BLOOD CLOTTING
1. Describe the major phases of hemostasis.
2. Describe the mechanisms that stimulate platelet activation.
3. Describe the roles of von Willebrandt factor and Glycoprotein IIb/IIIa in hemostasis.
4. Know the pathway involved in the formation of prostaglandins and thromboxanes and
how it affects platelet function. Know where aspirin inhibits this pathway.
5. Describe how thrombin cleavage of fibrinogen converts it to fibrin fibers and
facilitates blood clotting.
6. Describe the intrinsic and extrinsic pathways for activating blood clotting.
7. Describe how prothrombin is cleaved by factor X into active thrombin in the final
common pathway for blood clotting.
8. Describe the role of vitamin K in hemostasis.
9. Understand how -carboxyglutamate (Gla) resides in prothrombin facilitate its
cleavage to thrombin.
10. Describe the actions of warfarin and heparin in inhibiting blood clotting.
11. Describe how protein C and protein S function to limit blood clotting.
12. Understand how plasminogen activators function to dissolve blood clots.
BLOOD CLOTTING
1. Hemostasis- stop bleeding from damaged blood vessels
a. Rapid constriction of vessel
b. Aggregation of platelets to form plug
c. Clotting- web forms consisting of fibrin polymers; traps platelets and RBCs;
contracts to squeeze out fluid; generates seal
2. Platelet activation
a. Aggregation of platelets
 Produced by splitting off from megakaryocytes; no nucleus or protein
synthesis; replaced after 10 days
 Circulate in blood in inactive state
 Injury to blood vessel wall exposes extracellular matrix (ECM) components,
especially collagen, that bind to receptors on platelets to initiate activation;
platelet forms projections; dump secretary granules; contents of granules
activates additional platelets
 Activated platelets adhere strongly to ECM and to each other to form
aggregates
 Von Willebrandt factor (vWf) present in blood plasma; important for
associating platelets with ECM; binds collagen and receptors on platelets
(integrins and other receptor complexes)
 Glycoprotein IIb/IIIa (type of integrins) are receptors for fibrinogen; crosslinking of platelets occurs by fibrinogen and other proteins forming complex
with these receptors
 GPIIb/IIIa inhibitors inhibit platelet aggregation; used during angioplasty and
for acute coronary syndromes
b. Signaling pathways in platelet activation
 Activation of many signaling pathways; one important pathway involves
formation of prostaglandins and thromboxanes
 Activation of platelets involves activation of phospholipase A2; produces
arachidonic acid from cleavage of membrane phospholipids; cyclooxygenase
converts arachidonic acid to a prostaglandin (PGG2)
 In platelets, PGG2 converted to thromboxane A2 (TxA2); TxA2 acts both
within cell and released to neighboring cell; causes Ca2+ release in cytosol
which promotes dumping of secretary granules, platelet aggregation,
vasoconstriction
 In endothelial cells, PGG2 converted to PGI2 (a prostacyclin); PGI2 released
and enters platelets; inhibits platelet aggregation by activating adenylate
cyclase; inactivates platelets that leave damaged area; TxA2 inhibits adenylate
cyclase which prevents platelets in damaged area from being inactivated
c. Inhibiting platelet aggregation with aspirin
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TxA2 can be bad for some older people because platelet aggregation can
interfere with blood flow
Aspirin is cyclooxygenase inhibitor; inhibits platelet aggregation by lowering
TxA2 levels; also lowers PGI2 levels, but not dramatically because endothelial
cells, unlike platelets, have nuclei and continually synthesize more
cyclooxygenase
3. Formation of fibrin network
a. Conversion of fibrinogen to fibrin fibers
 Fibrinogen major blood plasma protein; conversion of soluble fibrinogen to
insoluble fibrin from proteolytic cleavage by thrombin; fibrin spontaneously
associates into long fibers
 Fibrinogen molecule composed of six polypeptide chains; two each of , ,
and  chains (2,2,2); three-stranded coiled-coil interaction of , , and 
chains; two  complexes linked by disulifde bonds
 Thrombin cleaves off four small peptides at N-terminus of each  and  chain
