Midterm #3: Coagulation Cascade Flashcards
1
Q
Purpose of Clotting Cascade
A
- Plasma proteins that rapidly form a clot at damage site to minimize blood loss (hemorrage) and to maintain **homeostasis **
2
Q
Homeostasis
A
- refers to the maintenance of the fluid state of the blood
- Requires a delicate balance between procoagulation (clot formation), anticoagulation, and fibrinlysis (clot dissolution)
- Dozens of plasma proteins are involved, as well as tissue protiens, blood platelets and surfaces of endotheilial cells that line vessels
- Defects in these can lead to pathological bleeding or thrombosis (clotting)
3
Q
The 5 phases of Coagulation: Overview
A
- Vasocontriction
- Formation of hemostatic plug
- Formation of a blood clot
- Control of coagulation
- Fibrinolysis
4
Q
Vasoconstricition
A
- Damage to vessel walls leads to smooth muscle spasm that results in narrowing of blood vessel diameter
- Limits blood flow to the damaged area and to allow localization of platelets and clotting factors
- Can last up to 30 min
- Vessel damage exposes collagen to the blood which is an important trigger for subsequent phases of coagulation
5
Q
Formation of a Hemostatic Plug
A
- platelets bind to the site of damage (adhesion)
- release signaling factors (activation)
- and stick together to form a scaffold for clotting (aggregation)
6
Q
Formation of a Blood Clot
A
- cross-linked fibrin reinforces the platelet plug to form a clot via a cascade of enzymatic reactions
7
Q
Control of Coagulation
A
- negative feedback signaling ensures that clot formation does not continue unchecked.
8
Q
Fibrinolysis
A
- the clot is removed after the underlying problem (injury, infection, etc.) is dealt with.
9
Q
Platelets
A
- circulating, anuclear cells that are fragments of megakaryocytes, which are produced in the bone marrow
- contain mitochondria and glycogen granules
- No DNA or protein synthetic capability
- contain ADP, serotonin (5-HT), and platelet factors (PFs) that play important roles in vasoconstriction, the formation of a platelet plug, and clotting.
10
Q
Adhesion
A
- damage to endothelial cells exposes collagen to which platelets bind, and releases the von Willebrand factor (vWF) which enhances collagen:platelet binding.
11
Q
Activation
A
- binding stimulates the platelets to release ADP, serotonin (5-HT), thromboxane A2 (TxA2) and “platelet factors”. These compounds mediate further vasoconstriction and the transition to a “sticky” platelet.
12
Q
Aggregation
A
- Sticky platelets form form a hemostatic plug that serves as a scaffold for the subsequent clotting cascade.
13
Q
prostacyclin (PGI2)
A
- produced by undamaged endothelial cells.
- causes vasodilation and inhibits platelet aggregation.
14
Q
Phase 2 Diagram
A
15
Q
The Anti-Platelet Activity of Aspirin
A
- covalently inactivates cyclooxygenase in both platelets and endothelial cells.
- Shuts down thromboxane synthesis and inhibits platelet aggregation
- Inhibition of endothelial COX shuts down prostacyclin synthesis, which has the opposite effect.
- At lower asprin concentration, platelet COX inactivated and cannot be regenerated
- Endothelial can synthesize more enzyme
16
Q
Name This Structure
A
Aspirin
17
Q
Plavix
A
- Clopidogrel
- Anti-platelet drug
- Inhibits a platelet ADP receptor and thereby inhibits aggregation
18
Q
Phase 3: Formation of a Blood Clot
A
- Cascade (amplified series of signals) results in proteolysis of fibrinogen (soluble plasma protein) to fibrin monomer
- Fibrin monomers polymerize to form a mesh network, the fibrin clot that traps blood cells and helps plug the leaking vessel
- Strengthened by cross-links between Lys and Arg residues catalyzed by a transglutaminase enzyme
19
Q
Getting to Know the Clotting Factors
A
- Most numbered I to XIII and are synthesized as inactive zymogens (proenzymes)
- Each factor must be activated often involving proteolysis
- Vitamin K Dependent Factors:
- IX, VII, X, and II (prothrombin)
- Require Ca++ for activity
- Thrombin Sensitive Factors
- V, VIII, XIII, and I (fibrinogen)
- Named clotting factors:
- **Fibrinogen **(I) and fibrin (Ia)
- **Prothrombin **(II) and thrombin (IIa)
- **Tissue Factor **(III)
- Transglutaminase (XIIIa)
- Prothrombinase (complex of Xa and Va)
- Tenase (VIIa or a complex of VIIIa and IXa; activate X and operate in extrinsic and intrinsic pathways respectively)
20
Q
Overview of the Clotting Cascade: Intrinsic and Extrinsic Pathways
A
- Intrinsic Pathway: triggered by exposed **collagen **and von Willebrand factor
- Extrinsic Pathway: triggered by tissue factor released by damaged epithelial cells
21
Q
Overview of Clotting Cascade: Diagram
A
22
Q
The Extrinsic Clotting Pathway
A
- a.k.a. the Tissue Factor Pathway
- Damaged blood vessels release Tissue Factor (factor III)
- TF binds to the Factor VII and activates it
- limits coagulation to injury site
- VIIa is a Ca2+-dependent protease that converts X to Xa (tenase activity)
- Factor Xa is a Ca2+-dependent protease that, with factor Va, cleaves prothrombin (factor II) to thrombin (factor IIa)
- Va also binds to activated platelets
- localizes clotting
- Thrombin is a Ca -dependent protease that cleaves fibrinogen (I) to fibrin (Ia)
- Fibrin polymerized to form a soft fibrin clot
- Thrombin also activates Factor XIII, which is a transglutaminase that cross-links fibrin polymers to form a mesh network: the hard fibrin clot
23
Q
The Intrinsic Clotting Pathway
A
- a.k.a. the Contact Clotting Pathway
- Damaged blood vessels expose collagen
- Factor XII binds to the site of the injury (aided by the other contact activation factors: prekallikrein and high-molecular-weight kininogen [HK]) and autoactivates to XIIa
- vWF promotes this process
- Factor XIIa proteolytically activates XI to XIa
- XIa activates factor IX which, together with factor VIIIa, activates factor X to Xa (tenase activity).
