Electrostatic steering and ionic tethering in the formation of thrombin-hirudin complexes: the role of the thrombin anion-binding exosite-I.
Electrostatic interactions between the thrombin anion-binding exosite-I (ABE-I) and the hirudin C-terminal tail play an important role in the formation of the thrombin-hirudin inhibitor complex and serves as a model for the interactions of thrombin with its many other ligands. The role of each solvent exposed basic residue in ABE-I (Arg(35), Lys(36), Arg(67), Arg(73), Arg(75), Arg(77a), Lys(81), Lys(109), Lys(110), and Lys(149e)) in electrostatic steering and ionic tethering in the formation of thrombin-hirudin inhibitor complexes was explored by site directed mutagenesis. The contribution to the binding energy (deltaG(degrees)b) by each residue varied from 1.9 kJ mol(-)(1) (Lys(110)) to 15.3 kJ mol(-1) (Arg(73)) and were in general agreement to their observed interactions with hirudin residues in the thrombin-hirudin crystal structure [Rydel, T. J., Tulinsky, A., Bode, W., and Huber, R. (1991) J. Mol. Biol. 221, 583-601]. Coupling energies (delta deltaG(degrees) int) were calculated for the major ion-pair interactions involved in ionic tethering using complementary hirudin mutants (h-D55N, h-E57Q, and h-E58Q). Cooperativity was seen for the h-Asp(55)/Arg(73) ion pair (2.4 kJ mol(-1)); however, low coupling energies for h-Asp(55)/Lys(149e) (deltadeltaG(degrees)int 0.6 kJ mol(-1)) and h-Glu(58)/Arg(77a) (deltadeltaG(degrees)int 0.9 kJ mol(-1)) suggest these are not major interactions, as anticipated by the crystal structure. Interestingly, high coupling energies were seen for the intermolecular ion-pair h-Glu(57)/Arg(75) (deltadeltaG(degrees)int 2.3 kJ mol(-1)) and for the solvent bridge h-Glu(57)/Arg(77a) (deltadeltaG(degrees)int 2.7 kJ mol(-1)) indicating that h-Glu(57) interacts directly with both Arg(75) and Arg(77a) in the thrombin-hirudin inhibitor complex. The remaining ABE-I residues that do not form major contacts in tethering the C-terminal tail of hirudin make small but collectively important contributions to the overall positive electrostatic field generated by ABE-I important in electrostatic steering.
Myles
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Department of Haematology, University of Cambridge, MRC Centre, Hills Road, Cambridge CB2 2QH, United Kingdom. tmyles@stanford.edu
Le Bonniec
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Betz
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Stone
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D019943:Amino Acid Substitution / Q000235:genetics; D000596:Amino Acids / Q000737:chemistry / Q000235:genetics; D000838:Anions / Q000737:chemistry; D000991:Antithrombins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology; D001667:Binding, Competitive / Q000235:genetics; D002863:Chromogenic Compounds / Q000378:metabolism; D004151:Dipeptides / Q000378:metabolism; D006629:Hirudins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D006868:Hydrolysis; D007700:Kinetics; D046911:Macromolecular Substances; D011485:Protein Binding / Q000235:genetics; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D055672:Static Electricity; D013816:Thermodynamics; D013917:Thrombin / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
D000596:Amino Acids; D000838:Anions; D000991:Antithrombins; D002863:Chromogenic Compounds; D004151:Dipeptides; D006629:Hirudins; D046911:Macromolecular Substances; D011994:Recombinant Proteins; C021129:S 2238; D013917:Thrombin