Hemostasis Flashcards
List the 6 serine proteases
- II (Thrombin)
- VII
- IX
- X
- XI
- XII
List the 3 Cofactor proteins
- III (tissue factor)
- V
- VIII
What does Ia stand for? I?
- Ia= fibrin
- I=fibrinogen
What does XIII represent?
Transgluatminase (zymogen)
Majority of coagulation proteins are synthesized where?
Synthesized by the liver and circulate freely in the plasma
Which 2 coagulation proteins can be stored in a-granules of platelets?
Factor V and Factor VIII
Where is Tissue Factor (III) located?
Tissue Factor is a transmembrane protein found on the plasma membrane of subendothelial cells
Within a proteolytic complex, what two additional elements are required for maximum proteolytic activity?
- Phospholipids (plamsa membrane)
- Calcium
Which coagulation factors have a vitamin K dependent carboylation of glutamate residues?
- II, VII, IX, X
What are the purpose of y-carboxyglutamate (GLA) residues?
The enhance the ability of serine proteases (II, VII, IX, X) to interact with the plasma membrane of platelets (CALCIUM neccessary)
GLA residues are synthesized by y-glutamate carboxylase; what serves as a cofactor for this enzyme?
The reduced form of vitamin K
What is the name of the enzyme complex that reduces vitamin K?
Vitamin K epoxide reductase
What is the mechanism of action of warfarin?
- Warfarin interferes with vitamin K reductase
- Without reduced vitamin K, GLA residues can’t be sythesized
- Without GLA resides, serine proteases are not able to interact with the plasma membrane of platelets
How do serine proteases with GLA residues interact with the plasma membrane of platelets?
- GLA residues are Negatively charged
- The phospholipid head groups of the plasma membrane are Negatively charged
- Calcium (+ charged) forms a bridge between the two negatively charged substances
- Calcium dependent conformational change also facilities insertion of hydrophobic region of the Protease into the core of the bilayer.
Where does warfarin impair the carboxylation of coagulation factors? How does it affect those coagulation factors that are in the circulation?
- Warfin only acts on NEWLY synthesized coagulation factors in the liver
- Functional coagulation factors (With GLA residues) that make it to circulation are not affected by warfarin treatment
What is the order of the half-lives (from shortest to longest) of the serine proteases that are able to have a GLA residue (thus can be affected by warfarin)?
- Factor VII (3-6 hours)
- Factor IX (18-24 hours)
- Factor X (40 hours)
- Factor II (70 hours)
What two coagulation factors make up the Initiation Complex? Where does this complex form?
- Tissue Factor (III)
- VIIa (protease)
- Ca2+ and Phospholipid needed
- Forms on subendothelial cells
What two coagulation factors make up the Tenase Complex? Where does this complex form?
- IXa (protease)
- VIIIa (cofactor)
- Ca2+ and phospholipid needed
- Forms on the surface of activated platelets
What two coagulation factors make up the Prothrombinase complex? Where does this complex form?
- Xa (Protease)
- Va (cofactor)
- Ca2+ and phopholipid needed
- Forms on surface of activated platelets
What initiates the process of coagulation?
- Damage to vascular exposes Tissue Factor (III)
- Tissue Factor acts as cofactor for Factor VIIa (available in circulation)
- TF/VIIa + Ca2+ + Lipid bilayer = INITIATION COMPLEX
On the subendothelial cell, the initiation complex activates which two Factors?
- Factor IX → IXa
- Factor X → Xa
When Protease Xa comes into contact with Va (+ Ca2+ and PL), what is produced?
Turns Prothrombin into Thrombin
Thrombin activates what coagulation factors?
- XI ⇒ XIa
- VIII ⇒ VIIIa
- V ⇒ Va
What is the action of Protease XIa? Does it require a cofactor?
- XIa activates IX ⇒ IXa
- IXa combines with cofactor VIIIa (activated via thrombin)
What is the action of Protease IXa? Does it require a cofactor?
- IXa together with cofactor VIIIa (activated via thrombin) create the TENASE COMPLEX
- Activates X ⇒ Xa
What is the action of Xa? Does it require a cofactor?
- Xa together with cofactor Va (via Thrombin) form the Prothrombinase complex
- Turn Prothrombin to Thrombin → creates burst of Thrombin production
When a mulititude of Thrombin has been created, what is its main purpose?
- Cleavage of Fibrinogen → Fibrin
- activates Factor XIII : transglutaminase enzyme
What is the function of fibrin?
Fibrin can self-assemble into a soft clot
What is the function of Factor XIIIa (activated by thrombin)?
Factor XIIIa is a transglutaminase enzyme that forms covalent bonds between glutamine and lysine residues to form a Hard clot
- clinical note: if clot can withstand treatment of 5M Urea, then it is a hard clot. If clot dissolves, then XIII is impaired
What is Antithrombin III (ATIII)? What does it act on?
