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Exosite-mediated substrate recognition of factor IX by factor XIa. The factor XIa heavy chain is required for initial recognition of factor IX. 2005

Taketoshi Ogawa, and Ingrid M Verhamme, and Mao-Fu Sun, and Paul E Bock, and David Gailani
Department of Pathology, Vanderbilt University, Nashville, Tennessee 37069, USA.

Studies of the mechanisms of blood coagulation zymogen activation demonstrate that exosites (sites on the activating complex distinct from the protease active site) play key roles in macromolecular substrate recognition. We investigated the importance of exosite interactions in recognition of factor IX by the protease factor XIa. Factor XIa cleavage of the tripeptide substrate S2366 was inhibited by the active site inhibitors p-aminobenzamidine (Ki 28 +/- 2 microM) and aprotinin (Ki 1.13 +/- 0.07 microM) in a classical competitive manner, indicating that substrate and inhibitor binding to the active site was mutually exclusive. In contrast, inhibition of factor XIa cleavage of S2366 by factor IX (Ki 224 +/- 32 nM) was characterized by hyperbolic mixed-type inhibition, indicating that factor IX binds to free and S2366-bound factor XIa at exosites. Consistent with this premise, inhibition of factor XIa activation of factor IX by aprotinin (Ki 0.89 +/- 0.52 microM) was non-competitive, whereas inhibition by active site-inhibited factor IXa beta was competitive (Ki 0.33 +/- 0.05 microM). S2366 cleavage by isolated factor XIa catalytic domain was competitively inhibited by p-aminobenzamidine (Ki 38 +/- 14 microM) but was not inhibited by factor IX, consistent with loss of factor IX-binding exosites on the non-catalytic factor XI heavy chain. The results support a model in which factor IX binds initially to exosites on the factor XIa heavy chain, followed by interaction at the active site with subsequent bond cleavage, and support a growing body of evidence that exosite interactions are critical determinants of substrate affinity and specificity in blood coagulation reactions.

UI MeSH Term Description Entries
D007611 Aprotinin A single-chain polypeptide derived from bovine tissues consisting of 58 amino-acid residues. It is an inhibitor of proteolytic enzymes including CHYMOTRYPSIN; KALLIKREIN; PLASMIN; and TRYPSIN. It is used in the treatment of HEMORRHAGE associated with raised plasma concentrations of plasmin. It is also used to reduce blood loss and transfusion requirements in patients at high risk of major blood loss during and following open heart surgery with EXTRACORPOREAL CIRCULATION. (Reynolds JEF(Ed): Martindale: The Extra Pharmacopoeia (electronic version). Micromedex, Inc, Englewood, CO, 1995) BPTI, Basic Pancreatic Trypsin Inhibitor,Basic Pancreatic Trypsin Inhibitor,Bovine Kunitz Pancreatic Trypsin Inhibitor,Kallikrein-Trypsin Inactivator,Kunitz Pancreatic Trypsin Inhibitor,Trypsin Inhibitor, Basic, Pancreatic,Trypsin Inhibitor, Kunitz, Pancreatic,Antilysin,Bovine Pancreatic Trypsin Inhibitor,Contrical,Contrykal,Dilmintal,Iniprol,Kontrikal,Kontrykal,Pulmin,Traskolan,Trasylol,Zymofren,Inactivator, Kallikrein-Trypsin,Kallikrein Trypsin Inactivator
D002384 Catalysis The facilitation of a chemical reaction by material (catalyst) that is not consumed by the reaction. Catalyses
D005164 Factor IX Storage-stable blood coagulation factor acting in the intrinsic pathway of blood coagulation. Its activated form, IXa, forms a complex with factor VIII and calcium on platelet factor 3 to activate factor X to Xa. Deficiency of factor IX results in HEMOPHILIA B (Christmas Disease). Autoprothrombin II,Christmas Factor,Coagulation Factor IX,Plasma Thromboplastin Component,Blood Coagulation Factor IX,Factor 9,Factor IX Complex,Factor IX Fraction,Factor Nine,Factor IX, Coagulation
D006801 Humans Members of the species Homo sapiens. Homo sapiens,Man (Taxonomy),Human,Man, Modern,Modern Man
D006868 Hydrolysis The process of cleaving a chemical compound by the addition of a molecule of water.
D001550 Benzamidines Amidines substituted with a benzene group. Benzamidine and its derivatives are known as peptidase inhibitors.
D013379 Substrate Specificity A characteristic feature of enzyme activity in relation to the kind of substrate on which the enzyme or catalytic molecule reacts. Specificities, Substrate,Specificity, Substrate,Substrate Specificities
D015945 Factor XIa Activated form of factor XI. In the intrinsic pathway, Factor XI is activated to XIa by factor XIIa in the presence of cofactor HMWK; (HIGH MOLECULAR WEIGHT KININOGEN). Factor XIa then activates factor IX to factor IXa in the presence of calcium. Coagulation Factor XIa,Factor XI, Activated,Activated Factor XI,Blood Coagulation Factor XI, Activated,Contact Activation Product,Factor 11A,Factor Eleven A,Factor XIa, Coagulation
D020134 Catalytic Domain The region of an enzyme that interacts with its substrate to cause the enzymatic reaction. Active Site,Catalytic Core,Catalytic Region,Catalytic Site,Catalytic Subunit,Reactive Site,Active Sites,Catalytic Cores,Catalytic Domains,Catalytic Regions,Catalytic Sites,Catalytic Subunits,Core, Catalytic,Cores, Catalytic,Domain, Catalytic,Domains, Catalytic,Reactive Sites,Region, Catalytic,Regions, Catalytic,Site, Active,Site, Catalytic,Site, Reactive,Sites, Active,Sites, Catalytic,Sites, Reactive,Subunit, Catalytic,Subunits, Catalytic

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