Fig. 4.8 ). Each reaction step involves an enzyme (an activated coagulation factor), a substrate (an inactive proenzyme form of a coagulation factor), and a cofactor (a reaction accelerator). These components are assembled on a negatively charged phospholipid surface, which is provided by activated platelets. Assembly of reaction complexes also depends on calcium, which binds to γ-carboxylated glutamic acid residues that are present in factors II, VII, IX, and X. The enzymatic reactions that produce γ-carboxylated glutamic acid use vitamin K as a cofactor and are antagonized by drugs such as Coumadin, a widely used anti-coagulant.
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Fig. 4.7 Thecoagulationcascadeinthelaboratoryandinvivo.(A)Clottingisinitiatedinthelaboratorybyaddingphospholipids,calcium,andeitheranegative-chargedsubstancesuchasglassbeads(intrinsicpathway)orasourceoftissuefactor(extrinsicpathway).(B)Invivo,tissuefactoristhemajorinitiatorofcoagulation,whichisamplifiedbyfeedbackloopsinvolvingthrombin(dotted lines).Thered polypeptidesareinactivefactors,thedark green polypeptides areactivefactors,whereasthelight green polypeptidescorrespondtocofactors.
Based on assays performed in clinical laboratories, the coagulation cascade has traditionally been divided into the extrinsic and intrinsic pathways (
Fig. 4.7A • The prothrombin time (PT) assay assesses the function of the proteins in the extrinsic pathway (factors VII, X, V, II (prothrombin), and fibrinogen). In brief, tissue factor, phospholipids, and calcium are added to plasma and the time for a fibrin clot to form is recorded.
• The partial thromboplastin time (PTT) assay screens the function of the proteins in the intrinsic pathway (factors XII, XI, IX, VIII, X, V, II, and fibrinogen). In this assay, clotting of plasma is initiated by the addition of negative-charged particles (e.g., ground glass) that activate factor XII (Hageman factor) together with phospholipids and calcium, and the time to fibrin clot formation is recorded.
Fig. 4.8 SequentialconversionoffactorXtofactorXabywayoftheextrinsicpathway,followedbyconversionoffactorII(prothrombin)tofactorIIa(throm-bin).Theinitialreactioncomplexconsistsofaprotease(factorVIIa),asubstrate(factorX),andareactionaccelerator(tissuefactor)assembledonaplateletphospholipidsurface.Calciumionsholdtheassembledcomponentstogetherandareessentialforthereaction.ActivatedfactorXathenbecomestheproteasecomponentofthenextcomplexinthecascade,convertingprothrombintothrombin(factorIIa)inthepresenceofadifferentreactionaccelerator,factorVa.
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Although the PT and PTT assays are of great utility in evaluating coagulation factor function in patients, they do not recapitulate the events that lead to coagulation in vivo. This point is most clearly made by considering the clinical effects of deficiencies of various coagulation factors. Deficiencies of factors V, VII, VIII, IX, and X are associated with moderate to severe bleeding disorders, and prothrombin deficiency is likely incompatible with life. In contrast, factor XI deficiency is only associated with mild bleeding, and individuals with factor XII deficiency do not bleed and in fact may be susceptible to thrombosis. The paradoxical effect of factor XII deficiency may be explained by involvement of factor XII in the fibrinolysis pathway (discussed later); although there is also some evidence from experimental models suggesting that factor XII may promote thrombosis under certain circumstances, the relevance of these observations to human thrombotic disease remains to be determined.