ANNALS OF CLINICAL AND LABORATORY SCIENCE, Vol. 8, No. 2 Copyright © 1978, Institute for Clinical Science General Considerations of Coagulation Proteins DAVID GREEN, M.D., Ph.D.* Atherosclerosis Program, Rehabilitation Institute of Chicago, Section of Hematology, Department of Medicine, and Northwestern University Medical School, Chicago, IL 60611. ABSTRACT The coagulation system is part of the continuum of host response to injury and is thus intimately involved with the kinin, complement and fibrinolytic systems. In fact, as these multiple interrelationships have un­ folded, it has become difficult to define components as belonging to just one system. With this limitation in mind, an attempt has been made to present the biochemistry and physiology of those factors which appear to have a dominant role in the coagulation system. Coagulation proteins in general are single chain glycoprotein molecules. The reactions which lead to their activation are usually dependent on the presence of an appropriate surface, which often is a phospholipid micelle. Large molecular weight cofactors are bound to the surface, frequently by calcium, and act to induce a favorable conformational change in the reacting molecules. These mole­ cules are typically serine proteases which remove small peptides from the clotting factors, converting the single chain species to two chain molecules with active site exposed. The sequence of activation is defined by the enzymes and substrates involved and eventuates in fibrin formation. Mul­ tiple alternative pathways and control mechanisms exist throughout the normal sequence to limit coagulation to the area of injury and to prevent interference with the systemic circulation. Introduction RatnofP4 eloquently indicates in an arti­ cle aptly entitled: “A Tangled Web. The Blood coagulation forms part of a con­ Interdependence of Mechanisms of tinuum of body response to injury. Acti­ Blood Clotting, Fibrinolysis, Immunity vation of blood coagulation occurs in and Inflammation,” the four systems are concert with the awakening of the kinin, extensively interrelated. Therefore, it is complement and fibrinolytic systems. As very difficult to arrive at a definition of a coagulation protein. For example, pro­ * Reprint requests: David Green, M.D., Ph.D., Rehabilitation Institute of Chicago, 345 East longed coagulation times as measured by Superior Street, Chicago, IL 60611. the partial thromboplastin time test are 95 96 GREEN observed in plasma deficient in prekal- factor, prekallikrein and high molecular likrein, and yet patients with this disor­ weight kininogen. The contemporary der may not suffer from a hemorrhagic version shown has not been accepted as diathesis. Is prekallikrein a coagulation official, but is simply included as a guide protein? By informal convention, it is now to the discussion of the coagulation pro­ listed among the clotting factors. teins which follows. Some years ago, the International Committee on Blood Clotting Factors as­ Biochemistry and Physiology signed Roman numerals to the then rec­ of the Coagulation Proteins ognized coagulation proteins. The official numbers and common synonyms are F ib r in o g e n (F a c t o r I) shown in table I. Adjacent to this list is a contemporary version which includes Fibrinogen is perhaps the best charac­ coagulation proteins characterized since terized of the coagulation proteins. It has the original list was formulated. The a molecular weight of approximately major modifications include (1) the rec­ 340.000 and consists of three pairs of ognition of specific platelet coagulant ac­ polypeptide chains called the A a, B ¡3 tivities, which now could be assigned and y chains.29 These are arranged in the under the previously unused Roman form of a dimer, linked by symmetrical numeral VI; (2) the indication that factor disulfide bonds.104 The A a chain has a VIII probably represents a complex of molecular weight of 64,000; the B /3 chain proteins designated anti-hemophilic fac­ 57.000 with approximately 480 resi­ tor, von Willebrand factor and the factor dues115; and the y chain 48,000 with 409 VIII related antigen; (3) and the multiple residues, the complete sequence of proteins involved in the initiation of which has been determined.46 Approxi­ coagulation, assigned under Roman mately 3 to 5 percent of the molecule is numeral XII and designated Hageman carbohydrate.11 A detailed description of fibrinogen structure has recently been published by Mosesson and Finlayson.78 T A B L E I Fibrinogen is synthesized by the The Coagulation Proteins hepatocyte30 and has a plasma half-life of between 3.2 and 4.5 days.3'72, 107 It is Official Number Synonym Contemporary version catabolized at the rate of 31 to 46 mg per kilogram per day. However, the site of I Fibrinogen I (Fibrinogen) fibrinogen catabolism has not been iden­ II Prothrombin II (Prothrombin) III Tissue Thromboplastin III (Tissue Factor) tified and the routes of excretion or IV