Overview Of Hemostasis (Schalm 87) Flashcards
What are the two components of coagulation complexes? At what site are they normally bound and act?
Cofactors and Enzymes
They bind and act at a procoagulant membrane surface
How are coagulation enzymes inhibited (4 mechanisms)?
Enzyme cleavage
Active site blockade
Stable complex formation
Substrate modification
How does membrane binding enhance coagulation enzyme activity?
Membrane binding properly aligns the participating proteins
How do coagulation factors VII, IX, X, prothrombin, and proteins C, S, and Z bind to a membrane surface?
They contain gammacarboxyglutamic acid (GLA) residues that interact with calcium and negatively charged phospholipids
What is required for carboxylation of glutamic acid precursors to GLA domains?
The vitamin K cycle in the liver
What complex formation characterizes the extrinsic coagulation pathway?
Tissue factor and activated factor VII (TF-FVIIa) - > generates activated factor X
What complex formation characterizes the intrinsic coagulation pathway?
FXIa-FVIIIa -> ultimately results in FXa generation
What complex formation characterizes the common pathway?
FXa-FVa -> activates prothrombin to generate thrombin, which cleaves fibrinogen to fibrin
Which coagulation cofactors have regions that interact with phospholipids, allowing fully functional enzymatic complexes to assemble on a membrane surface?
Factors V and VIII
What two zygomogens and single cofactor make up the contact pathway?
Factor XII and prekallikrein (zymogens)
High-molecular-weight Kininogen (HK)
When does contact activation of coagulation occur?
When FXII, PK, and HMW spontaneously assemble on a suitable negatively charged surface - this occurs when blood is removed from the vascular system and comes into contact with an artificial surface (resulting in the “intrinsic” clotting potential of blood)
How is the contact pathway of coagulation affected by calcium chelators (citrate, EDTA)?
It is not affected as it is not calcium dependent (and may influence viscoelastic tests that utilize a holding period);
Seems most pronounced in equine, then canine, then human samples
What role does the contact pathway play during normal in vivo coagulation?
Little to none;
It may play a role in pathological thrombus formation, potentially binding to RNA, polyphosphate (e.g. ADP, ATP), or misfolded proteins
What physiologic roles does the contact pathway play?
Bradykinin generation (after activation on negatively charged phospholipid membranes)
Angiogenesis
Profibronlyitic, proinflammatory, and anti-adhesive properties
What mineral is required for HK binding to membranes surfaces?
Zinc
Name three functions of kinongens?
Inhibition of atrial natriuretic peptide
Prevent calpain-mediated platelet aggregation
Prevent thrombin binding to platelets
Release bradykinin (after cleavage by kallikrein) - a vasoactive peptide
What are two cell types that express abundant tissue factor (FIII)?
Adventitial cells surrounding blood vessels larger than capillaries
Differentiating skin keratinocytes/other epithelial cells
(This distribution is consistent with TF’s function as a protective “hemostatic envelope”)
What is the likely source of blood-borne tissue factor?
Monocytes or microvesicles from TF-bearing monocytes
What outer membrane phospholipid may play a role in maintaining TF in an encrypted state?
Sphingomyelin
Which coagulation factor zymogen has the shortest half-life?
Factor VII - thus it’s levels decrease rapidly if liver function or vitamin-K cycle is impaired
The activated form of Factor VII (FVIIa) has the LONGEST half-life of the enzymes, approximately 2 hours (others are measured in seconds)
How does the presence of circulating FVIIa affect hemostasis?
Exposure of blood to TF allows formation of both TF-FVIIa and TF-FVII complexes. THe TF-FVIIa complexes probably generate sufficient enzymatic activity to trigger the clotting cascade
What are the main inhibitors of the TF-FVIIa complex?
Tissue factor pathway inhibitor (TFPI) - FXa complex
Heparin - antithrombin
Where does the intrinsic cascade primarily occur?
It primarily occurs on the surface of activated platelets, which expose binding sites for FIXa, FVIIIa, and FX, greatly increasing the rate of FXa generation by many millionfold
What is the primary binding site for FXI? What is the preferred substrate for FXIa?
