Coagulation System & Fibrinolysis

• These notes cover topics related to the Coagulation System and Fibrinolysis

Coagulation System (Part 1)

• Plasma transports at least 16 procoagulants, also known as coagulation factors made in the liver
• Eight of these factors are enzymes known as zymogens, and some are cofactors
• In 1958, the International Committee officially named the plasma procoagulants, using Roman numerals based on their discovery order

Fibrinogen Molecule

• Functions as a mirror-image dimer
• Each half contains three nonidentical polypeptide chains (Aα, Bβ, y) bonded by disulfide bonds
• The E domain contains six N-terminals, forming a large central region
• Two D domains each have three carboxyl terminals at each outer end

Clotting Factor II: Prothrombin

• Thrombin is its activated form
• Thrombin acts as the main enzyme of the coagulation pathway
• Primary function of thrombin is to cleave fibrinopeptides (FP) A and B from alpha and beta chains

Thrombin's Role

• Cleaves fibrinopeptides
• Thrombin cleaves fibrinopeptides A and B from the N-termini of the α and β chains of fibrinogen
• Results in formation of fibrin monomer
• Alpha and beta chain ends of fibrin monomer attract portions of the D domain of other monomers

Fibrinogen - Clotting Factor I

• Substrate of thrombin
• Enzyme of the coagulation system
• Essential for platelet aggregation
• The normal plasma concentration is 200-400 mg/dL
• The most concentrated of all plasma procoagulants
• An acute-phase reactant protein

Thrombin Functions

• Activates Coagulation Factors V, VIII, and XI via positive feedback
• Activates Factor XIII
• Initiates platelet aggregation
• Activates the protein C pathway

Thrombomodulin-Thrombin Complex

• Thrombomodulin and thrombin activate Protein C
• Results in thrombin losing procoagulant ability to activate Factors V & VIII
• Activation of Protein C results in the destruction of FV & FVIII
• The complex also activates TAFI

Vitamin K-Dependent Prothrombin Group

• Named for structural resemblance to prothrombin
• Contains proteins with 10-12 glutamic acid units near their amino terminal end
• Only protein S and Z act as cofactors

Vitamin K

• It's a quinone from leafy green vegetables, produced in the intestines by organisms Bacteroides fragilis and Escherichia coli
• It catalyzes the modification of the prothrombin group, where glutamic acid transforms into γ-carboxyglutamic acid
• Necessary addition of a second carboxyl group to gamma carbon

Proteins Induced by Vitamin K Antagonists (PIVKA)

• Contain des-y-carboxyl proteins
• They do not participate in the coagulation reaction without the second carboxyl group
• Deficiency in Vitamin K or the presence of Coumadin causes this

Clotting Factor III: Tissue Factor

• It occurs on membranes of fibroblasts and smooth muscle cells
• Expressed in high levels within the brain, lung, placenta, heart, kidney, and testes
• Not normally expressed on blood vessel cells
• Upon injury, TF exposure leads to coagulation through VIIa

Clotting Factor IV: Ionized Calcium

• Vital for coagulation complexes on platelets or cell membranes
• Serine proteases adhere to negatively charged phospholipid surfaces through positively charged calcium ions (Ca2+)
• It is also referred to by this name and chemical symbol, not by its numeral

Clotting Factor V: Labile Factor

• Glycoprotein in plasma and platelet alpha granules
• During platelet activation, partially activated Factor V is released
• In coagulation, factor Va is a cofactor to Xa in the prothrombinase complex

Prothrombinase Complex

• Accelerates thrombin generation more than 300,000-fold
• Generates a small initial amount of thrombin.

Clotting Factor VI

• Was originally thought to be a procoagulant, but later identified as activated Factor V
• The name was withdrawn and never reassigned

Clotting Factor VII: Stable Factor

• Small amounts, about 1-2%, are present in the blood, but remains inert if unbound to tissue factor
• The coagulation cascade activates through FVII when its exposed to tissue factor during vessel injury
• Injury itself determines the rate of FVII activation

Clotting Factor VIII: Antihemophilic Factor

• Major production by hepatocytes mostly, also by microvascular ECs
• Unstable in plasma in free form, always travelling bound to vWF
• Production by genes located on the X chromosome
• vWF is its large carrier protein.
• FVIII and vWF are essential for hemostasis
• Thrombin separates FVIII from vWF to activate FVIII for coagulation
• FVIIIa subsequently binds to activated platelets for intrinsic tenase complex

Von Willebrand Factor

• Large multimeric glycoprotein involved in platelet adhesion, transports procoagulant factor VIII and comprised of subunits from ECs and megakaryocytes.
• Stored in alpha granules in platelets and Weibel-Palade bodies in EC.
• Acute phase reactant protein and those with group O blood have lower levels.

