Chok Biochem 3rd Shift Reviewer Blood Coagulation

BLOOD COAGULATION Hemostasis – cessation of bleeding from a cut or severed vessel

Thrombosis – endothelial damage or removal

Four Phases

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Local vasoconstriction - ↓ blood flow distal to injury

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Hemostatic clot (thrombus) formation – via bifurcated cascade of proteolytic proteins

Platelet plug formation – thrombin activation  platelet adhiesion to exposed collagen  release of activated platelet substances = ↑ vasoconstriction and adherence  platelet shape change, aggregation Fibrinolysis – dissolution of fibrin clot

Blood Coagulation Factors Facto Name r I Fibrinogen II Prothrombin III Tissue Factor IV Calcium V Proaccelerin VII Proconvertni VIII Antihemophilic IX Christmas Factor X Stuart Factor XI Plasma Thromboplastin Antecedent XII Hageman factor XII Fibrin stabilizing factor Coagulation Pathways Pathway Intrinsic Contact Phase Intrinsic Tenase Complex Extrinsic Final Common

Substrate Prekallikrein XII XI IX VIII X VII X Prothrombi n Fibrinogen Fibrin

Pathway

Function

Both Both Extrinsic Both Both Extrinsic Intrinsic Intrinsic Both Intrinsic

Becomes fibrin by removal of 2A and 2B peptides Becomes thrombin with Xa Activates X with VIIa Enzyme activator Catalyzes thrombin formation by Xa X  Xa X  Xa with IXa, hemophilia A Gla-containing, X  Xa, hemophilia B Prothrombin  Thrombin IX  IXa with Ca

Intrinsic

Prekallikrein  kallikrein, XI  XIa Crosslinks precipitated fibrin monomers

Product Kallikrein XIIa XIa IXa VIIIa Xa VIIa Xa Thrombin Fibrin Crosslinked clot

Enzyme Damage surface Kallikrein, HMK XIIa, HMK, Ca XIa, Ca, PI Thrombin VIIIa, Ca, PI Trauma, Ca, PI Tissue factor, VIIa, Ca, PI Xa, Ca, PI, Va Thrombin XIIIa

Notes 2-6 mins, components intrinsic to blood

within 15 sec

Soft clot Hard clot

VItK dependent clotting factors

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Modification of II, VII, IX, X  γ carboxyglutamate (Gla) Dihydroquinone (reduced) + O2  Epoxide (oxidized) = proton abstractor = Gla residues Epoxide + VitK reductases + dithiol cofactors  Dihydroquinone active Gla residues much stronger Ca chelators  Ca bridges between II, VI, IX, X and membrane

Final Common Pathway Factors

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Prothrombin – single chain glycoprotein from liver, active α molecule @ N terminal, propeptide with 3 domains: Gla, Kringle 1, Kringle 2 o Conversion – prothrombinase complex (platelet anionic phospholipids, prothrombin, Va, Ca, Xa); cleave Arg-Thr bond = release propeptide

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Thrombin – A & B chains with disulfide bonds, serine protease, release fibrinopeptides, converts XIII  XIIIa Fibrinogen – soluble, 3 pairs of non-identical, but homologous chains (Aα, Bβ, γγ) with disulfide bonds; central αβγ, terminal βγ Cterminals; highly anionic fibrinopeptides (-8 central, -4 peripheral) o Conversion – A&B cleavage by thrombin  +5/-4 charge = non-covalent aggregation; with FSF-XIIIa = covalent isopeptide bonds

Fibrinolysis

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Demolishing of fibrin, elimination of blood clots for wound repair, activated by fibrin Fibrin activates plasminogen activators  plasmin (Arg-Val cleaving)  cleave 3x stranded coiled segments @ covalent crosslinkages Plasmin active only with fibrin, inactivated by α2 antiplasmin Activated by urokinase (kidneys), streptokinase (streptococci with ASA), tissue plasminogen activator (vascular endothelium during injury or stress)

Anticoagulants

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Calcium ion chelators – fluoride, oxalate, citrate Antithrombin coagulants (in vivo) o Antithrombin III – inhibits all active proteases except VIIa, 75% antithrombin activity

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α2 macroglobulin – 20-25% antithrombin activity

o

Heparin cofactor II, α1-antitrypsin – minor inhibitors, @ physiologic conditions

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Heparin (synthetic) – enhances antithrombin III 100x via conformational change, promotes thrombin binding, coats endothelium = ↓ intrinsic pathway

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Protein C (PC) – Gla-containing, activated by thrombin, inactivates V&VIII

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Thrombomodulin (antithrombin) – binds and decreases thrombin activity, 1000x capacity to activate PC VitK antagonists o Dicoumarol – competitive inhibitor of vitK reductase, prevents conversion to active dihydroquinone

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Warfarin – rat poison, same as dicoumarol

Lipoprotein associated coagulation inhibitor (LACI) / Anticonvertin – inhibits extrinsic pathway, VIIa-Xa complex Aspirin - ↓ cyclooxygenase  ↓ TXA2 & prostacyclin (PGI2, vasodilator) but PGI2 regenerates faster