NBME - 6/16 - Blood plasma proteins, Coagulation, and Fibrinolysis

Question 1

Primary function of Albumin, which makes up 60% of total protein

Answer 1

Osmotic pressure maintenance brings water back out of tissues

Question 2

Functions of Globulin (4), which make up 35% of total protein

Answer 2

Transport nutrients, vitamins, metals, hormones, metabolites

Defense against infection

Regulatory processes

Blood Clotting – hemostasis

Question 3

What initiates coagulation?

Answer 3

Coagulation is initiated by TF binding to factor VIIa. The TF-VIIa complex activates small amounts of factors IX and X. Factor X associates with its cofactor Va forming a complex on the TF-bearing cell that converts prothrombin to thrombin. Active Xa is localized to the site of its activation.

Question 4

Feedback processes going on during the initiation phase of Coagulation

Answer 4

Free Xa is inhibited in the circulation by antithrombin III (ATIII) and other serpins.

ATIII, the major inhibitor of thrombin, also inactivates most of the other coagulation proteases as well, including IXa, Xa and XIa, ensuring that fibrin clots are not formed in the circulation.

Question 5

When does Amplification take place?

Answer 5

Amplification takes place only when damage to the vasculature allows platelets and factor VIII/vWF access to the TF-bearing cells at the site of injury.

Question 6

What happens now that we have Thrombin floating around in the Amplification phase?

Answer 6

Platelets release additional factor V, and thrombin activates it forming more active Va/Xa complexes on the platelets. Hence thrombin levels are amplified.

Thrombin also releases VIIIa from the VIII/vWF complex. VIIIa then binds to activated platelets. Factor XI is also activated.

Question 7

Where does Propagation occur?

Answer 7

Propagation occurs on the surface of large numbers of platelets at the site of injury.

Question 8

Discuss the role of factors 9, 10, and 11 i nthe formation of the hemostatic plug

Answer 8

Factor 9a activated during the initiation phase binds to factor 8a on the platelet surface. Additional 9a is supplied by platelet-bound 11a.

Factor 10 from the plasma is activated to 10a at many sites on platelets, generating a burst of thrombin large enough to clot fibrinogen and form a hemostatic plug

Question 9

Discuss how platelets adhere to the subendothelium

Answer 9

Adhesion of platelets to the subendothelial layers. (1) GPIa initially binds to exposed collagen, which results in conformational changes of the complex and cells round up. (2) Conformational changes allow GPIb to interact with vWF (3) The second binding event exposes the GPIIb/GPIIIa complex, which also binds to vWF and interacts with fibrinogen.

Question 10

How are factor complex proteins bound to the plasma membrane?

Answer 10

Factor complex proteins contain γ-carboxyglutamate residues that bind to Ca2+ ions coordinated to phospholipid (PL) head groups on the plasma membrane, thereby anchoring the complexes to the cell surface.

Question 11

What key ingredient is needed to form the γ-carboxyglutamate residues needed to anchor the factor complexes.

Answer 11

Vitamin K

For exorbitant uneeded detail see below.

Question 12

How does Warfarin work?

Answer 12

Warfarin (Coumadin) is a slow and long-acting anticoagulant. It is an analog of vitamin K, that blocks the gammacarboxylation reaction (inhibits vitamin K epoxide reductase).

The non-carboxylated blood clotting proteins are unable to promote blood coagulation because they cannot bind calcium and thus cannot bind to their phospholipid sites of activation.

Question 13

Endothelial cells have three anticoagulant functional abilities. What are they? No detail yet.

Answer 13

1. Protein C–Protein S–Thrombomodulin system
2. Antithrombin III (ATIII) and tissue factor pathway inhibitor (TFPI)
3. Endothelial cells themselves inhibit platelet activation and aggregation

Question 14

Discuss the Protein C–Protein S–Thrombomodulin system

Answer 14

Protein C–Protein S–Thrombomodulin system inactivates factor Va and VIIIa

Thrombin, bound to thrombomodulin on a healthy endothelial cell surface, activates protein C. Activated protein C, in complex with protein S, binds to the platelet membrane, and the activated protein C complex (APC) destroys Factors Va and VIIIa by proteolytic cleavage, thereby inhibiting the coagulation cascade.

Question 15

Discuss Antithrombin III (ATIII) and tissue factor pathway inhibitor (TFPI)

Answer 15

Belong to the class of serpins (serine protease inhibitors) always present bound to heparan sulfates on the vascular endothelial cell surface (also present in the circulation).

ATIII inhibits thrombin, and most other factor serine proteases especially, IXa and Xa

TFPI inhibits TF-VIIa complex, also Xa.

Question 16

How does Heparin work?

Answer 16

ATIII inhibits thrombin, and most other factor serine proteases especially, IXa and Xa – effect is potentiated by heparin.

Question 17

How do Endothelial cells themselves inhibit platelet activation and aggregation?

Answer 17

Endothelial cells themselves inhibit platelet activation and aggregation by

• releasing prostacyclin (PGI2), nitric oxide (NO), and
• degrading ADP using an membrane ecto-ADPase.

Question 18

How is plasminogen, and thus, Fibrinolysis, regulated?

Answer 18

Plasminogen can be activated by either tissue plasminogen activator (t-PA) or urokinase plasminogen activator (u-PA).

Plasminogen activator inhibitor 1 (PAI-1) is the major physiological inhibitor of both t-PA and u-PA.

Circulating alpha-2-antiplasmin (the primary inhibitor of plasmin) blocks the activity of any plasmin free in the blood, but plasmin bound to fibrin is protected