Substrate-Assisted Catalysis of the PAR1 Thrombin Receptor

• Published in: The Journal of biological chemistry Vol. 275; no. 52; pp. 40671 - 40678
• Main Authors: Jacques, Suzanne L., LeMasurier, Meredith, Sheridan, Paul J., Seeley, Stacy K., Kuliopulos, Athan
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 29.12.2000; American Society for Biochemistry and Molecular Biology
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M004544200

Platelet activation and aggregation are mediated by thrombin cleavage of the exodomain of the PAR1 receptor. The specificity of thrombin for PAR1 is enhanced by binding to a hirudin-like region (Hir) located in the receptor exodomain. Here, we examine the mechanism of thrombin-PAR1 recognition and cleavage by steady-state kinetic measurements using soluble PAR1 N-terminal exodomains. We determined that the primary role of the PAR1 Hir sequence is to reduce the kinetic barriers to formation of the docked thrombin-PAR1 complex rather than to form high affinity ground-state interactions. In addition, the exosite I-bound Hir motif facilitates the productive interaction of the PAR138LDPR/SFL44 sequence with the active site of thrombin. This locking process is the most energetically unfavorable step of the overall reaction. The subsequent irreversible steps of peptide bond cleavage are rapid and allosterically enhanced by the presence of the docked Hir sequence. Furthermore, the C-terminal exodomain product of thrombin cleavage, corresponding to the activated receptor, binds tightly to thrombin. This would suggest that an additional role of the Hir sequence in the thrombin-activated receptor is to sequester thrombin to the platelet surface and modulate cleavage of other platelet receptors such as the PAR4 thrombin receptor, which lacks a functional Hir sequence.