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Cleavage of the thrombin receptor: identification of potential activators and inactivators
Insights
Thrombin receptor peptide cleavage kinetics were studied for thrombin and 12 other proteases. While thrombin shows high efficiency, other proteases like chymotrypsin can cleave the peptide, potentially inactivating the receptor.
Area of Science:
- Biochemistry
- Enzymology
- Protease activity
Background:
- The thrombin receptor plays a crucial role in cellular signaling.
- Understanding protease interactions with the thrombin receptor is vital for drug development and disease research.
Purpose of the Study:
- To determine the kinetic parameters for the hydrolysis of a thrombin receptor peptide (residues 38-60) by thrombin and 12 other proteases.
- To compare the cleavage specificities and efficiencies of various proteases on the thrombin receptor activation site.
- To develop a model that reconciles peptide cleavage kinetics with cellular responses to thrombin.
Main Methods:
- Kinetic analysis (kcat, Km, kcat/Km) of peptide hydrolysis by purified proteases.
- Site-directed cleavage mapping using a synthetic peptide representing the thrombin receptor activation site.
- Comparison of cleavage efficiencies across a panel of 13 proteases.
Main Results:
- Thrombin exhibited high catalytic efficiency (kcat/Km) for cleaving the TR39-40 peptide at the activation site.
- Trypsin and chymotrypsin efficiently cleaved the peptide, with chymotrypsin causing inactivation by cleaving after Phe43.
- Other proteases like factor Xa and plasmin showed significantly lower cleavage efficiencies, while some proteases were unable to cleave the peptide.
- Neutrophil cathepsin G also inactivated the receptor by cleaving after Phe55, but less efficiently than chymotrypsin.
Conclusions:
- Thrombin-mediated cleavage of its receptor peptide is highly efficient.
- Non-specific cleavage by other proteases, such as chymotrypsin and cathepsin G, can lead to receptor inactivation.
- The kinetic data provide insights into the regulation of thrombin receptor signaling by various proteases in vivo.
Abstract:
The kinetic parameters were determined for the hydrolysis of a peptide based on the activation site of the thrombin receptor (residues 38-60) by thrombin and 12 other proteases. The kcat and Km values for the cleavage of this peptide (TR39-40) by thrombin were 107 s-1 and 1.3 microM; the kcat/Km of TR39-40 is among the highest observed for thrombin. A model is presented that reconciles the parameters for cleavage of the peptide with the concentration dependence of cellular responses to thrombin. Cleavage of TR39-40 was not specific for thrombin. The pancreatic proteases trypsin and chymotrypsin hydrolysed TR39-40 efficiently (kcat/Km > 10(6) M-1.s-1). Whereas trypsin cleaved TR39-40 at the thrombin activation site (Arg41-Ser42), chymotrypsin hydrolysed the peptide after Phe43. This chymotryptic cleavage would result in inactivation of the receptor. The efficient cleavage of TR39-40 by chymotrypsin (kcat/Km approximately 10(6) M-1.s-1) was predominantly due to a low Km value (2.8 microM). The proteases factor Xa, plasmin, plasma kallikrein, activated protein C and granzyme A also hydrolysed TR39-40 at the Arg41-Ser43 bond, but exhibited kcat/Km values that were at least 10(3)-fold lower than that observed with thrombin. Both tissue and urokinase plasminogen activators as well as granzyme B and neutrophil elastase were unable to cleave TR39-60 at appreciable rates. However, neutrophil cathepsin G hydrolysed the receptor peptide after Phe55. Like the chymotryptic cleavage, this cleavage would lead to inactivation of the receptor, but the cathepsin G reaction was markedly less efficient; the kcat/K(m) value was almost four orders of magnitude lower than that for thrombin. In addition to the above cleavage sites, a secondary site for thrombin and other arginine-specific proteases was identified at Arg46, but the cleavage at this site only occurred at very low rates and is unlikely to be significant in vivo.