s e c t io n 36.9
Fibrinolytic Subsystem
859
Pittsburgh), in which an Arg has replaced Ala at the reac-
tive site, is a good inhibitor of thrombin.
Because inactivation of factors Va and Villa by
activated protein C promotes the dissociation of the pro-
téinases, cofactor protein inactivation complements the
action of the proteinase inhibitors. This eliminates the
“protection” that the proteinases have when bound to
their cofactor proteins and substrates. Inactivation of pro-
teinases by SERPINS occurs via a common mechanism
that involves a Michaelis complex between the proteinase
and the inhibitor (Figure 36-16). This mechanism applies
to all serine proteinases of the hemostatic system, i.e.,
the procoagulant, anticoagulant, and fibrinolytic subsys-
tem proteinases.
Mechanism of Action of Heparin as a
Therapeutic Anticoagulant
The inactivation of procoagulant proteinases can be
catalyzed by glycosaminoglycans, which are sulfated
polysaccharide molecules found on the surface of the nor-
mal endothelial cells and in the basophilic granules of mast
cells. The glycosaminoglycans act as catalysts, increas-
ing the rates of inactivation of the proteinases as much
as 100,000-fold. The therapeutic benefits of heparin in
preventing and treating deep vein thrombosis are derived
from its altering the balance between the procoagulant and
anticoagulant subsystem reactions in favor of the antico-
agulant subsystem.
Heparin is a polymer of repeating disaccharide “build-
ing blocks.” All heparins bind to antithrombin; however,
heparin molecules that contain a unique pentasaccharide
sequence bind with particularly high affinity (designated
high-affinity heparins).
Heparins that contain this pen-
tasaccharide sequence are also the most effective in cat-
alyzing the inactivation of proteinases by antithrombin.
This increased effectiveness of high-affinity heparins is
due to a conformation change that they cause in the an-
tithrombin molecule that makes the binding between an-
tithrombin and heparin tighter.
The magnitude of the heparin-catalyzed increases in
the rates of proteinase inactivation by antithrombin de-
pends on the molecular weight of the high-affinity hep-
arin molecules. Up to a molecular weight of about 20,000,
the higher the molecular weight, the greater the increase
in the rate of proteinase inactivation. Thrombin inactiva-
tion requires heparin molecules of sufficient molecular
weight to permit the formation of a thrombin-heparin-
antithrombin complex. Factor Xa inactivation is less sensi-
tive to the molecular weight of the heparin molecules, and
factor Xa is the only proteinase that can be efficiently in-
activated by the very low-molecular-weight high-affinity
pentasaccharide (Figure 36-17). Here, in contrast to the
situation of factor XI activation, the glycosaminoglycan is
acting as a catalyst of an anticoagulant (proteinase inacti-
vation) process.
Inhibitors of the Contact Phase Proteinases
Several proteinase inhibitors inactivate the proteinases of
the contact phase. Among the SERPINS are C-l inactiva-
tor, a i-proteinase inhibitor, and antithrombin. The target
proteinases for these inhibitors are factor Xlla, kallikrein,
and factor XIa. The molecular mechanisms are the same
as those described for the procoagulant and fibrinolytic
system proteinases.
Another inhibitor is present in plasma that can inactivate
thrombin, plasmin, and, to a much lesser extent, the other
proteinases as well. This inhibitor, o^-macroglobulin, in-
hibits by a completely different mechanism from that of
the SERPINS. It entraps the proteinases in a “cavity” that
is created by the four subunits of the «
2
-macroglobulin
molecule. The active sites of the entrapped proteinases are
sterically hindered from protein substrates but are accessi-
ble to low-molecular-weight chromogenic substrates that
are used in some laboratory coagulation tests for heparin
and antithrombin.
36.9 Fibrinolytic Subsystem
The fibrinolytic system removes the fibrin of the hemo-
static plug and thus is responsible for the temporary ex-
istence of the fibrin clot. The proteolytic action of plas-
min on fibrin and fibrinogen is extensive and more like
the digestive proteolysis catalyzed by trypsin and Chy-
motrypsin than the proteolysis involved in proteinase pre-
cursor activation. The fibrinolytic subsystem includes the
reactions of plasminogen activation, plasmin inactivation,
and fibrin digestion. As is common throughout the hemo-
static system, irreversible activation reactions of the fib-
rinolytic system are opposed by irreversible proteinase
inactivation.
Plasminogen Activation
Plasminogen is converted to plasmin as the result of the
cleavage of a single peptide bond, Arg
161
-Val562. Two dif-
ferent proteinases are responsible for the physiological ac-
tivation of plasminogen: tissue-type plasminogen activator
(t-PA) and urinary plasminogen activator (u-PA). t-PA is
the principal activator of plasminogen and is synthesized
primarily in endothelial cells, the principal source of t-PA
in the circulating blood.