The GPIb-alpha subunit of the GPIb/IX/V complex
The preferred substrate is FIX (will also cleave additional FXI)
What are the two activators of FIX bound to a procoagulant membrane surface?
TF-FVIIa complex and FXIa
What factor enables proper association of two FVIII chains, improving its intracellular and plasma stability?
VonWillibrand Factor (VWF)
What two factors form the prothrombinase complex?
FXa (enzyme) and FVa (cofactor)
What are the two strong inhibitors of the prothrombinase complex?
AT-HSPG
TFPI
What are the two products after the first cleavage of prothrombin
alpha-thrombin (most active on fibrinogen
meizothrombin (also contains enzyme sites, but less active)
Name three places Tissue factor pathway inhibitor is found?
Freely circulating in the active form
Attached to endothelial cell surfaces
Platelet alpha-granules, released on activation
What factors are inhibited by TFPI?
TF
FVIIa
FXa
What is the main function of C1-inhibitor? What role does it play in hemostasis?
C1-inhibitor inhibits the complement system
It also inhibits the contact pathway, acting on FXIIa, kallikrein, and FXIa; deficiency has no impact on hemostasis, but results in episodic production of bradykinin and angioedema
What is required for antithrombins inhibitory activity?
Endothelial-bound HSPG
Pharmaceutical heparin molecules
(It’s ability to inhibit varies with heparin structure and the target protease)
What is the activator of protein c?
Thrombin or meizothrombin to APC
What two cofactors are inactivated by protein c?
FVa
FVIIa
Where is thrombomodulin found? What are its activities?
Endothelial transmembrane molecule
Inhibits thrombin’s interaction with fibrinogen
Increases ability of thrombin to cleave PC (activating it)
What is the cofactor of APC-mediated cleavage of FVa and FVIIa?
Protein S
Which pathway plays a major role linking inflammation and coagulation?
The protein C pathway
What are four ways endothelial cells prevent thrombus formation?
Expression of thrombomodulin
Expression of HSPG, which binds to AT
Production of urokinase (Upa) and tissue plasminogen activators -> fibrinolysis
Expression of tissue factor pathway inhibitor
What is a prothrombotic property of endothelial cells?
Store VWF in Weibel-Palade bodies, which they secrete on activation by thrombin
What role does nitric oxide secreted by endothelial cells play in coagulation?
Inhibits platelet activation
Causes vasodilation
What phospholipid is procoagulant, markedly increasing the rate of coagulation reactions?
Phosphatidylserine
What are the three phases of coagulation in the cell based model?
Initiation (exposure of TF via injury, formation of TF-FVIIa complex, activation of FIXa)
Amplification (small amount of thrombin activates platelets that have escaped vasculature, activation of FXIa, FVa, and cleavage of VWF from FVIII ->FVIIIa)
Propagation (recruitment of additional platelets, platelet aggregation, binding of FIXa-FVIIIa complex on platelet surface and formation of FXa-FVa prothrombinase complex)
What is the primary function of thrombin? What are three other activities of thrombin?
Conversion of fibrinogen to fibrin, which results in spontaneous polymerization into an insoluble fibrin gel
- feedback, both generating and inhibiting it’s own formation and fibrin degradation
- platelet activation
- activation of factor XIII, which crosslinks fibrin strands
Where is plasminogen synthesized? Where is it found?
Synthesized in the liver
Found in plasma with long half-life of 2 days
What are the products of fibrin degradation by plasmin called?
Fibrin degradation products
What are D-dimers?
D-dimer fragments are produced by degradation of the linked region between D domains of adjacent fibrin molecules
Where is tissue plasminogen activator produced?
By endothelial cells in response to bradykinin, histamine, acetylcholine, and PAF
What is the major plasminogen activator in the extravascular space? What is its major role?
Urokinase plasminogen activator
Produced by fibroblast-like cells, epithelial cells, monocytes, and endothelial cells to degrade ECM in the process of cell migration and healing
Elevated levels of plasminogen activator inhibitor-1 are associated with what?
Prothrombotic disease
What is the primary plasmin inhibitor?
Alpha2-antiplasmin
