title: Molecular determinants of the mechanism underlying acceleration of the interaction between antithrombin and factor Xa by heparin pentasaccharide.
abstract: The control of coagulation enzymes by antithrombin is vital for maintenance of normal hemostasis. Antithrombin requires the co-factor, heparin, to efficiently inhibit target proteinases. A specific pentasaccharide sequence (H5) in high affinity heparin induces a conformational change in antithrombin that is particularly important for factor Xa (fXa) inhibition. Thus, synthetic H5 accelerates the interaction between antithrombin and fXa 100-fold as compared with only 2-fold versus thrombin. We built molecular models and identified residues unique to the active site of fXa that we predicted were important for interacting with the reactive center loop of H5-activated antithrombin. To test our predictions, we generated the mutants E37A, E37Q, E39A, E39Q, Q61A, S173A, and F174A in human fXa and examined the rate of association of these mutants with antithrombin in the presence and absence of H5. fXa(Q61A) interacts with antithrombin alone with a nearly normal k(ass); however, we observe only a 4-fold increase in k(ass) in the presence of H5. The x-ray crystal structure of fXa reveals that Gln(61) forms part of the S1' and S3' pocket, suggesting that the P' region of the reactive center loop of antithrombin is crucial for mediating the acceleration in the rate of inhibition of fXa by H5-activated antithrombin.
authors.affiliation: Department of Biochemistry and Molecular Biology, School of Biomedical Sciences, Monash University, Clayton, Victoria 3800, Australia.
authors.forename: Noelene S
authors.initials: NS
authors.lastname: Quinsey
authors.forename: James C
authors.initials: JC
authors.lastname: Whisstock
authors.forename: Bernard
authors.initials: B
authors.lastname: Le Bonniec
authors.forename: Virginie
authors.initials: V
authors.lastname: Louvain
authors.forename: Stephen P
authors.initials: SP
authors.lastname: Bottomley
authors.forename: Robert N
authors.initials: RN
authors.lastname: Pike
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000991:Antithrombins / Q000378:metabolism*; D001426:Bacterial Proteins / Q000494:pharmacology; D002460:Cell Line; D029968:Escherichia coli Proteins; D015951:Factor Xa / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006493:Heparin / Q000737:chemistry / Q000494:pharmacology*; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009844:Oligosaccharides / Q000737:chemistry / Q000378:metabolism; D033903:Periplasmic Proteins; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D015842:Serine Proteinase Inhibitors / Q000494:pharmacology; D014162:Transfection
chemical_list: D000991:Antithrombins; D001426:Bacterial Proteins; C039076:Eco protein, E coli; D029968:Escherichia coli Proteins; D009844:Oligosaccharides; D033903:Periplasmic Proteins; D011994:Recombinant Proteins; D015842:Serine Proteinase Inhibitors; D006493:Heparin; D015951:Factor Xa