BL Hemostasis Pt 2

Question 1

Antithrombin what is it and which factors does it inhibit?

Answer 1

is a serpin (serine protease inhibitor) that targets:
prekallikrein
Factor XIIa
Factor XIa
Factor IXa
Factor Xa
Factor VIIa
Thrombin (Factor II)

• Antithrombin is free floating. Only has to see factors to inactivate them

Question 2

How does heparin affect antithrombin?

Answer 2

• Heparin is a cofactor for antithrombin. It speeds up antithrombins ability to inactivate the serine proteases by several thousand fold.

(faster anticoagulation) - blood thinner

Question 3

Two mechanisms that heparin acts as antithrombin cofactor?

Answer 3

1) Longer form of heparin molecule that binds to both antithrombin and its serine protease target to bring them close in proximity and inactivate the protease.
- Thrombin is inactivated this way.

2) Pentasaccharide (short) sequence of heparin induces conformational change in antithrombin that allows it to binds better, and thereby inhibit.

Inactivates factor Xa but not thrombin. (heparin is too short)

Question 4

Explain how protein C is activated and how the protein C - protein S system regulates coagulation. What does that result in?

Answer 4

Thrombin (factor II) + thrombomodulin (transmembrane protein on endothelial cells) cleaves and activates protein C
- becomes APC

Protein S is a cofactor for protein C → inhibits cofactors VIIIa and Va → anti-coagulation

Question 5

Factor V Leiden

Answer 5

○ Factor V Leiden is Factor Va resisting inactivation by activated Protein C. → risk factor for venous thromboembolism.

• One of protein C’s target is factor V, and if factor V gets mutated (point mutation) from glutamine to arginine at position 506, it becomes resistant to the proteolytic activity of Protein C.

Question 6

tissue factor pathway inhibitor (TFPI) functions in regulating coagulation.

Answer 6

TFPI can complex with other factors to play a role in anticoagulation and protection from atherosclerosis by turning off the initiation phase (extrinsic pathway)

Quaternary complex:

1. TFPI
2. Factor Xa
3. Factor VIIa
    4. Tissue factor

Question 7

Fibrinolysis
what is it?
when does it begin?
What is key enzyme for this to occur?

Answer 7

process of clot breakdown that occurs following clot formation → allows eventual repair of damaged blood vessel following injury.

begins as soon as the clot forms

Need plasmin

Question 8

Plasmin

• what is it?
• what activates it?

Answer 8

Serine protease that can cleave both fibrinogen and fibrin.
The fragments can be detected in blood, called fibrin degradation products, FDP.

• It is activated when plasminogen is cleaved by tissue plasminogen activator (t-PA) or urokinase (u-PA)

Question 9

Serine proteases

Answer 9

(7) Serine Proteases:
	• Prekallikrein
	• Factor XII
	• Factor XI
	• Factor IX
	• Factor X
	• Factor VII
	• Factor II (prothrombin)

(These are the guys that get directly made from the previous step - minus Prekallikrein, VII and XII since they come first)

others: plasmin, protein C, t-PA, u-PA

Question 10

Where is plasminogen produced?

Answer 10

Plasminogen is produced in liver and circulates plasma.

Question 11

How does tPA activate plasminogen to plasmin?

Answer 11

1. After clot is formed, plasminogen is embedded in the clot.
2. tPA binds to the plasminogen →converts them to plasmin → chews up the clot (forms fibrin split products and D-dimer)
   ○ Only way you find D-dimer is if someone formed a clot and has broken them down

Question 12

Regulators (inhibitor) of fibrinolysis:

Answer 12

plasminogen activation inhibitor 1 (PAI-1)
TAFI (exopeptidase)
Alpha2-antiplasmin (serpin):

Question 13

plasminogen activation inhibitor 1 (PAI-1)

Answer 13

○ inactivates t-PA so that it doesn’t activate plasminogen to plasmin (which occurs especially in presence of fibrin)

Question 14

Thrombin-activatable fibrinolysis inhibitor (TAFI) (exopeptidase)

Answer 14

○ Covalently attaches to fibrin with the help of Factor XIIIa, and is cleaved (activated) when it binds to the thrombin-thrombomodulin complex. →
After TAFI (an exopeptidase) is activated, it removes amino acids (Arg and Lys) from the C terminal of fibrin molecules and FDPs. This reduces the # of plasminogen binding sites on fibrin so plasminogen is not able to bind and act →
inhibit fibrinolysis

(remember plasminogen embeds itself into the clot)

Question 15

Alpha2-antiplasmin (serpin):

Answer 15

○ primary free plasmin inhibitor in blood
- Bound plasmin (bound to fibrin) is protected.

Factor XIIIa can help link alpha2-antiplasmin to fibrin and stabilize the clot.

Question 16

List some of the mechanisms the endothelial cell lining uses to prevent clot formation in the resting state.

Answer 16

• Anticoagulation mechanisms

* Antiplatelet mechanisms

Question 17

Anticoagulation mechanisms

Answer 17

a. Express heparin sulfate and dermatan sulfate which are cofactors for antithrombin and heparin cofactor II, respectively.
b. Expression of thrombomodulin, cofactor with thrombin for activation of protein C.
c. Expression of TFPI to inhibit extrinsic Xase complex (i.e. tissue-factor VIIa)

• goal: prevent clot formation

Question 18

Antiplatelet mechanisms

Answer 18

Synthesis and secretion of PGI2 and NO, which prevent adhesion of activated Plts and cause vasodilation.

Express enzyme that metabolizes ADP (potent plate agonist) to AMP and adenosine (a potent inhibitor of platelet function).

-goal: prevent clot formation

Question 19

Primary hemostasis

Answer 19

○ BV gets damaged → Subendothelium exposed → VWF binds → allows platelets to adhere b/c of VWF’s interactions w/ platelet integrin receptor→ platelet changes shape and platelet aggregation is promoted →release coagulation proteins, platelet agonists, and vasoactive substances to help with vasoconstriction.

Phosphatidylserine is also translocated to outer leaflet so coagulation reactions can take place.

Question 20

Secondary hemostasis

Answer 20

Tissue factor becomes exposed to factor VIIa circulating in plasma→
initiation of extrinsic coagulation pathway→
activation of factors XI and X →
prothrombinase complex is formed (made up of factor Xa and Va) bind to anionic phospholipid surface of platelet and in presence of calcium to activate prothrombin to thrombin →
activate factors to amplify coagulation process →
propagation phase of coagulation →
generation of burst thrombin →
cleaves fibrinogen to fibrin to form fibrin network around area of injury and cleaves XIII to covalently cross link fibrin.