Hemostasis and Coagulation Cascade Flashcards
Zymogen and Serine Protease
Zymogen, aka proenzyme – inactive enzyme precursor
Serine protease: an enzyme that cleaves peptide bonds
Serine is the nucleophilic amino acid in the enzyme’s active site
Cleavage of disulfide bonds causes activation
Extrinsic 10ase
TF (VIIa), factor 10 (inactive), Ca2+, and phosphotidylserine to cleave the factor 10 and make it active
Gamma - Carboxyglutamate (GLA)
aka Gla residue
Post-translational modification of glutamate
Found in Prothrombin, Factors VII, IX, and X, and proteins C, S and Z
Binds calcium ions:
Aids binding of coagulation cascade proteins to membrane components
Necessary for cleavage activity of the serine proteases
GLA = binds Ca2+ to allow molecules to bind to membrane surfaces
Factor 7 in extrinsic
9- intrinsic pathway
10 – cleaves thrombin
Proteins C, S, and Z – involved in anti-coagulations
Synthesis of GLA
Requires vitamin K
Oxidation forms reactive carbanion and vitamin K epoxide
Non-enzymatic carboxylation forms γ-carboxyglutamate
Epoxide is reduced to quinone and then hydroquinone
Warfarin prevents recycling of vitamin K
Excess vitamin K can overcome the inhibition because second reductase is insensitive to Warfarin
Activation of Protein C Pathway
Thrombin is inactivated by thrombomodulin on endothelial surface and also complex activates protein C and combines with protein S and inactivates factors 5 and 8
Inhibits coagulation cascade
Activation of Antithrombin
Antithrombin is serine protease inhibitor; inactivates any of the factors and inactivation activity is accelerated by cell surface heparin – derived from heparin and can stimulate antithrombin (inactivation of thrombin and factors)
Inhibits coagulation cascade
Factor Z and Z Related Protease Inhibitor
Factor Z and Z related protease inhibitor – inhibits factor XIa
Inhibits coagulation cascade
Fibrinolysis
Resolve fibrin clots via plasmin
Degrades fibrin and pulls apart clots
T-PA activated
Lysine residues bound to plasminogen to keep it in clot so readily available and cleave fibrin
Urokinase: ECM degradation and fibrin clot degradation
Regulation of Fibrinolysis: Prevention
Primary defense: alpha2 antiplasmin circulation in bloodstream to act on plasmin to inhibit it
Thrombin-activatable fibrinolysis inhibitor
(TAFIa) – acts negatively on plasmin so stops dissolution of the clots; is a carboxypeptidase; prevents plasmin from binding to fibrin
Plasminogen activator inhibitor: negatively affects t-PA so plasminogen cannot be converted to plasmin
Thrombin: Coagulation Action
Cleavage of fibrinogen to form fibrin
Activation of factor XIII (to crosslink fibrin)
Activation of factors V and VIII (Cofactors for factor Xa and factor IXa, respectively)
Activation of factor XI
Thrombin: Anti-Coagulation Action
Activation of protein C
Requires binding to thrombomodulin
Thrombin: Fibrinolysis Inhibition
Activation of thrombin-activatable fibrinolysis inhibitor (TAFI)
Requires binding to thrombomodulin
NO
Nitric Oxide (NO): inhibits platelet adhesion and aggregation by elevating levels of cGMP
Thrombomodulin
Thrombomodulin: glycoprotein that binds thrombin, which then cleaves protein C to yield activated protein C; this is in combination with protein S degrades factors Va and VIIIa, limiting their actions
t-PA
t-PA: tissue plasminogen activator; activates plasminogen to plasmin, which digests fibrin; the action of t-PA is opposed by plasminogen activator inhibitor-1 (PAI-1)
IX Activation
Intrinsic pathway via XIa
Extrinsic pathway via VIIa
Endogenous Anticoagulants
- Antithrombin III: inhibits IXa, Xa, XIa, XIIa
- Protein C or S: vitamin K dependent; inactivates Va and VIIIa
- TFPI: produced via endothelium; inactivates VIIa, therefore inhibiting IX and X
VIIa Activation
activates Xa and IXa
IX Activation
IX activates X
PT
Extrinsic Pathway (initiating pathway) 2, 5, 7, 10, and fibrinogen
PTT
Intrinsic Pathway (amplifying pathway) 2, 5, 8, 9, 10, 11, 12, and fibrinogen
GPCR
G protein coupled receptors
Ligands bind to cause conformation change to make GTP from GDP in order to activate and dissociate complex
alpha is a GTPase to make GTP to GDP and inactivates everything again
2 regulatory pathways affected: PLC and adenylyl cyclase
Platelet Activation: Adhesion
Exposed collagen and subendothelial vWF
Collagen binds to GP Ia-IIa and GPVI
vWF binds to GP Ib-IX-V
Thrombin binds GP Ib-IX-V, PAR-1, PAR-4
Collagen and thombin activate phospholipase to cleave PIP2 to form 2 second messengers:
- DAG: activates protein C to cause platelet responses and release granules
- IP3: stimulates Ca2+ release to cause platelet responses (affects shape)
Release of Platelet Factors
- Serotonin
- TxA2: Ca2+ stimulates phospholipase to cleave arachidonic acid to form TxA2 and activate other platelets
- ADP: released by DAG to stimulate platelets
Platelet Aggregation
GP IIb-IIIa binds fibrinogen to bring platelets together to make primary platelet plug
Fibrinogen cleaved to fibrin via thrombin to make secondary stronger plug
Platelet Inactivation
Prostacyclin inhibits platelet activation and released by endothelial cells
Acts through GPCR to increase cAMP
Aspirin inhibits COX-1 to prevent TxA2
Fibrinogen Structure
N terminals are held together at the center by fibrinopeptides with a high negative charge bound together by disulfide bonds
When activated, negative charge decreases so platelets can interact and fibrin can build
Factor XIIIa (activated by thrombin) is a transglutaminase and cross links fibrin molecules via covalent bonds