known as fibrinopeptides A and B; resulting molecule called fibrin monomer
 Monomers associate as half-staggered arrays to form fibers; results in unstable
“soft clot”
 Stable “hard clot” formed by cross-linking monomers; transamidation reaction
forms isopeptide bonds between side chains at C-termini of  chains;
catalyzed by factor XIII
b. Signaling pathways for stimulating fibrinogen cleavage
 Cascade of proteases; proteases present in blood plasma as inactive precursors
prior to cleavage; most synthesized in liver; each protease cleaves the next
protease in pathway, which activates it; results in amplification of signal
 Two signaling pathways activated by different stimuli; both feed into to
common pathway for final steps
- Intrinsic- everything needed for pathway contained in blood; stimulated by
contact of blood with negatively charged surface, such as glass tube or
surface exposed from ruptured vessel; end step is cleavage of factor X to
activate it
- Extrinsic- requires component from surrounding tissue to stimulate; need
tissue factor (thromboplastin) that is normally buried below endothelium;
exposed by injury; activates factor VII which cleaves factor X to activate
it
 Common pathway
- Activated factor X cleaves inactive prothrombin to generate active
thrombrin, which then cleaves fibrinogen
- Factor X cleaves at two site on prothrombin; N terminal peptide released;
peptides designated A and B linked through disulfide bond in active
thrombin
c. Role of vitamin K in clotting
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Various blood clotting factors undergo functionally important
posttranslational modification in liver before release; addition of carboxyl
group to glutamic acid residues to produce -carboxyglutamate (Gla)
Prothrombin has ten Gla residues at N-terminal region; prothrombin cannot be
cleaved without Gla modification
Vitamin K is cofactor for enzyme that produces Gla modification
Dicoumarol (found in spoiled sweet clover) and warfarin (used as rat poison)
interfere with vitamin K cycle and inhibit clotting; can be used as blood
thinner drugs
d. Function of Gla residues in prothrombin cleavage
 Prothrombin must associate with negatively charged phospholipid membrane
to be cleaved by factor X, which occurs at site vessel injury
 Prothrombin-membrane association requires Ca2+, which forms bridges
linking Gla residues to phospholipids
 Higher negative charge from Gla modification provides stronger attraction for
Ca2+
 Ca2+ chelating agents prevent clotting of blood samples
4. Blood clotting disorders
a. Hemophilia A; inherited deficiency of factor VIII; X-linked
b. Von Willebrandt disease; inherited deficiency of vWf
 Platelet adhesion depends on vWf
 Factor VIII forms complex with vWf which increases stability of factor VIII;
loss of vWf decreases stability of factor VIII
5. Controlling blood clotting
a. Inappropriate blood clots major cause of heart attacks and strokes
b. Protein C – Protein S
 Excess thrombin binds to endothelial cell receptor called thrombomodulin
 Thrombin bound to thrombomodulin activates protein C, which then forms
complex with protein S called activated protein C complex (APC)
 APC anchored by protein S to platelet surface at clot by calcium bridges
 APC destroys activated factor V and VIII by proteolysis
 Inherited deficiency of protein C or protein S, or point mutation in APC
cleavage site of factor V increase risk of venous thromboembolism
c. Heparin
 Antithrombin is protein present in blood that inhibits clotting by inhibiting
most proteases of clotting system; weak activity strongly enhanced by binding
glycosaminoglycan heparin
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Heparin secreted in response to injury by mast cells that line blood vessels to
limit spread of clot; useful as blood thinner drug
d. Plasminogen activators
 Plasmin is protease that cleaves fibrin to dissolve blood clots; inactive
plasminogen present in blood and cleaved into active plasmin by various
proteases known as plasminogen activators
 Tissue-type plasminogen activator released from endothelial cells; useful drug
for dissolving blood clots in stroke and heart attack patients
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