haemostasis: clotting cascade, how fibrin network is generated & role of vitamin K

Question 1

What is blood coagulation/clotting?

Answer 1

• it is the transformation of blood into a solid gel called a clot or thrombus

Question 2

What does the ‘clot’ contain?

Answer 2

• it contains a protein polymer called fibrin as well as trapped blood cells

Question 3

what kind of enzymes are most clotting factors?

Answer 3

most are serine proteases
* they contain a serine residue in their catalytic protease domain

Question 4

What is the function of clotting that is relevant to the platelet plug?

Answer 4

it functions to support and reinforce the platelet plug to solidify the blood that remains in the wound

Question 5

How is clotting initiated?

Answer 5

• Like platelet aggregation, clotting is initiated when there is an injury to the blood vessel wall which disrupts the endothelium and expresses the underlying collagen

Question 6

What is the clotting cascade?

Answer 6

the clotting cascade is a series of of enyzme activation events in plasma which resullts in the formation of an insoluable fibrin clot

Question 7

In general terms, what happens at each step of the cascade?

Answer 7

• at each step of the cascade, an inactive plasma protein or ‘factor’ is converted into a proteolytic enzyme which then goes on to catalyse the generation of the next enzyme

Question 8

What is a very important step in the clotting cascade?

Answer 8

the conversion of prothrombin into the enzyme thrombin which has many important functions

Question 9

why are activated platelets essential for the clotting cascade?

Answer 9

they are important because several of the cascade reactions** take place on the surface of the platelets**, the clotting factors can bind to specific plasma membrane receptors on the platelet

Question 10

What are the 2 different clotting cascade pathway & which one do we focus on more?

Answer 10

• the intrinsic pathway - in which the physiological relevance is not clear
• the extrinsic pathway ( ie the components in the pathway are not in the blood) - we focus on this pathway

Question 11

At what step do both pathways cross over?

Answer 11

• the 2 pathways merge at the factor Xa step which is the enzyme that catalyses prothrombin to thrombin

Question 12

What important factor does the extrinsic pathway begin with?

Answer 12

• This pathway begins with a protein called tissue factor (TF)

Question 13

Is tissue factor a plasma protein?

Answer 13

• no TF is not a plasma protein
• it is located on the outer plasma membrane of various tissue cells, including fibroblasts and other cells in the wall of blood vessels

Question 14

When an injury occurs to the blood vessel, what factor does the exposed TF bind to?

Answer 14

• tissue factor binds to a plasma protein called factor XII which then becomes activated to factor XIIa

Question 15

What does the complex of the tissue factor and factor XIIa on the plasma membrane catalyse?

Answer 15

the complex catalyses the conversion of factor X into factor Xa and it also catalses the conversion of IX into IXa

Question 16

What important effects/functions does thrombin have?

Answer 16

• activates platelets
• acts as a serine protease to cleave fibrinogen and convert it to fibrin
• it is also involved in its own positive feedback loop - ie the formation of thrombin stimulates the formation of more thrombin
• it also activates XIII to form XIIIa

Question 17

How does thrombin act on fibrinogen, and what happens as a result?

Answer 17

• Thrombin is a serine protease that cleaves off small peptides (A &B) of fibrinogen
• the remnants of fibrinogen then bind together to form fibrin
• Once fibrin is formed, it undergoes polymerisation

Question 18

Describe the process of fibrin polymerisation

how does it become stabilised?

Answer 18

• Fibrin undergoes polymerisation to form a loose mesh of strands
• it then becomes stabilised and strengthened by the formation of covlaent cross linkages by the enzyme factor XIIIa

Question 19

What is the liver’s role in haemostasis?

Answer 19

• the liver produces many clotting factors and it also produces bile salts
• the bile salts also aid with the absorption of vitamin K from the intestines
• Vitamin K aids in the formation of clotting proteins through carboxylation reactions

Question 20

what is the general structure of clotting factors?

Answer 20

they contain a protease domain and a gla domain

Question 21

What is the function of the protease domain on the clotting factor?

Answer 21

• this is the catalytic part of the clotting factor
• it cuts into other proteins and converts them to active proteins

Question 22

What is the function of the gla domain of clotting factors?

Answer 22

• the gla domain is involved in membrane attachment to the platelets
• it has a strong negative charge an a high affinity for Ca2+ ions which bind to the surface of activated platelets as they have a negatively charged membrane

Question 23

What is the important component in the gla domain that allows the binding of positively charged Ca2+ ions to the clotting factors?

Answer 23

gamma (y)- caroxyglutamate