end. Thrombin, which converts fibrinogen to fibrin
monomers, also converts protransglutaminase (Factor XIII) to transglutaminase (Factor XIIIa). |
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Term
Study Guide and Objectives
7) Explain how prothrombin is converted to thrombin, including in the explanation the roles of Ca2+; phospholipid, Factor V, and Factor X. |
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Definition
Formation of Thrombin
Thrombin is normally present in the blood as the proenzyme, prothrombin (Factor II). It is
formed from prothrombin by two hydrolyses catalyzed by Factor Xa. An Arg-Thr bond is
cleaved, releasing the amino terminal section of prothrombin and an Arg-Ile bond is cleaved,
opening a disulfide loop. The result is a two-chain thrombin molecule held together by a
disulfide bond.
Activated Factor X (Factor Xa) converts prothrombin to thrombin only in the presence of Ca2+,
phospholipid, and Factor Va. Phospholipids are available in the plasma membrane of platelets.
A complex (Xa-Va) of Factors Xa and Va is the catalytic entity.
Characteristics of Thrombin
Thrombin shows specificity for Arg-Gly bonds, is a serine endopeptidase, and resembles
trypsin in mechanism of action. Its molecular weight is 33,700. It has an A chain and B chain
connected by disulfide bonds. The B chain is similar in sequence to trypsin.
Function of Thrombin
Thrombin catalyzes the conversion of fibrinogen to fibrin and of protransglutaminase to
transglutaminase. Thrombin is also involved in the activation of Factors V, VII, and VIII. |
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Term
Study Guide and Objectives
8) Distinguish between extrinsic activation and intrinsic activation of blood clotting.
Describe the role of tissue factor in extrinsic activation. Identify the first factor
activated when blood clotting is initiated by intrinsic activation. |
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Definition
The Triggers for Clotting
Factor X is activated by two routes, one known as extrinsic because
the factor is extravascular (for example, in vitro extract from brain) and a second known as
intrinsic, because all components are in blood.
Extrinsic System
When tissues are damaged, tissue factor (Factor III), a membrane glycoprotein in the subendothelial
layer, is released. A complex of tissue factor and Factor VIIa then catalyzes the
activation of Factor X.
Intrinsic System
The initial event in the intrinsic sequence is the exposure of Factor XII to surface proteins when
the blood vessel has been damaged. Factor XII undergoes a conformational change, increasing
its catalytic activity at least 10,000-fold.
Activation of Factor XII sets off a chain of activations involving Factors XI, IX, and VIII, and
finally the activation of Factor X. In vitro, contact with a glass surface activates Factor XII and
prekallikrein. |
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Term
Study Guide and Objectives
9) List three factors in the clotting cascade that are not enzymes. |
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Definition
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Term
Study Guide and Objectives
10) Describe the role of vitamin K in blood clotting. Describe the glutamyl carboxylase
reaction. Identify the factors in the blood clotting cascade which are vitamin Kdependent. |
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Definition
In the absence of vitamin K, blood ceases to clot. A rat on a vitamin K-deficient diet, for
example, dies within 7-10 days. The molecular role of vitamin K was elucidated in 1974.
It was discovered that vitamin K serves as a cofactor for a liver microsomal enzyme that carboxylates peptide bound glutamyl residues and converts them to γ-carboxyglutamyl (Gla) residues. This is a posttranslational modification which is independent of the activation of these factors.
In the overall blood clotting sequence, there are four factors (prothrombin, Factor VII, Factor
IX, and Factor X) that are vitamin K-dependent. |
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Term
Study Guide and Objectives
11) Describe the mechanism of action of antithrombin III and the role that antithrombin III plays in regulation of blood coagulation. |
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Definition
Control of Clotting
Clotting must be finely regulated; it must occur rapidly and yet remain confined to the area of
injury. What contributes to the control of clotting? The lability of the activated clotting factors
contributes to the control of clotting. The activated clotting factors have a short lifetime becausethey are diluted by blood flow, removed by liver, degraded by proteases, and inactivated by
specific inhibitors. The inhibitors include:
Antithrombin III - a plasma protein that inactivates thrombin, Factor VIIa, and several other activated factors by forming irreversible complexes with them. |
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Term
Study Guide and Objectives
12) Identify the most common types of hemophilia. Identify the biochemical defect involved. |
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Definition
Hemophilia - Hemophilia is a category of diseases characterized by abnormal or excessive bleeding resulting from deficiency or absence of a clotting factor. It is genetically transmitted as a sex-linked recessive characteristic. Heterozygous females
are asymptomatic carriers. Of clinical cases resulting from clotting factor deficiencies,
80% involve Factor VIII, 10-20% involve Factor IX. Persons with hemophilia must receive injections of the deficient factor.
