Structure-function relationships in the interaction of alpha-thrombin with blood platelets.

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• Platelet activation by alpha-thrombin is crucial for hemostasis.
• Understanding the molecular interactions between thrombin and platelets is key to comprehending thrombotic processes.

Purpose of the Study:

• To identify specific regions of alpha-thrombin involved in platelet binding and activation.
• To elucidate the roles of the catalytic site and fibrinogen-binding site in thrombin-platelet interactions.

Main Methods:

• Chemical modification of highly purified alpha-thrombin using specific agents targeting catalytic site residues (serine, histidine) and tryptophan residues.
• Assessment of modified thrombin's binding characteristics to platelets.
• Evaluation of platelet activation, aggregation, and serotonin release following thrombin interaction.

Main Results:

• Modification of the catalytic site did not significantly alter thrombin binding to platelets but abolished platelet activation.
• Modification of tryptophan residues near the fibrinogen-binding site led to loss of high-affinity thrombin binding to platelets.
• Low-affinity binding remained unaffected, and thrombin's ability to induce platelet aggregation and serotonin release was altered but not abolished.

Conclusions:

• Catalytic site residues are not essential for thrombin binding to platelets.
• The macromolecular substrate-binding site of alpha-thrombin is critical for high-affinity platelet binding.
• Evidence suggests at least two types of thrombin binding sites on platelets and multiple binding regions on thrombin.