PAR-3 is a low-affinity substrate, high affinity effector of thrombin.
A polypeptide corresponding to the extracellular domain of protease-activated receptor 3 (PAR-3) is hydrolyzed by thrombin slowly because of high K(M) (>100 microM). However, thrombin is found to bind two PAR-3, one without catalyzing hydrolysis or blocking the active site, while the other is hydrolyzed. In a solvent lacking Na(+), hydrolysis of a nitroanilide substrate is enhanced 1.6-fold by addition of PAR-3 polypeptide, with half-saturation at 2.5 microM. In contrast, the fibrinogen clotting activity of thrombin is inhibited completely by PAR-3, with a K(I) of 3 microM. None of the activities of thrombin are affected by addition of 50 microM PAR-4 polypeptide. Thus, PAR-3 in low concentrations binds thrombin in a configuration that blocks the anion-binding exosite but not the catalytic site, while hydrolysis of PAR-3, PAR-4, and other substrates that do not interact with exosite I persists. The allosteric effect of PAR-3 is characteristic of that of Na(+).
Owen
Whyte G
WG
Department of Biochemistry and Molecular Biology, Mayo Foundation for Medical Education and Research, Rochester, MN 55905-0002, USA. wgo@mayo.edu
D000494:Allosteric Regulation; D006801:Humans; D006868:Hydrolysis; D008954:Models, Biological; D010455:Peptides / Q000737:chemistry / Q000378:metabolism; D017434:Protein Structure, Tertiary; D018179:Receptors, Thrombin / Q000737:chemistry / Q000378:metabolism*; D013917:Thrombin / Q000378:metabolism*
D010455:Peptides; D018179:Receptors, Thrombin; C105295:protease-activated receptor 3; D013917:Thrombin; C112908:protease-activated receptor 4