Disease Deficiency
Hemophilia A Factor VIII (classical hemophilia)
Hemophilia B Factor IX (Christmas disease)
Hemophilia C Factor XI
Congenital parahemophilia Factor V |
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Term
Study Guide and Objectives
13) Define thromboembolism. List several methods of treatment for thromboembolism. |
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Definition
Thromboembolism - the obstruction of blood vessels by detached clots. It is observed
to a greater extent in women taking oral contraceptives than in other women. The pill
has been reported to cause a decrease in antithrombin III. |
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Term
Study Guide and Objectives
14) State why the anticoagulant effect of heparin is more rapid than that of warfarin. |
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Definition
Heparin - used for immediate effect
Coumarins (warfarin and dicoumarol) - used in patients prone to abnormal venous
clot formation; slower acting, but longer lasting inhibitors of clotting compared with heparin. Warfarin - resistance has been reported in several families. In humans, resistance to warfarin is only a problem if one needs anticoagulant therapy. Warfarin has been used extensively as a rat poison and warfarin-resistance also occurs in rats. Each
year in the USA, two million patients start warfarin treatment. |
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Term
Study Guide and Objectives
15) Describe the function of bradykinin. Describe its production from the kininogens. |
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Definition
Kallikrein enzymes convert kininogens (large precursor proteins) to bradykinin by hydrolyzing a
peptide sequence out of the middle of the kininogens. Bradykinin is a peptide containing nine
amino acids. It dilates blood vessels and increases their permeability. Dilation of the blood vessel
by bradykinin and dissolution of the clot by plasmin serve to undo the hemostatic constriction
and clot formation that occur immediately after injury. |
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Term
Study Guide and Objectives
16) Describe the role of plasmin. Identify the trigger for its activation. |
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Definition
Formation of Plasmin
The dissolution of the clot may be viewed as a final stage of the clotting process. Typically, lysis
of clots occurs within a few days of their formation and involves the proteolytic cleavage of fibrin by the enzyme plasmin (fibrinolysin). Plasmin, a plasma protease, is formed by the activation of its zymogen, plasminogen (profibrinolysin), which is produced by the kidneys.
Plasminogen has a high affinity for fibrin. |
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Term
Study Guide and Objectives
17) Describe the function and mechanism of action of proteins C, S, and LACI. |
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Definition
Control of Clotting
Protein C - a γ-carboxyglutamic acid containing protein which functions as an anticoagulant by inactivating factors VIIIa and Va. Protein C is activated by thrombin, when it is bound to thrombomodulin, a membrane protein present on the surface of the endothelial cells that line blood vessels.
Protein S - a γ-carboxyglutamic acid protein that acts as a cofactor for activated protein C.
Lipoprotein-associated coagulation inhibitor (LACI) -inactivates the Factor VIIa-TF-Xa complex; also known as tissue factor pathway inhibitor (TFPI). |
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Term
Study Guide and Objectives
18) List five genetic defects that will lead to excessive clotting. |
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Definition
Genetic Risk Factors for Thrombosis
Antithrombin III - An autosomal dominant deficiency of antithrombin III has been reported in several families.
Protein C - For cases of venous thrombosis in individuals under 45 years of age, 4% are due to protein C deficiency.
Protein S - For cases of venous thrombosis in individuals under 45 years of age, 5% are due to protein S deficiency.
Factor V Leiden - A genetic defect such that Factor Va is not inactivated by Protein C; occurs in 3% of the population in Western countries.
Factor II 20210A - A change in prothrombin mRNA (G--->A) that leads to increased messenger stability and increased levels of prothrombin. |
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Term
Study Guide and Objectives
19) Describe the activation of plasminogen. What agents are involved physiologically
and in the medical setting? |
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Definition
Tissue plasminogen activator (tPA), a serine protease which is synthesized in endothelial cells
and released following damage to blood vessels, is a major physiological activator of plasminogen. TPA has a high affinity for fibrin. The activity of tPA is regulated through plasminogen activator-inhibitor Type 1 (PAI-1) and Type 2 (PAI-2). tPA is used clinically as a plasminogen activator.
