s e c t io n 36.5
The Procoagulant Subsystem of Coagulation
853
Third, as the proteinases complete their catalysis of pre-
cursor activation, they dissociate more completely from
their respective complexes and are inactivated by the
proteinase inhibitors present in the blood. Dissociation
from the complexes is important; in the complexes and
in the presence of substrates, the proteinase inhibitors
are “blocked” from reacting with the proteinases because
the substrate obstructs access to the active site of the
proteinase.
Fourth, complete dissociation is promoted by the pro-
teolytic inactivation of the activated cofactor proteins by
activated protein C. Proteolytic inactivation of the acti-
vated cofactors ensures that reformation of the activation
complex does not occur to any appreciable extent. Because
the system is dynamic, all of these processes occur simul-
taneously. However, in the initial stages of the secondary
hemostatic response, the first two “steps” predominate. As
the hemostatic plug becomes consolidated by fibrin rein-
forcement, the third and fourth steps predominate.
The Prothrombin Activation Complex
(Prothrombinase)
The first and most completely characterized of the acti-
vation complexes of the procoagulant subsystem is that
of prothrombin activation. Because of this, the prothrom-
bin activation complex will be described in greater detail
than the other, similar activation complexes (Figure 36-9)
The general principles described below apply to all of the
complexes.
The prothrombin activation complex consists of factor
Xa (the proteinase), prothrombin (the substrate), factor Va
(the cofactor protein in an “activated” form), phospholipid
bilayer surface, and Ca2+ ion. Ca2+ ion is involved in the
binding of prothrombin and factor Xa to the phospholipid
bilayer and cell membrane surfaces. Prothrombin and fac-
tor Xa associate with the membrane surface as the result of
a Ca2+ ion-dependent conformation that provides a “face”
on the molecule that interacts with the lipid molecules of
the membrane bilayer. Ca2+ ion binding to these two pro-
teins and the lipid-binding conformation are related to the
presence of several (14-12) unique amino acid residues,
p-carboxyglutamate,
in
these
proteins
(see
section
“Vitamin K, Oral Anticoagulants, and Their Mechanisms
of Action”) The shapes of the proteins and their relative
dimensions, and the half-thickness of the phospholipid bi-
layer are based on studies of the hydrodynamic properties
of the proteins and on x-ray crystallographic structure data.
Association of factor Xa and prothrombin with factor Va
involves the EGF-like and kringle structures of the two
proteins and regions (sites) on the A domains of factor Va.
F I G U R E 3 6 - 9
(Also see color figure.) Archetypal activation complex of coagulation:
p ro th ro m b in a se.
Complex formation precedes rapid proteolysis of
prothrombin. The presence of the specific domains and the binding sites
associated with them are responsible for the formation of the complexes
between the correct proteins. The A domains of factor Va provide the
interacting sites for both factor Xa and prothrombin, the C domains the
sites for binding to the surface. The Gla domains bind Ca2+ (white dots)
and bind factor Xa and prothrombin to the surface. The locations of the
various sites further orient the molecules optimally with respect to the
surface and to each other to give the localized, large increases in rates of
proteolysis. The conversion of the proenzyme form of a proteinase to its
catalytically active form commonly requires proteolysis of a peptide bond
that frees a Leu, lie, or Val residue from the peptide linkage. This permits a
conformation change within the active proteinase domain that is stabilized
by the formation of a salt bridge between the newly freed residue and
Asp102 (chymotrypsin numbering). Proteolysis,
p e r se,
is not the key
element in “enabling” of the catalytic apparatus of the proteinase, but
rather the conformation that is stabilized by the salt bridge. The
dimensions indicated on the figure are measured distances between
fluorescent labels on the proteins (active sites for factor Xa and
meizothrombin) and a fluorophore in the membrane.
Activation of Prothrombin
The activation of prothrombin to thrombin and the conver-
sion of fibrinogen to fibrin are frequently described as the
final, common pathway of blood coagulation. This des-
ignation, the result of the convergence of the “intrinsic”
and “extrinsic” pathways with the formation of factor Xa,
is common in older literature. Prothrombin activation is
shown in Figure 36-10. After proteolytic cleavage, frag-
ment 1-2 retains the Gla domain and the two kringles of the
prothrombin molecule. Although it may remain associated
with the surface and ultimately contribute to displacement
of other proteins, the amount of prothrombin converted to
thrombin and fragment
1
- 2
makes this a minor role of the
fragment.
The a-thrombin dissociates from the prothrombin
fragment
1 - 2
and diffuses to its substrates: platelets
(the platelet thrombin receptor), factor V, factor VIII,
