c h a p t e r 36
Biochemistry of Hemostasis
Heparin Structure (Repeating Disaccharides)
High Affinity for Antithrombin
F I G U R E 3 6 - 1 7
(Also see color figure.) Structure of heparin. Heparin is a polymer of repeating disaccharide units that contain one uronic
acid and one hexosamine residue. The uronic acid residues may be either glucuronic acid or iduronic acid, both of which
are monosaccharide acids that differ in their stereochemistry. The hexosamine residue is glucosamine. Both the uronic
and hexosamine residues can be modified by O- and N-sulfation and the glucosamine residue by N-acetylation. All
heparins bind to antithrombin; however, heparin molecules that contain a unique pentasaccharide sequence bind with
particularly high affinity (high-affinity heparins). Approximately 30% of the heparin molecules present in the commonly
used therapeutic heparins have high affinity for antithrombin.
Activation of plasminogen occurs as a result of the bind-
ing of both plasminogen and t-PA to fibrin, which results
in a nearly 400-fold decrease in the
for the activation
reaction, from about 65
to 0.16 /zM. In addition to this
enhancement of ES binding, the common binding to the
same “surface” localizes the activation reaction to fibrin.
Binding to fibrin occurs via the lysyl residues in fibrin and,
as a result of fibrinolysis, plasmin proteolysis of Lys-X
peptide bonds increases the number of available sites to
which the plasminogen and t-PA can bind. Kringles 1
and 4 of plasminogen and plasma bind to fibrin via their
lysine binding sites. The activation of plasminogen by
t-PA and u-PA is opposed by two protein inhibitors, plas-
minogen activator inhibitors 1
and 2 (PAI-1 and PAI-2;
Figure 36-18). PAI-1
inactivates t-PA very rapidly, but less
so when the t-PA is bound to fibrin. Higher than normal
concentrations of PAI-1 are associated with the occurrence
of thrombosis, indicative of the necessity for balance be-
tween tPA activation of plasminogen and t-PA inactivation
Plasminogen can be activated rapidly by the formation
of a “stoichiometric” complex between the plasminogen
molecule and proteins from several strains of hemolytic
plasmin” complex (SK-plasmin) converts plasminogen to
plasmin as well as digesting fibrin. In contrast to normal
plasmin, SK-plasmin is not inhibited by a^-antiplasmin.
Streptokinase-plasmin(ogen) is used therapeutically in
fibrinolytic therapy to remove thrombi. Another therapeu-
tically useful product has been prepared from SK-plasmin.
In this product, the active Ser of the plasmin is reacted with
an acylating chemical to form an unstable acyl ester. Slow,
spontaneous hydrolysis of the acylated SK-plasmin results
in a more sustained level of SK-plasmin in the circulation
during fibrinolytic therapy.
Plasmin is inactivated by a
-antiplasmin in a very rapid
is approximately 2 x 10
is analogous to
the best indicator of the efficiency
of an enzyme-catalyzed reaction). Blocking of the kringle
Lys binding sites reduces the rate of plasmin inactivation.
This is most evident from the “protection” of plasmin that
occurs when the plasmin is bound to the fibrin strands.
This protection tips the balance between the proteolytic
action of plasmin on its fibrin substrate and its inactiva-
tion by o'
-antiplasmin to proteolysis. The half-life (time
for half of the plasmin to be inactivated by a
is greater than
s for plasmin bound to fibrin, while it is
approximately 0.1 s in the absence of fibrin. This is another