Summary
The serine protease thrombin is the effector enzyme of blood coagulation. It has many
activities critical for the formation of stable clots, including cleavage of fibrinogen
to fibrin, activation of platelets and conversion of procofactors to active cofactors.
Thrombin carries-out its multiple functions by utilising three special features: a
deep active site cleft and two anion binding exosites (exosite I and II). Similarly,
thrombin inhibitors have evolved to exploit the unique features of thrombin to achieve
rapid and specific inactivation of thrombin. Exogenous thrombin inhibitors come from
several different protein families and are generally found in the saliva of haematophagous
animals (blood suckers) as part of an anticoagulant cocktail that allows them to feed.
Crystal structures of several of these inhibitors reveal how peptides and proteins
can be targeted to thrombin in different and interesting ways. Thrombin activity must
also be regulated by endogenous inhibitors so that thrombi do not occlude blood flow
and cause thrombosis. A single protein family, the serpins, provides all four of the endogenous thrombin inhibitors found in man. The crystal
structures of these serpins bound to thrombin have been solved, revealing a similar
exosite-dependence on complex formation. In addition to forming the recognition complex,
serpins destroy the structure of thrombin, allowing them to be released from cofactors
and substrates for clearance. This review examines how the special features of thrombin
have been exploited by evolution to achieve inhibition of the ultimate coagulation
protease.
Keywords
Exosite - inhibition - protease - regulation - structure