MGD - clotting cascade Flashcards
what is the intrinsic pathway triggered by
damaged endothelial lining of blood cells promotes binding factor XII
(autocatalytic activation)
what is the extrinsic pathway trigged by
trauma releases tissue factor (III)
(membrane damage exposes extracellular domain of tissue
what happens after intrinsic / extrinsic pathways are activated?
cascade
factor X is activated (last factor)
thrombin activation
formation of fibrin clot
what are the tissue factors activated in intrinsic pathway?
12, 11, 9, (8), 10, (5), thrombin, fibrin, (13), cross linked fibrin
8 & 5 are cofactors
what are the tissue factors activated in the extrinsic pathway?
3, 7, 10, thrombin, fibrin, (13), cross-linked fibrin
what is required for sustained thrombin activation?
Gla domains, Ca2+
Gla targets factor 9, 10 to membrane
what is the role of gamma-carboxyglutamate (Gla) residues in clotting?
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
how can clots be localised to region of damage?
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
what is the structure of fibrinogen?
2 sets of tripeptides: a,ß,gamma joined at N termini by disulphide bonds
3 globular domains linked by rods
what prevents fibrinogen from aggregating together?
the ends of N-terminal regions of a,ß chains are highly negatively charged preventing aggregation of fibrinogen
how do fibrin clots form?
- thrombin cleaves fibrinopeptides A & B from the central globular domain of fibrinogen
- 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
what is the C-terminus and which is the N-terminus?
the C-terminus is at the central globular domain: fibrinopeptides A & B
the N terminus is at the ends: ß & gamma globular subunit
how are newly formed clots stabilised?
by formation of amide bonds between side chains of lysine and glutamine residues in different monomers
cross-linking reaction is catalysed by transglutaminase
how is transglutaminase used in formation of fibrin clot activated?
from protransglutaminase by thrombin
what is the repeating structure of fibrin molecule? (allowing the formation of a clot)
Glys-His-Arg sequence
amide bonds form between side chains of Lysine and glutamine
what is classic haemophilia a defect of? what happens and how do you treat it?
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
which clotting factors does thrombin positively feedback on?
3, 5, 7, 11, 13
what does prothrombin stimulate which in turn inhibits what?
prothrombin stimulates antithrombin which negatively feedbacks on factor 10 and prothrombin
what inhibits factors 5 and 8 (both not protease)
protein C
what is the function of factor 13?
to help form fibrin strands
thrombin –> fibrinogen –> fibrin strands
thrombin +ve feedback on factor 13 –> fibrin strands
what does thrombin activates which breaks down the fibrin mesh? through what?
fibrinolysis:
thrombin activates plasminogen –> plasmin –> breaks down fibrin mesh
plasminogen –> via streptokinase and t-PA–> plasmin –> breakdown fibrin mesh (from fibrin)
how is protein C activated? what can defects in protein C cause?
by thrombin binding to endothelial receptor (thrombomodulin)
thrombotic disease
how can clotting process be stopped?
- localisation of prothrombin
- digestion by proteases
- specific inhibitors
how does localisation of prothrombin stop the clotting process?
dilution of clotting factors by blood flow and removal by liver
what is a specific inhibitor that regulates the clotting process?
antithrombin III
enhanced by heparin binding
AT3-heparin does not act on thrombomodulin-bound thrombin
what is an essential cofactor for carboxylase of glutamic residue?
what is it antagonised by?
vitamine K
antagonised by warfarin - which greatly reduce the clotting cascade if not stopped
