Term
| List the three mechanisms by which hemostasis is accomplished |
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Definition
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
| Describe the first event in platelet aggregation. Describe how adhering platelets induce further aggregation. |
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Definition
1) as blood flows out of a severed blood vessel, its platelets come into contact with the vessel wall and adhere tightly to collagen fibers.
This 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.
2)Changes in the shape and structure of the platelets follow - they become spiny spheres - and the contents of the platelets' granules are secreted releasing
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.
3)Thus platelet aggregation is enhanced (by the released ADP). 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
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Definition
Platelets are disk-shaped cells derived from megakarocytes in the bone marrow. They lack
nuclei and contain distinctive granules or dense bodies. |
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Term
| List the physical characteristics of fibrinogen |
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Definition
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-helicalregions).
It is comprised of six polypeptide chains. Two sets of three unlike chains, α2 β2γ2 ,are linked head to head and side to side by disulfide bonds near their amino terminal regions. |
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Term
| Describe the role of thrombin in the conversion of fibrinogen to fibrin monomers. |
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Definition
| Thrombin (a serine endopeptidase) 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. |
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Term
| 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? Because
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
| State the function of transglutaminase and write the reaction that it catalyzes. |
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Definition
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).
Lysine loses its NH3+ group and glutamine loses a H from its amide group |
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Term
| What regulates transglutaminase? |
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Definition
Thrombin, which converts fibrinogen to fibrin
monomers, also converts protransglutaminase (Factor XIII) to transglutaminase (Factor XIIIa). |
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Term
| 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
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.
The presence of Ca2+,phospholipid, and Factor Va are required for activated Factor X (Factor Xa) to convert prothrombin to thrombin. Phospholipids are available in the plasma membrane of platelets.
A complex (Xa-Va) of Factors Xa and Va is the catalytic entity. |
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Term
| What are the two routes of Factor X activation? |
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Definition
| 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. |
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Term
| What is extrinsic activation of Factor X? |
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Definition
| 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. |
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Term
| What is the intrinsic activation of Factor X? |
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Definition
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. |
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Term
| What are the characteristics of thrombin? |
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Definition
| 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. |
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Term
| What are the functions of thrombin? |
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Definition
| 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
| List three factors in the clotting cascade that are not enzymes. |
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Definition
Factor III - tissue factor
Factor V - activator protein
Factor VIII - activator protein |
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Term
| Describe the role of vitamin K in blood clotting. |
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Definition
| Vitamin K serves as a cofactor for a liver microsomal enzyme that carboxylates peptide bound glutamyl residues and converts them to gamma-carboxyglutamyl (Gla) residues. This is a post-translational modification which is independent of the activation of these factors. |
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Term
| Describe the glutamyl carboxylase reaction. |
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Definition
There is a three-step mechanism for the vitamin K-dependent carboxylase reaction:
1) addition of O2 to the reduced vitamin (vitamin K hydroquinone)
2) interaction of this oxygenated intermediate with the glutamic acid residue of the substrate to form a
γ-carbanion and vitamin K epoxide.
3) addition of CO2 to the carbanion. |
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Term
| Identify the factors in the blood clotting cascade which are vitamin K dependent. |
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Definition
| prothrombin, Factor VII, Factor IX, and Factor X |
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Term
| 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
Antithrombin III - a plasma protein that inactivates thrombin, Factor VIIa, and several other activated factors by forming irreversible complexes with them.
Antithrombin III is a specific inhbitor that controls clotting. By forming the irreversible complexes with those factors it prevents too much clotting and confines clotting to the area of injury. |
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Term
| Identify the most common types of hemophilia. Identify the biochemical defect involved. |
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Definition
Hemophilia is a category of diseases characterized by abnormal or excessive bleeding resulting from deficiency or absence of a clotting factor. It is a sex-linked recessive disease. Heterozygous females
are asymptomatic carriers.Persons with hemophilia must
receive injections of the deficient factor
Hemophilia A is deficient of Factor VIII (classical hemophilia) - 80% of cases
Hemophilia B is deficient of Factor IX (Christmas disease) - 10-20% of cases
Hemophilia C is deficient of Factor XI - less common
Congenital parahemophilia is deficient of Factor V - less common |
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Term
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Definition
Thromboembolism is 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
| How is thromboembolism treated? |
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Definition
Heparin - immediate effect
Coumarins (warfarin and dicoumarol) - slower acting but long lasting
Aspirin - at low conc. inhibits formation of thromboxane A2, a stimulant for platelet aggregation. at high conc. induces a decrease of the plasma concentrations of vitamin K-dependent clotting factors, probably because it inhibits vitamin K epoxide reductase.
Clopidogrel (Plavix) - inhibitor of ADP-induced platelet aggregation. Clopidogrel irreversibly modifies the platelet ADP receptor and inhibits ADP binding. |
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Term
| State why the anticoagulant effect of heparin is more rapid than that of warfarin. |
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Definition
Heparin works more rapidly because of what it acts on. It directly increases the rate of formation of the thrombin-antithrombin III complex. This will stop clotting quickly.
Coumarins such as warfarin prevent the recycling of the Vitamin K epoxide to the Vitamin K hydroquinone. (Coumarins mimic the Vitamin K core) This will slow down clotting because Vitamin K is necessary for several factors - prothrombin, Factor VII, Factor IX, and Factor X |
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Term
| Describe the function of bradykinin. |
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Definition
| 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
| Describe how bradykinin is produced 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.
Note bradykinin must be formed continuously to maintain its effect because ten percent is destroyed in one pass of blood through the lungs, where a peptidase removes the C-terminal Phe-Arg groups. |
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Term
| Describe the role of plasmin. |
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Definition
Typically, lysis of clots occurs within a few days of their formation and involves the proteolytic cleavage of fibrin by the enzyme plasmin (fibrinolysin).
Once plasmin has formed, it can, by cleavage, convert fibrin to soluble peptides (fibrin degradation products, FDP). Plasmin has a high affinity for fibrin.
Extra Notes
Fibrinolysis should be regulated and there is an alpha- 2-antiplasmin which can inactivate plasmin.
Rare deficiencies of PAI-1 or alpha-2-antiplasmin result in excessive bleeding, caused by excessive plasmin, leading to lysis of hemostatic plugs. |
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Term
| Identify the trigger for plasmin activation. |
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Definition
Plasmin, a plasma protease, is formed by the activation of its zymogen, plasminogen (profibrinolysin), which is produced by the kidneys.
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. |
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Term
| Describe the function and mechanism of action of proteins C, S, and LACI. |
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Definition
Protein C - a gamma-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 gamma-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
| List five genetic defects that will lead to excessive clotting |
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Definition
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
| 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
| Indicate why von Willebrand disease results in abnormal bleeding? |
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Definition
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.
Increased degradation of Factor VIII will prevent clotting and lead to excessive bleeding similar to classic hemophilia (deficient in Factor VIII) |
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Term
| Where do the intrinsic and extrinsic pathways converge? |
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Definition
| At the step involving conversion of Factor X to Factor Xa |
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Term
| Function of the gamma-Carboxyglutamyl Residues |
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Definition
The gamma-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. |
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