MGD - clotting cascade

Question 1

what is the intrinsic pathway triggered by

Answer 1

damaged endothelial lining of blood cells promotes binding factor XII
(autocatalytic activation)

Question 2

what is the extrinsic pathway trigged by

Answer 2

trauma releases tissue factor (III)

(membrane damage exposes extracellular domain of tissue

Question 3

what happens after intrinsic / extrinsic pathways are activated?

Answer 3

cascade
factor X is activated (last factor)
thrombin activation
formation of fibrin clot

Question 4

what are the tissue factors activated in intrinsic pathway?

Answer 4

12, 11, 9, (8), 10, (5), thrombin, fibrin, (13), cross linked fibrin
8 & 5 are cofactors

Question 5

what are the tissue factors activated in the extrinsic pathway?

Answer 5

3, 7, 10, thrombin, fibrin, (13), cross-linked fibrin

Question 6

what is required for sustained thrombin activation?

Answer 6

Gla domains, Ca2+

Gla targets factor 9, 10 to membrane

Question 7

what is the role of gamma-carboxyglutamate (Gla) residues in clotting?

Answer 7

Ca2+ is attached to phospholipid of platelets (calcium bridge)
addition of COOH groups to glutmate residues forms carboxyglutamate
this allows interaction of site of damage with clotting factors as addition of COO- to glutamate makes carboxyglutamate even more negative to bind to Ca2+ bridge - so can clot

Question 8

how can clots be localised to region of damage?

Answer 8

prothrombin binds to Ca2+ ions via gla residues (activate the clotting amplification)
only prothrombin next to site of damage will be activated
therefore, clotting will be localised to site of damage

Question 9

what is the structure of fibrinogen?

Answer 9

2 sets of tripeptides: a,ß,gamma joined at N termini by disulphide bonds
3 globular domains linked by rods

Question 10

what prevents fibrinogen from aggregating together?

Answer 10

the ends of N-terminal regions of a,ß chains are highly negatively charged preventing aggregation of fibrinogen

Question 11

how do fibrin clots form?

Answer 11

1. thrombin cleaves fibrinopeptides A & B from the central globular domain of fibrinogen
2. globular domains at the C-terminal ends of the ß & gamma chains interact with exposed sequences at the N-termini of the cleaved ß & alpha chains to form a fibrin mesh or clot

Question 12

what is the C-terminus and which is the N-terminus?

Answer 12

the C-terminus is at the central globular domain: fibrinopeptides A & B
the N terminus is at the ends: ß & gamma globular subunit

Question 13

how are newly formed clots stabilised?

Answer 13

by formation of amide bonds between side chains of lysine and glutamine residues in different monomers
cross-linking reaction is catalysed by transglutaminase

Question 14

how is transglutaminase used in formation of fibrin clot activated?

Answer 14

from protransglutaminase by thrombin

Question 15

what is the repeating structure of fibrin molecule? (allowing the formation of a clot)

Answer 15

Glys-His-Arg sequence

amide bonds form between side chains of Lysine and glutamine

Question 16

what is classic haemophilia a defect of? what happens and how do you treat it?

Answer 16

factor VIII (8) - not a protease but stimulates factor IX (9) - serine protease
(factor 8 activity is markedly increased by limited proteolysis by thrombin and factor 10a - positive feedback amplifies clotting signal and accelerates clot formation)
treatment with recombinant factor VIII

Question 17

which clotting factors does thrombin positively feedback on?

Answer 17

3, 5, 7, 11, 13

Question 18

what does prothrombin stimulate which in turn inhibits what?

Answer 18

prothrombin stimulates antithrombin which negatively feedbacks on factor 10 and prothrombin

Question 19

what inhibits factors 5 and 8 (both not protease)

Answer 19

protein C

Question 20

what is the function of factor 13?

Answer 20

to help form fibrin strands
thrombin –> fibrinogen –> fibrin strands
thrombin +ve feedback on factor 13 –> fibrin strands

Question 21

what does thrombin activates which breaks down the fibrin mesh? through what?

Answer 21

fibrinolysis:
thrombin activates plasminogen –> plasmin –> breaks down fibrin mesh
plasminogen –> via streptokinase and t-PA–> plasmin –> breakdown fibrin mesh (from fibrin)

Question 22

how is protein C activated? what can defects in protein C cause?

Answer 22

by thrombin binding to endothelial receptor (thrombomodulin)

thrombotic disease

Question 23

how can clotting process be stopped?

Answer 23

1. localisation of prothrombin
2. digestion by proteases
3. specific inhibitors

Question 24

how does localisation of prothrombin stop the clotting process?

Answer 24

dilution of clotting factors by blood flow and removal by liver

Question 25

what is a specific inhibitor that regulates the clotting process?

Answer 25

antithrombin III
enhanced by heparin binding
AT3-heparin does not act on thrombomodulin-bound thrombin

Question 26

what is an essential cofactor for carboxylase of glutamic residue?
what is it antagonised by?

Answer 26

vitamine K

antagonised by warfarin - which greatly reduce the clotting cascade if not stopped