- ATIII is a serine protease inhibitor
- Targets: Thrombin, Xa, and IXa
Activity of ATIII is increased 1000-fold by binding to what?
- Heparan Sulfate (present of surface of endothelial cells)
- Provides docking site to bring ATIII into close proximity with its substrates and increases the rate of reaction
What drug can be used to immediately inactive coagulation factors?
Heparin
Heparin comes in what two forms?
- Unfractionated heparin: 45 sugars long; binds to ATIII, IXa, Xa, and IIa (Thrombin)
- Low molecular weight heparin: 18 sugars long; binds to ATIII and Xa
What is the action of Tissue Factor Pathway Inhibitor (TFPI)?
- circulating anticoagulant protein that inhibits factors VIIa and Xa on the surface of subendothelial cells
What is impaired in Hemophilia A or B? Why are the actions of TFPI problematic in these individuals?
- Impaired tenase complex
- Inhibits activation of Xa (of prothrombinase complex)
- Puts much greater significance of Xa created by initiation complex on subendothelial cells
- TFPI eliminates this only source of Factor Xa
Endothelial cells have two membrane receptors which utilize thrombin to create a natural anticoagulant. When binds to Protein C, binds to Thrombin which hides its binding site for its platelet receptor. Name these two receptors.
- Endothelial Protein C receptor (EPCR) binds to Protein C
- Thrombomodulin binds to Thrombin
How does Protein C become activated? What is its function?
- On the endothelial cell, Thrombin (bound to Thrombomodulin) activates Protein C
- Activated Protein C binds to platelet membranes
- With help of cofactor (Protein S), APC degrades Factor Va and Factor VIIIa on platelet membranes
In Leiden mutation, what Coagulation factor is mutated? What does this impair?
- Factor Va is mutated
- Becomes resistent to APC cleavage
- Present with hypercoagulability and are at increased risk for venous thrombi formation
How are arterial thrombi formed? What other name are they referred to?
- Result of rupture of atherosclerotic plaques which expose collagen
- High shear forces of arterial circulation activate vWF and cause platelet deposition at these sites
- Referred to as white clots; comprised of mostly platelets
What drug is used to arterial thrombi? How does it do it?
Plavix interupts platelet plug formation and thus reduce risk of thrombus formation
How are Deep Vein Thrombi formed? What other name are they referred to?
- DVT occurs is large veins of legs - enviroment of LOW SHEAR that favors COAGULATION
- Develop of non-ruptured endothelial surfaces that have lost some anticoagulant properties
- Red clots: contain primarily fibrin with trapped RBC
What coagulation factor is deficient in Hemophilia A? What other disease mimicks this deficiency?
- Deficiency in Factor VIII (Tenase cofactor)
- mimicked by von Willebrand disease; vWF stabilizes Factor VIII in the circulation
What coagulation factor is deficient is Hemophilia B?
Deficiency in Factor IX (Tenase Protease)
Patients with Hemophilia are at increased risk of bleeding into what?
Joints (ankles, knees, elbow) because joints have limited amounts of tissue factor
What types of drugs should Hemophiliacs stay away from ?
NSAIDs which inhibit platelet plug formation; which in hemophiliac patients becomes the sole means to stop bleeding
Test tubes for blood sample collection have what?
Citrate: an anticoagulant
In a blood sample, how can the effect of citrate by reversed?
Adding additional calcium into the sample, which aloows coagulation to be measured
Plasma is composed of what?
- coagulation factors + citrate (anticoagulant)
- No RBC, WBC, or platelets
In serum collection, what is left out so that RBC’s and clotted blood may be removed?
Does not contain an anticoagulant
What is bleeding time?
- Measure of the time it takes to stop the flow of blood from a standarized wound
- Assesses platelet function to perform platelet plug
- Does not assess clotting cascade function
What pathways does the PT (prothrombin time) test evaluate?
In the lab, Calcium and Thromboplastin (tissue factor and phospholipids) are added to the sample to examine the Extrinsic (VII) and Common pathways (Factors V, X, II [thrombin], and I [fibrinogen])
Why is the PT test best for monitoring warfarin therapy?
- Of the 4 vitamin K dependent clotting factors, Factor VII has the shortest half life and will be the first to be replaced with newly-made GLA-residue deficient Factor VII from the liver
- PT test measures extrinsic pathway and is very sensitive to decreases in Factor VII
What two pathways does the activated partial thromboplastin time (aPTT) test evalutate?
- Intrinsic (XII, XI, IX, VIII) and Common Pathway (V, X, II, I)
- A negatively charged surface activates Factor XII which starts the coagulation cascade
- Calcium and Phospholipid (not tissue factor) is added