Von Willebrand Four Sites

• GP Ib/IX/V (platelet surface receptor: adhesion)
• GP IIb/IIIa (platelet surface receptor: aggregation)
• Binds collagen
• Binds factor VIII

ADAMTS-13

• It has a disintegrin and also metalloprotease and thrombospondin type 1 motif.
• Also known as a metalloprotease
• Degrades vWF into multimers

Clotting Factor IX: Christmas Factor

• Both Factor VIII and Factor IX production are influenced by X chromosome genes.
• The extrinsic tenase activates it and forms the intrinsic tenase complex alongside with Factor VIII.

Clotting Factor X: Stuart-Prower Factor

• Also triggered by the extrinsic tenase and forms the prothrombinase complex in conjunction with Factor V

Clotting Factor XI

• Triggered by the contact factor complex
• Then more significantly by thrombin and activates Factor IX

Contact Factor Complex

• The complex includes Factor XII, HMWK, and Pre-K
• Consists of Hageman, Fitzgerald, and Fletcher Factors.
• They are triggered by contact with negatively charged surfaces.
• Factor XI then triggers the complex, but deficiencies do not lead to clinical bleeding

Clotting Factor XII: Hageman Factor

• Negatively charged surfaces such as non-siliconized glass, kaolin and ellagic acid can trigger activation in vitro.
• Activation by stents and bacterial cell membranes can be triggered in vivo.

Clotting Factor XIII

• The complex is activated in 2 steps and consists of heterodimers.
• The steps are, Factor XIIa transforms pre-K into Kallikrein and Kallikrein cleaves HMWK to Bradykinin
• The alpha subunit is produced by mostly megakaryocytes and monocytes and the bets subunit is produced in the liver
• After its stimulated by thrombin it crosslinks the fibrin polymers to form a stable clot

Coagulation Pathway Complexes

• 3 pathway complexes are the Intrinsic tenase, Extrinsic tenase and Prothrombinase
• Both consist of 3 factors, which vitamin K-dependent serine protease, nonenzyme cofactor, Calcium and phospholipids

Intrinsic Pathway

• Reaction starts with Factor XII, ending with fibrin polymerization.
• Intrinsic due to Factor XII presence in blood.
• Reaction involves Factors XII, pre-K, HMWK, XI, IX, VIII, X, V, II, I
• The process is measured by the Activated Partial Thromboplastin Time (APTT)

Extrinsic Pathway

• Initiated by TF:VIIa and is key for in vivo coagulation
• Called extrinsic given absence of TF in blood.
• Involves Factors VII, X, V, II, I
• Tested by Prothrombin Time (PT)

Common Pathway

• Links both Intrinsic and Extrinsic, involves Factors X, V, II and I
• Measured and tested by both APPT and PT

Hemostatic System

• Requires two cell types for normal coagulation, cells that express TF in extravascular regions and platelets that exist in intravascular regions.

Cell-Based Physiologic Coagulation

• The process is in 2 phases, which are Initiation and Propagation
• Initiation starts on tissue factor cells producing 3-5% thrombin
• Propagation starts on platelets producing 95% thrombin
• Deficiencies in proteins of coagulation complexes can trigger significant bleeding disorders

Initiation Phase

• Begins when the extrinsic tenase complex forms also known as contact activation
• During the extrinsic tenase formation, small amounts of Factor Xa, Factor IXa and thrombin are generated
• Injury severity highly determines the amount of thrombin generation

Low Level Thrombin

• During initiation stage thrombin activates platelets, releases factors, dissociates vWF, activates factor XI and splits fibrinogen peptides
• At the end of initiation phase and into propagation, fibrinopeptides are cleaved
• Only 3% of thrombin or 10-30 nmol/L is generated

Propagation

• The reactions involved starts on the platelet surface
• During the trauma or injury platelets can either be activated or adhere to exposed collagen
• Therefore due to this coagulation is dependent on activated platelets and tissue factor bearing cells which localizes clotting to an injured location