The role of the insertion loop around tryptophan 148 in tthe activity of thrombin.
Thrombin has trypsin-like specificity for Arg-Xaa and Lys-Xaa peptide bonds; however, it is much more specific than trypsin, cleaving far fewer peptide bonds in macromolecular substrates. To probe the nature of the specificity of thrombin, a mutant has been constructed in which the Trp148 loop of thrombin has been replaced with the same loop of bovine trypsin. This mutant was expressed in Escherichia coli as prethrombin-2(148) using a T7 expression system previously described for wild-type prethrombin-2 [DiBella et al. (1995) J. Biol. Chem. 270, 163-169]. After refolding and purification, prethrombin-2(148) was activated to thrombin(148) with Echis carinatus snake venom. The k(cat)/K(m) for the release of fibrinopeptide A from fibrinogen was 4.5 +/- 0.5 microM(-1)s(-1) for thrombin(148), which was approximately 20% of that of recombinant thrombin (25 +/- 2.0 microM(-1)s(-1)). Thrombin(148) was inhibited less well by hirudin with a K(i) of 500 pM compared to a value of 12 pM determined for recombinant thrombin. The mutant thrombin was also compared to trypsin and wild-type recombinant thrombin for the ability to cleave small peptide substrates. The Michaelis constants (K(m)) were found to be between 5- and 10-fold higher for thrombin(148) relative to wild-type recombinant thrombin, although the catalytic constants (k(cat)) for thrombin(148) and recombinant thrombin remained relatively unchanged for all three substrates. Thrombin(148) had a specificity constant (k(cat)/K(m)) 2-fold higher for the hydrolysis of H-D-phenyalanyl-L- pipecolyl-L-arginine-p-nitroaniline (a thrombin substrate) than that of trypsin. For N-benzoyl-L-isoleucyl-L-glutamylglycyl-L-arginine- p-nitroaniline (a trypsin substrate) and N-carbobenzoxyglycylprolyl-L-arginine-p-nitroaniline (a substrate for both enzymes), the specificity constants for trypsin were 1000- and 16-fold higher, respectively. Although replacement of the Trp(148) loop does not yield an enzyme with more trypsin-like specificity, the Trp(148) loop is important in the substrate binding and specificity of thrombin (on the basis of K(m) and K(i)).
DiBella
E E
EE
Baker Laboratory of Chemistry, Cornell University, Ithaca, New York 14853-1301, USA.
Scheraga
H A
HA
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D002417:Cattle; D004247:DNA / Q000235:genetics; D004792:Enzyme Precursors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics; D066298:In Vitro Techniques; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009842:Oligopeptides / Q000737:chemistry; D011487:Protein Conformation; D011516:Prothrombin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013379:Substrate Specificity; D013917:Thrombin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D014357:Trypsin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D014364:Tryptophan / Q000737:chemistry
D004792:Enzyme Precursors; D009842:Oligopeptides; D011994:Recombinant Proteins; D014364:Tryptophan; D011516:Prothrombin; D004247:DNA; C012007:prethrombins; D014357:Trypsin; D013917:Thrombin