Thrombin is produced in the liver
Thrombin is
formed from
Prothrombin.
Thrombin is
produced by the
enzymatic
cleavage of two
sites on
prothrombin by
activated X
factor.
Thrombin is a protein
Cellular Response
• Thrombin is an enzyme
• Catalyzes the final step in the
clotting process:
o the conversion of fibrinogen
to fibrin
• Fibrin forms threads that are
woven into a clot, stopping
bleeding.
• Thrombin also activates a
factor that catalyzes more
fibrin production
o Positive feedback
Thrombin
Is the result of the pathway demonstrating positive or negative
feedback?
Thrombin is a factor that facilitates the clotting of blood
by catalyzing conversion of fibrinogen to fibrin.
Positive feedback mechanisms like blood clotting
enhance the original stimulus.
 Blood clotting is a positive feedback loop because
the clot keeps growing until it is fully formed and must
form quickly, especially if the blood vessel is large. Once
a vessel is damaged, platelets start to cling to the
injured site and release chemicals that attract more
platelets. The platelets continue to pile up and release
chemicals until a clot is formed.
The pathway takes blood from a nonclotting state to a
clotting state for as long as it’s needed for the clot to
form. When the clot has formed, the mechanism for
clotting just stops, there is no reverse mechanism that
slows and stops clotting. It keeps growing until stopped.
Consequence of Alteration
 Insufficient bleeding and
prolonged bleeding
 In hemophilia there is a
factor 8 deficiency. If this
protein is not found in the
pathway, thrombin is never
able to change fibrinogen to
fibrin and blood clots cannot
form
Pathway/mechanism
 Platelets release clotting factors which trigger the
formation of thrombin.
 Thrombin activates a factor that catalyzes the
formation of more thrombin, which drives clotting to
completion.
 Thrombin then activates fibrin which helps form
webs to clot.
In other words…
 Thrombin uses a second messenger to help clot the
blood and stop the bleeding. It never directly comes
in contact with the cell to communicate.
 Clotting factors: platelets, damaged cells, plasma.
video
 http://www.hopkinsmedicine.org/hematology/coag
ulation.swf