- Remainder of pathway leading to fibrin activation is the same as the extrinsic pathway
24
Q
The Central Role of Thrombin
A
- Thrombin proteolytically activates V, VIII and XIII
- promotes coagulation via feedback activation
- Thrombin also activates platelets by proteolysis of the thrombin receptor in the platelet cell membrane
- Cascade is amplifying because each protease is a catalyst for the activation of another catalyst
- Very small amount of Factor VIIa can generate larger amount of Factor Xa, which can then generate an even larger amount of thrombin
- Tissue factor pathway more important in vivo and generates a “thrombin burst” required for efficient & timely clotting
25
Phase 4: Control of Coagulation and Clot Retraction
* Initation of coagulation cascade activates am=nti-coagulation proteins (feedback inhibition)
* **Tissue Factor Pathway Inhibitor (TFPI):** is a plasma protein that inhibits thrombin and Xa. The Xa:TFPI complex further inhibits VIIa
* **Anti-Thrombin (AT)** inactivates thrombin and Xa. AT inhibition is strongly stimulated by **heparan sulfate** a _glycoaminoglycan_ composed of repeats of sulfated disaccharides
* **Protein C and Protein S:** Protein C is activated by a complex of thrombin and **thrombomodulin** (found on the surface of endothelial cells) and then binds to Protein S. The complex inhibits Va and VIIIa
* activated platelets in the clot physically contract, shrinking the fibrin network and making the clot denser and mechanically stronger
26
Phase 5: Fibrinolysis
* After clot formation, underlying problem can be repaired
* Blood clot must be removed to restore normal blood flow
* Funx of **fibrinolytic pathway**
* Thrombin:thrombomodulin complex induces the release of **tissue plasminogen activator (tPA)** which activates **plasminogen** to **plasmin**
* Plasmin is a protease that degrades fibrin to soluble degradation products
* **Urokinase** (produced in the kidneys) and **streptokinase** (produced by bacteria) can also activate plasmin
* Fibrinolysis can be inhibited in 2 ways:
* direct inhibition of plasmin by **alpha2-antiplasmin**
* inhibition of plasminogen activation by **plasminogen activator inhibitor (PAI)**
27
Hemophilia
* hereditary disease that impair coagulation process
* vonWillebrand diesase: deficiency of vWF
* hemophilia A: Factor VIII deficiency
* hemophilia B: Factor IX deficiency
* Can get recombinant factors VIII, and IX
28
Hypercoagulability disorders (a.k.a. congenital thrombophilias)
* defects in anticoagulation
* such as Factor V Leiden thrombophilia
* mutation in factor V that prevents inactivation by PC:PS complex
29
Atherosclerosis
* Accumulation of cholesterol rich plaques in vessels
* Can become calcified
* Can narrow vessel
* Block occur when clot forms from when plaques rupture of when inflammation causes tissue factor exposure
30
myocardial infarction, stroke, pulmonary embolism
* A clot in coronary artery: myocardial infarction
* A clot in cerebral artery: stroke
* Deep vein thrombosis during periods of stasis or immobilization. Can dislodge and travel to the lung and cause pulmonary embolism
31
Heparin
* glycosaminoglycan similar to heparan sulfate, but is more highly sulfated and is produced solely by basophils and mast cells
* promotes inhibition of thrombin (IIa) and Xa by anti-thrombin
* Must be given via injection, can have significant side effects like thrombocytopenia and needs closely monitored therapy
* Unfractionated heparin (12-15 kDa)
* Low Molecular Weight heparins (\<8 kDa): more predictable activity, shorter half-life, and lower incidiences of side effects
32
Name this structure
heparin
33
Fondaparinux
* pentasaccharide that binds to anti-thrombin and inhibits Factor Xa, but not thrombin
* Used to treat DVTs and pulmonary embolism
34
Recombinant tPA, streptokinase, urokinase
* may be used as **thrombolytics**
* disrupt blood clots in the first few hours following a stroke or MI
35
Name this structure
fondaparinux
36
gamma-glutamyl carboxylase
* Factors II (thrombin), VII, IX, and X contain multiple glutamate residues that must be postranslationaly modified to gamma-carboxyglutamate residue via gamma-glutamyl carboxylases
* These dicarboxylate side chains tightly bind Ca enabling tight binding to phospholipids at the platelet surface and anchoring these proteins to the damaged site
* Vitamin K is essential co-factor for gamma-glutamyl carboxylase
37
Warfarin
* inhibits vitamin K epoxide reducatse (VKOR)
* Vary widely depending on the genotype of CYP2C9
* Patients with cerebral infarct, DVT or PE are started on heparin then transitioned to warfarin
* Direct thrombin inhibitors might be used in place of heparin or warfarin in certain clnical situations
38
Name this structure
Vitamin K
39
Name this structure
Warfarin
