852
c h a p t e r 36
Biochemistry of Hemostasis
fibrin. This dramatic change occurs as the result of re-
moval of less than 0.2% of the fibrinogen mass. Polymer-
ization of FnIIm occurs with both side-to-side and end-
to-end association. Interaction between fibrin monomers
is best visualized as occurring between specific sites that
are structurally complementary to one another. Polymer-
ization begins with the formation of a dimer. Two fibrin
monomers lie side-to-side with each monomer overlap-
ping one-half of the other. This produces a dimer that is
approximately 1.5 times longer than the fibrin monomer or,
because of the lack of contribution of the fibrinopeptides
to the length of the fibrinogen, 1.5 times longer than fib-
rinogen (Figure 36-8). The third fibrin monomer is be-
lieved to lie adjacent to the “open half’ of a monomer in the
fibrin dimer, thus leading to a trimer with a length equal to
approximately two times that of the fibrin monomer. Con-
tinued side-to-side and end-to-end association leads to the
formation of the fibrin polymer. The regular association
described above is not always maintained and thus “de-
fects” cause formation of branches on the polymerizing
fibrin. The fibrin monomer molecules are noncovalently
associated in the growing fibrin polymer. Consequently,
in the circulation where blood is flowing past the hemo-
static plug and its fibrin mesh, dissociation of the fibrin
monomers from the polymer occurs and the clot dissolves.
Fibrinogen is an acute phase reactant
3
and thus its con-
centration is substantially increased in several clinical sit-
uations. When the fibrinogen concentration is increased,
the action of thrombin on fibrinogen is faster—the conse-
quence of the greater extent of saturation of thrombin with
fibrinogen.
Red blood cells are responsible for the color of the blood
clot
in vitro
and also the clot that forms from blood released
onto the skin. A clot formed from fibrin in the absence
of entrapped red blood cells is a white, translucent gel.
In vitro,
fibrin forms a stringy mass that is easily wrapped
around a glass rod.
Noncovalently associated fibrin is physiologically un-
satisfactory because the dissociation of the fibrin results
in recurrent bleeding. Fibrin monomer dissociation is pre-
vented by formation of covalent cross-links between dif-
ferent FnIIm molecules. The result of this covalent cross-
linking is an insoluble fibrin and a stable hemostatic plug.
These cross-links are formed by the action of factor XHIa,
plasma, and/or platelet transglutaminase (see below). Mul-
tiple cross-links are formed among a chains of several dif-
ferent fibrin monomers. This creates a molecular species
designated
a
polymer (see “Fibrinolysis” below). Two
3 Acute phase reactants are plasma proteins that undergo large increases in
synthesis and thus concentration in plasma in response to acute inflammation
caused by surgery, myocardial infarction, and infections.
adjacent
y
chains from two different fibrin monomers are
cross-linked also to form a species designated
y-y
dimer.
36.5 The Procoagulant Subsystem of Coagulation
Activation Complexes
The proteolytic reactions of the hemostatic system are nei-
ther catalytically efficient nor localized when proteinase
and proteinase precursor only are present. The rapid, lo-
calized proenzyme activation required for normal hemo-
static response occurs only in a complex of proteinase,
proteinase precursor, and cofactor protein assembled on
the surface of a damaged cell membrane, or
in vitro,
on
the surface of phospholipid bilayers. The catalytic effi-
ciency of an enzyme-catalyzed reaction is expressed by
the ratio of the kinetic constants
kc
and
Km
(
kJK
m ) . 4
In
the activation complexes,
kJKm
values can be greater than
10
7
M
“ 1
s ' 1. With proteinase and proenzyme alone, the
kc/Km
values are only approximately 100 M
- 1
s
- 1
and
thus the reactions are 10
5
times less efficient. Expressed
in terms of the same amount of product formed in the two
situations, a
1 0 5
increase represents the difference between
requiring
1
minute and about
6
months to form the product!
It is helpful in the effort to understand activation com-
plexes to consider complex formation, the reactions that
occur in the complexes, and the “demise” of the com-
plexes as proceeding in a sequence. First, a reversible,
noncovalent association of proteinase, cofactor protein
(strictly, activated cofactor protein), proteinase precursor,
and membrane surface occurs to form the activation com-
plex. This spontaneous association occurs as the result of
complementary interaction sites on the protein molecules,
e.g., the binding sites between proteinase and protein sub-
strate, proteinase and cofactor protein, substrate and co-
factor protein, and all three proteins with the membrane
surface. Tissue factor normally exists as an integral mem-
brane protein and is always associated with the membranes
of cells in the vessel wall. The same processes are involved
in the anticoagulant subsystem and, with a different sur-
face, fibrin in the fibrinolytic system as well.
Second, irreversible proteolytic action in the complex
converts the proteinase precursor in the complex into an
active proteinase. This is followed by dissociation of the
product proteinase from the complex in which it has been
formed. Association of this proteinase with the next co-
factor protein and the next protein substrate to form the
next complex in the coagulation cascade then occurs.
4 K m
is the Michaelis constantand
k c
is the turnover rate Constantin the
classical mechanism for enzyme catalyzed reactions (See Chapter 6).
