title: Important role of the cys-191 cys-220 disulfide bond in thrombin function and allostery.
abstract: Little is known on the role of disulfide bonds in the catalytic domain of serine proteases. The Cys-191-Cys-220 disulfide bond is located between the 190 strand leading to the oxyanion hole and the 220-loop that contributes to the architecture of the primary specificity pocket and the Na+ binding site in allosteric proteases. Removal of this bond in thrombin produces an approximately 100-fold loss of activity toward several chromogenic and natural substrates carrying Arg or Lys at P1. Na+ activation is compromised, and no fluorescence change can be detected in response to Na+ binding. A 1.54-A resolution structure of the C191A/C220A mutant in the free form reveals a conformation similar to the Na+-free slow form of wild type. The lack of disulfide bond exposes the side chain of Asp-189 to solvent, flips the backbone O atom of Gly-219, and generates disorder in portions of the 186 and 220 loops defining the Na+ site. This conformation, featuring perturbation of the Na+ site but with the active site accessible to substrate, offers a possible representation of the recently identified E* form of thrombin. Disorder in the 186 and 220 loops and the flip of Gly-219 are corrected by the active site inhibitor H-D-Phe-Pro-Arg-CH(2)Cl, as revealed by the 1.8-A resolution structure of the complex. We conclude that the Cys-191-Cys-220 disulfide bond confers stability to the primary specificity pocket by shielding Asp-189 from the solvent and orients the backbone O atom of Gly-219 for optimal substrate binding. In addition, the disulfide bond stabilizes the 186 and 220 loops that are critical for Na+ binding and activation.
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Leslie A
authors.initials: LA
authors.lastname: Bush-Pelc
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Francesca
authors.initials: F
authors.lastname: Marino
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Zhiwei
authors.initials: Z
authors.lastname: Chen
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Agustin O
authors.initials: AO
authors.lastname: Pineda
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: F Scott
authors.initials: FS
authors.lastname: Mathews
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110. Electronic address: enrico@wustl.edu.
authors.forename: Enrico
authors.initials: E
authors.lastname: Di Cera
mesh_terms: D000495:Allosteric Site; D003460:Crystallization; D003545:Cysteine; D004220:Disulfides / Q000737:chemistry*; D011487:Protein Conformation; D012964:Sodium / Q000378:metabolism; D013329:Structure-Activity Relationship; D013917:Thrombin / Q000737:chemistry* / Q000502:physiology*
chemical_list: D004220:Disulfides; D012964:Sodium; D013917:Thrombin; D003545:Cysteine