858
c h a p t e r 36
Biochemistry of Hemostasis
thromboembolism. The inability of activated protein C to
cleave Gin
5 0 6
slows the cleavage of Arg
3 0 6
in factor Va. A
second mutation, factor V (Cambridge), that confers ac-
tivated protein C resistance is at Arg306. As a result, the
cleavage essential for complete inactivation of factor Va
does not occur.
The factor V (Leiden) mutation is very common in in-
dividuals of western European origin. The prevalence is
approximately 5% in the Western Hemisphere in Cau-
casians; the mutation is almost completely absent from
Africans and almost entirely absent from Asians.
F I G U R E 3 6 - 1 5
(Also see color figure.) Anticoagulant subsystem. Activation of protein C
occurs adjacent to the injury site; inactivation occurs on the exposed
surface at the injury site. Protein C is activated through cleavage by
thrombin at Arg169-Ile170. Protein S is a 635-residue vitamin K-dependent
protein that functions as a cofactor. There is a Gla domain and four
EGF-like domains in the molecule, but no proteinase domain.
Thrombomodulin is a 554-residue integral membrane protein. The
extracellular region comprises residues I -494, the transmembrane region
residues 495-518, and the intracellular region residues 519-554. EGF-like
structures are found from residue 224 to 459. In the color version of this
figure, motifs and domains are color coded as follows: Gla domain (blue),
EGF-like domain (magenta), activation peptide (yellow), and proteinase
domain (green). Light chains are indicated in dark gray, heavy chains in
light gray. Regions connecting the motifs are black.
become nonfunctional catalysts. The interaction of the ac-
tivated cofactors with the proteinases in their respective
complexes is eliminated because the interaction is a prop-
erty of the factor Va and factor Villa heavy chains. The
rates of proteolytic inactivation of factors Va and Villa are
increased by participation of the cofactor protein, protein
S. The increased rate of proteolysis with protein S (only
about 500 times) is much less than observed with the co-
factor proteins Va and Villa of the procoagulant subsystem
(more than
1 0 , 0 0 0
times).
Mutations that result in amino acid substitution at the
cleavage site in a protein substrate cause two changes in
the proteolytic process. First, the rate of cleavage of the
peptide bond is reduced because of the altered orientation
of the substrate within the active site. Second, the altered
substrate is very likely to be a competitive inhibitor of
the proteinase responsible for the cleavage. Both of these
effects are evident in the phenomenon called
activated
protein C resistance.
A mutation in the factor V gene, G to A at 1691, results
in the replacement of the normal Arg residue at position
506 in the heavy chain of factor Va by a Gin residue. In-
dividuals carrying this mutation, called factor V (Leiden)
are at increased
risk
of venous thrombosis and venous
Anticoagulant Subsystem—Proteinase Inhibitors
Proteinase inactivation occurs by a stoichiometric reaction
between proteinase and inhibitor that results in the forma-
tion of a “covalent” ester bond between the reactive site
residue of the inhibitor (Arg
3 9 3
in antithrombin) and the ac-
tive site residue (Ser
195
in the proteinase). The proteinases
thrombin, factor Xa, factor IXa, and, less effectively, fac-
tor Vila and factor XIa are all inactivated by antithrombin
(Figure 36-16). Other SERPINS can inactivate procoagu-
lant proteinases, heparin cofactor II can inactivate throm-
bin, and
a
i -proteinase inhibitor can inactivate factor Xa.
An altered
a
i -proteinase inhibitor (a i -proteinase inhibitor
FIGURE 36-16
(Also see color figure.) Proteinase inactivation by SERPINS. Proteinase
inactivation occurs by reaction between proteinase and inhibitor, e.g.,
antithrombin. The proteinase is a stoichiometric reactant in this instance
but is not a catalyst. This reaction results in the formation of a covalent
bond between the reactive site residue of the inhibitor (Arg'193 in
antithrombin) and the active site residue (Ser195 in the proteinase). This
complex formation prevents the proteinase from hydrolyzing any other
peptide bond. Proteinases, thrombin, factor Xa, factor IXa, and, less
effectively, factor Vila and factor XIa are all inactivated by the plasma
protein inhibitor antithrombin (previously designated antithrombin III).
The product ATm
is the cleaved form of antithrombin. It is formed in both
the absence and presence of heparin, but more so in the presence. In the
color version of this figure, the proteinase is indicated by green, the
inhibitor by red, and the inactivated proteinase by gray. Stripes on the
inhibitor represent the helices (Figure 36-7).
