8-11. Hemostasis - Grossclose Flashcards
Where does factor X occur in the coagulation scheme? What does it do?
Factor X plays an important role in the beginning part of the common pathway. It can be activated either by the intrinsic or extrinsic pathway. Accelerated by a protein cofactor (Factor Va), Factor X activates prothrombin to thrombin.
What is the importance of vitamin K to the coagulation cascade? Does vitamin K participate directly at the site of clotting?
Vitamin K is an important part of the cascade because several factors (II, VII, IX, protein C and protein S) use it as a cofactor for post-translational modification. Vitamin K allows the vitamin-K dependent factors to add an extra carboxyl group to the glutamate residue near the N-terminus that can then bind a molecule of Ca2+. The Ca2+-complexed portions of the clotting factors can interact with phospholipids. Vitamin K does not participate directly at the site of clotting.
What is the role of gamma-carboxyglutamate residues in clotting factors?
After synthesis of clotting factors, an extra carboxyl group is added to glutamate residues near the N-terminus by gamma glutamyl carboxylase. It does not act directly at the site of clotting. The extra carboxyl group gives these factors a divalent anion (2-) at the glutamyl residue, enabling Ca2+ to bind. These calcium complexes enable the factors to interact with phospholipids of plasma membranes.
[Glu proteins to Gla proteins]
Many activated coagulation factors are enzymes. What class of reactions do they catalyze?
Many of the coagulation factors are proteolytic enzymes (proteases) that converts the other proenzymes (zymogens) into their active enzymatic forms.
What is the “antihemophilic factor”, and at what stage does it function?
Factor VIII is the “antihemophilic factor” It is a protein cofactor that often circulates with vonWillebrand factor (vWF). Together with Factor IXa, they work to activate Factor X, thus initializing the common pathway. It works at the end stage of the intrinsic pathway.
Name the K-dependent factors.
Factor II (prothrombin), VII, IX, X, Protein C and Protein S
How does factor XIIIa differ from the other enzymes of the coagulation cascade?
Factor XIIIa is the only non-proteolytic enzyme of the clotting cascades. It is a transamidase which catalyzes the formation of peptide bonds between the side chains of lysine and glutamine residues in fibrin chains. (aka Fibrin Stabilizing Factor)
Name the factors of the common pathway of coagulation and discuss their roles.
Factor X, which is activated by Factor IXa, to Factor Xa. Factor Xa activates Prothrombin (Factor II) –> Thrombin (Factor IIa). Thrombin activates fibrinogen (Factor I) –> fibrin.
Thrombin ALSO converts Factor V –> Factor Va (not an enzyme), which is a cofactor for Factor Xa, making Xa hundreds to thousands of times more active. [allosteric enhancer]
In addition, activates protein c, platelets, factor VIII/vWF and conversion of Factor XIII to Factor XIIIa, which makes Fibrin cross-linked.
Which (2) proteins of the cascade are not enzymes?
Factor VIII and Factor V are not enzymes. Factor VIII is a cofactor for Factor IXa.
At which amino acid does K-dependent carboxylation occur?
Glutamate residues near the N-terminus
Where are the components of the cascade synthesized?
The liver synthesizes plasma-clotting factors. Renal tissue and megakaryocytes produce Factor XIII and vWF.
Discuss the ways in which coagulation is regulated.
Coagulation can be limited in several ways:
Blood flow can “sweep away” clotting elements, diluting their effects
Uptake by the liver and inactivation of activated factors
Endogenous antiproteases; inhibition of coagulation process during the very process of coagulation
Undamaged endothelial cells generate antiaggregant
Some activated factors can break down other factors
Discuss the merits and drawbacks of anticoagulant therapy with heparin versus vitamin K antagonists.
Heparin enhances the inhibitory action of antithrombin III (it binds to the active site of thrombin and inhibits its activity). Once the complex of antithrombin III and thrombin is made, the heparin molecule leaves and finds another antithrombin III and enhances its binding to thrombin. There is the drawback that heparin therapy will not work, and that most likely signifies an antithrombin III deficiency.
Vitamin K antagonists (ie. Warfarin, coumadin, etc.) prevent the K-dependent carboxylation of factors II, VII, IX and X. They are used for long-term control of thrombotic tendencies and their effectiveness depends on protein turnover.
Name the sequence of participation of factors in the intrinsic and extrinsic pathways.
Extrinsic Pathway (uses Tissue Factor Pathway) **Factor VII --> Factor VIIa (Factor VII is the only factor that is already slightly active before converted to VIIa). When Factor VIIa meets tissue factor (thromboplastin/Factor III), they convert IX to IXa, which converts Factor X to Xa. Aiding as cofactors, Ca2+ & phospholipid helps converts Factor IX --> IXa and Factor IXa with Factor VIIIa/vWF converts Factor X --> Factor Xa
Intrinsic Pathway
Factor XII –> Factor XIIa by contact with a negatively charged surface (like uncoated glass) and via a conformational change.
Factor XIIa with prekallikrein (PK and HMWK (HK)) converts Factor XI –> Factor XIa, which converts Factor IX –> Factor IXa
Factor IXa along with Factor VIIIa/vWF converts Factor X –> Factor Xa. It is important to note that Factor XIIa also converts plasminogen to plasmin, and starts the healing process as well.
Describe the mode of action of heparin.
Heparin does not directly cause clotting. Heparin enhances the inhibitory action of antithrombin III.
