Biochemistry-Coagulation & Fibrinolysis Flashcards
What is the difference between plasma and serum?
Serum has the clotting proteins removed
What are the major proteins that make up plasma?
Albumins (60%) and Globulins (35%)
Function of albumin
Osmotic pressure and transportation of fatty acids, bilirubin and drugs.
Function of globulins
Transport of lipoproteins, retinol, transferrin, steroids; host defense (Ig and complement); regulation (hormones, enzymes)
How much thrombin is activated in the clotting cascade?
Not very much. You only need a little bit to activate fibrin.
What are the intrinsic and extrinsic clotting cascades? What types of enzymes predominate in this cascade?
Serine proteases make up the majority of the activated enzymes in the clotting cascade.
Initiation phase of the clotting cascade
Sub-endothelial tissue factor is exposed after vascular injury. Factor VIIa contacts tissue factor and forms a complex. TF-VIIa complex activates factors IX and X. Factor X forms a complex with cofactor Va (now prothrombinase) on the TF-bearing cell and converts prothrombin to thrombin. Thrombin then goes on to activate factors V, VII, VIII and XI.
Where does the active Xa come from in the clotting cascade?
Local site of activation, prothrombinase is resistant to ATIII and serpins. Free Xa, IXa and XIa in the circulation are inactivated by antithrombin III and serpins (serine protease inhibitors) so clots aren’t formed in circulation.
Amplification phase of the clotting cascade
Platelets and factor VIII/vWF access the TF-bearing cells at the site of injury. Platelets release factor V, thrombin activates it and more prothrombinase complexes (activated V/X complex) is formed and thrombin levels are amplified. Thrombin also releases VIIIa from the VIII/vWF complex. VIIIa binds activated platelets and factor XI is activated
What happens to platelets when they are bound by thrombin? What happens when they are bound by collagen.
They change shape and degranulate. When collagen binds they shoot out filaments and become pseudopods.
What role do factors XIII and IX play on the platelets?
They activate X so that it can form a prothrombinase complex with Va. “VIII and IX are Xase”
Hemophilia A
Factor VIII deficiency
Hemophilia B
Factor IX deficiency
Most common inherited bleeding disorder?
Von Willebrand’s
Propagation phase of the clotting cascade
Factor IXa, which was activated in the initiation phase, binds factor VIIIa on the platelet surface. Factor X is activated to Xa at many sites on platelets, generating a burst of thrombin large enough to clot fibrinogen and form a hemostatic plug
How does damage to the blood vessel cause platelet aggregation?
Platelets are exposed to vWF and collagen. GP I/II surface molecules allow for platelet adhesion and degranulation. Platelets release ADP (activates surrounding platelets), TxA2 & 5-HT (vasoconstriction) and factor V (stays on surface to form prothrombinase w/Xa).
How does clopidogrel work?
Inhibits ADP binding site on platelets to prohibit activation of other platelets
Mechanism of fibrinogen activation by thrombin?
Thrombin removes the negatively charged Glu and Asp residues that repel fibrinogen molecules from one another, allowing the fibrin molecules to aggregate to form a soft clot. Factor XIIIa (transamidase) converts the soft clot into the hard clot by cross-linking fibrin molecules
What clotting factors require Ca2+? Deficiency in what nutrient could cause decreased function by these clotting factors?
II, VII, IX and X. Vitamin K. These clotting factors are gamma-carboxyglutamate residues. You need Vitamin K to go from Glu to the gamma-carboxyglutamate.
What action on vitamin K is essential in the clotting cascade?
Reduction of the epoxide by vitamin K epoxide reductase. This allows Vitamin K to get back to its active form by vitamin K reductase so it can continue to activate clotting factors II, VII, IX, X and protein S. Note that warfarin blocks Vitamin K epoxide reductase.
What can you give a patient who is hemorrhaging because they took too much warfarin?
You can’t just supplement with vitamin K because it takes too much time to resynthesize all the clotting factors. Instead you would give fresh frozen plasma or factor concentrates to replace clotting factor deficiency.
What is thrombin’s role in preventing intravascular clotting after endothelial injury?
Thrombin binds thrombomodulin on healthy endothelial cells. This complex activates protein C. Protein C forms an activated complex with protein S from the platelet surface that goes on to destroy factors Va and VIIIa, stopping the coagulation cascade.
How does heparin work?
ATIII is a serpin that is always present bound to heparan sulfates on endothelial cells. ATIII inhibits thrombin and most other serine proteases to stop coagulation. Heparin gets into the blood and massively activates ATIII.
How does the body stop the coagulation cascade at the very beginning?
Tissue factor pathway inhibitor (TFPI) inhibits the TF-VIIa complex and Xa.
What role do the endothelial cells play in inhibition of the clotting cascade?
PGI2 and NO release. Degradation of ADP via membrane ADP-ase.
How does the fibrin network get dissolved?
When the network forms, it already has plasminogen and t-PA inside of it. Normally t-PA is kept inactive by plasmin activator inhibitor (PAI-1). Under the effects of protein C, PAI is degraded, activating t-PA. t-PA is a serine protease and converts plasminogen to the active plasmin. Plasmin degrades the fibrin clot until it is exposed to the external clot and is then deactivated by alpha2-antiplasmin.
