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Channeling during prothrombin activation.
D S Boskovic1, L S Bajzar, M E Nesheim
1Department of Biochemistry, Botterell Hall, Queen's University, Kingston, Ontario K7L 3N6, Canada.
The Journal of Biological Chemistry
|June 1, 2001
Summary
The conversion of prothrombin to thrombin involves intermediates, but studies suggest direct conversion occurs. This "channeling" bypasses some intermediate steps in blood clotting.
Area of Science:
- Biochemistry
- Hematology
- Enzymology
Background:
- Prothrombin (II) is a plasma zymogen crucial for blood clotting.
- Its conversion to thrombin (IIa) by prothrombinase involves two proteolytic cleavages.
- Meizothrombin (mIIa) and prethrombin-2 (P2) are potential intermediates in this process.
Purpose of the Study:
- To investigate the reaction pathway of prothrombin to thrombin conversion.
- To determine if intermediates meizothrombin and prethrombin-2 are obligatory.
- To quantify the extent of direct prothrombin to thrombin conversion.
Main Methods:
- Kinetic measurements of prothrombin, meizothrombin, prethrombin-2, and thrombin over time.
- Analysis using a mathematical model with parallel reaction pathways.
- Steady-state experiments with continuous prothrombin infusion and radiolabeling.
Main Results:
- Time-course data indicated thrombin formation preceded intermediate accumulation, suggesting channeling.
- Model analysis revealed approximately 40% of consumed prothrombin bypassed free intermediates.
- Steady-state experiments showed consumption rates of free intermediates were significantly lower than thrombin formation rate.
Conclusions:
- Evidence supports a channeling phenomenon in prothrombin-to-thrombin conversion.
- A significant fraction of prothrombin is directly converted to thrombin, bypassing free meizothrombin and prethrombin-2.
- This direct conversion pathway plays an important role in blood coagulation kinetics.