Streptokinase, obtained from hemolytic streptococci, is also used for treatment of acute myocardial infarction and thromboembolism. It activates plasminogen by complexing with it. Urokinase (uPA) circulates in plasma and is converted from a single chain form (pre-urokinase, scuPA) to a more active double chain form (tcuPA, urokinase) by Factor XII. Urokinase is synthesized in the kidney and found in the urine, hence its name. |
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Term
Study Guide and Objectives
20) Indicate why von Willebrand disease results in abnormal bleeding? |
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Definition
| von Willebrand disease - von Willebrand disease is an autosomal dominant disorder that leads to impaired platelet adhesion. vWF and factor VIII circulate together in a noncovalent complex. A deficiency of vWF will result in increased degradation of factor VIII (classic hemophilia --> 80% of cases) |
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Term
Hemostasis, the cessation of bleeding, is accomplished by three mechanisms:
1. rapid constriction of the injured vessel to reduce flow through the vessel
2. clumping of platelets to form a plug on the injured surface of the blood
vessel
3. clot formation |
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Definition
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Term
The Fibrin Clot
A clot is formed by a series of transformations involving more than ten different proteins. The
final result is the fibrin clot. Clotting occurs when fibrinogen, a soluble blood plasma protein,
is converted to fibrin monomers. The monomers align end-to-end and side-to-side to create a
fibrin lattice. The monomers are crosslinked to form the insoluble clot (fibrin). |
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Definition
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Term
An Overview of Blood Coagulation
A striking feature of the blood clotting process is that it is a series of zymogen activations
(conversion of proproteins to their active state). In this enzymatic cascade, the activated form of
one factor catalyzes the activation of the next factor. Very small amounts of the initial factors
are needed because each catalytic step yields a large amplification.
The clotting proteins are synthesized in the liver and must be exported into general circulation.
They are, thus, synthesized as preproproteins. Most clotting proteins are glycoproteins and are
therefore glycosylated as they traverse the ER and the Golgi membranes. In addition,
prothrombin and Factors VII, IX, and X are carboxylated in a vitamin K-dependent reaction as
they traverse the ER.
Role of Vitamin K in Blood Coagulation
In the overall blood clotting sequence, there are four factors (prothrombin, Factor VII, Factor
IX, and Factor X) that are vitamin K-dependent.
Tentative |
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Definition
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Term
Vitamin K epoxide is reduced to vitamin K (quinone form) by a warfarin - sensitive enzyme
(vitamin K epoxide reductase) that has sulfhydryl groups. The quinone form of vitamin K can
be reduced to the hydroquinone by either a warfarin-sensitive vitamin K reductase (sulfhydryl
enzyme) or a NAD-linked dehydrogenase. The sulfhydryl enzyme is considered the more
important one at physiological concentrations of the vitamin. Coumarins, such as warfarin and
dicoumarol, exert their anticoagulant activity by preventing the recycling of the epoxide to the
hydroquinone. |
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Definition
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Term
Function of the γ-Carboxyglutamyl Residues
Calcium is essential to the blood clotting process and calcium chelating agents will prevent
blood coagulation. The γ-carboxyglutamyl residues (Gla) in prothrombin and Factors VII, IX,
and X are located near the amino-terminal ends of these proteins and endow these proteins with
an ability to strongly bind Ca2+. Calcium ions serve as a bridge between these proteins and the
phospholipids of the platelet membrane. These proteins are brought in close proximity to each
other on the platelet membrane, thus augmenting their activation. When prothrombin is
activated to thrombin, the amino terminal fragment, containing Gla residues, is cleaved, freeing
thrombin from the phospholipid surface so that it can activate fibrinogen. Gla residues of VII,
IX, and X are not cleaved upon activation. |
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Definition
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Term
Clotting must be finely regulated; it must occur rapidly and yet remain confined to the area of
injury. What contributes to the control of clotting? The lability of the activated clotting factors
contributes to the control of clotting. The activated clotting factors have a short lifetime because
they are diluted by blood flow, removed by liver, degraded by proteases, and inactivated by
specific inhibitors. The inhibitors include:
Antithrombin III - a plasma protein that inactivates thrombin, Factor VIIa, and several
other activated factors by forming irreversible complexes with them.
Heparin - a negatively charged sulfated polysaccharide found in mast cells near the
walls of blood vessels. It increases the rate of formation of the thrombin-antithrombin III
complex.
Protein C - a γ-carboxyglutamic acid containing protein which functions as an
anticoagulant by inactivating factors VIIIa and Va. Protein C is activated by thrombin,
when it is bound to thrombomodulin, a membrane protein present on the surface of the
endothelial cells that line blood vessels.
Protein S - a γ-carboxyglutamic acid protein that acts as a cofactor for activated protein
C.
Protein Z - Human protein Z (PZ), another vitamin K-dependent plasma protein, was
purified and first described in 1984. PZ is synthesized in the liver and the N-terminal
domain of PZ is homologous to that of factors VII, IX, and X. His and Ser residues of
the catalytic triad are missing in the C-terminal domain of protein Z. PZ inhibits
coagulation by serving as a cofactor for plasma PZ-dependent protease inhibitor. The
complex inactivates factor Xa.
Anti-trypsin - an inhibitor of a variety of proteases, including the clotting factors which
are serine proteases. |
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Definition
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