Calcium IV (Calcium) V Labile Factor V (Labile Factor) reutilization of the molecule are un­ --- VI :PF-j (Platelet coagulant activities) known. An important, unanswered ques­ VI:PF4 VII Stable Factor VII {Stable Factor) tion is what percentage of fibrinogen is VIII Antihemophilic Factor VIII:AHF (Antihemophilic Factor) normally converted to fibrin in the non­ VIII :VWF (von Willebrand Factor) bleeding state. VIII:RAg (Related-Antigen) IX Christmas Factor IX (Christmas Factor) X Stuart-Prower Factor X (Stuart-Prower Factor) P r o t h r o m b in (F a c t o r II) XI Plasma Thromboplastin XI (Plasma Thromboplastin Antecedent Antecedent) XII Hageman Factor XII:HF (Hageman Factor) Prothrombin has a molecular weight of XIIsPK (Prekallikrein, Fletcher) 68.000 and consists of a single polypep­ XII:HMWK (High Molecular Weight Kininogen) tide chain of 522 amino acid residues. It XIII Fibrin Stabilizing Factor XIII (Fibrin Stabilizing Factor) is a glycoprotein containing hexose, hex- osamine and sialic acid.58 A characteristic The use of the suffix "a" after any factor indicates its activated form. feature is a calcium binding region in the GENERAL CONSIDERATIONS OF COAGULATION PROTEINS 97 a helical portion of the molecule.112 This cent carbohydrate, mainly sialic acid, binding is due to the presence of hexoses and other neutral sugars.42 A •y-carboxyglutamic acid residues, incor­ heavy (125,000) and light (64,000) chain porated into the molecule through the ac­ have been identified and are present in a tion of vitamin K.105 Prothrombin is 1:2 ratio.21 Factor V is synthesized by the synthesized by the liver4'86 and has a liver36,86 and has a plasma half-life of ap­ plasma half-life of 2.8 to 4.4 days.54,103 proximately 20 hours.99 Shapiro and Martinez103 estimated a synthetic rate of 2.5 mg per kilogram per day. The sites of catabolism and excretion P l a t e l e t C o m p o n e n t s are unknown. Platelet factor 3, a lipoprotein compo­ nent of the platelet membrane, consists T is s u e F a c t o r (F a c t o r III) principally of phosphatidyl serine and Tissue factor is a complex material phosphatidyl ethanolamine.70,71 Platelet consisting of approximately 49 percent factor 4 has been extensively charac­ protein, 42 percent phospholipid and terized and has a molecular weight of 7 percent cholesterol.81 Phosphatidyl 9600. It consists of a single polypeptide ethanolamine appears to be the principal chain of 92 amino acid residues having lipid component.80 The protein compo­ two disulfide bridges.113 As it occurs in nent has a molecular weight of approxi­ the platelet, it is probably a tetramer.76 Other platelet coagulant activities have mately 55,000.8 Tissue factor activity is been identified83'111 but not as com­ mainly found in the lung, brain and pletely characterized. placenta117 and may also be recovered from cultured skin fibrinoblasts40 It is principally localized to the microsomal F a c t o r VII fractions.118 Factor VII is a glycoprotein with an apparent molecular weight of 60,000.57 It C a l c iu m (F a c t o r IV) is a single chain polypeptide with an ac­ According to Tullis,109 calcium in a cessible enzymatic site; however, as will concentration of 0.5 x 10-3 M is a prere­ be discussed subsequently, the form of quisite for each phase of hemostasis. A the molecule active in clotting reactions simple experiment indicates the essential has two chains. The amino acid sequence role of calcium in coagulation. If fresh of the molecule shows considerable simi­ whole blood is diluted 1:20 in imidazole larity to that of prothrombin and the light buffer, pH 7.4, it will not clot. However, chain of factor X.56 It also is synthesized by if the buffer contains calcium, 5 mEq per the liver and vitamin K is required for the liter, clotting occurs in 2 to 3 minutes formation of the complete molecule. Fac­ (unpublished observation). This experi­ tor VII has a half-life of only 100 to 300 ment indicates that calcium concentra­ min.47 tion is the limiting factor in coagulation, in vitro. F a c t o r VIII Factor VIII is a glycoprotein with a F a c t o r V molecularweight in excess ofl millionand Factor V is a large protein with a consists of a series of subunits held to­ molecular weight in excess of 200,000.27 gether by disulfide bonds.59,101 V III: AHF Its activity is labile but can be stabilized (anti-hemophilic factor) can be separated by divalent cations and reducing from V III: VWF (vonWillebrand factor) by agents.2'9 It contains from 10 to 20 per­ cryoprecipitation,48 Polyelectrolytes,65 IM 98 GREEN NaCl116 and 0.25 M CaCl2.90 The VIII: F a c t o r XI AHF formed by CaCl2 treatment has subunits of 30,000 to 100,000 molecular Current evidence indicates that factor weight73 while the V III: VWF has 200,000 XI is a protein of molecular weight molecular weight subunits.59 It has not yet 175.000.108 It normally exists in the circula­ tion complexed with high molecular been resolved as to whether VIII: VWF weight kininogen.