Page 336 - Essential Haematology

322 / Chapter 24 Platelets, blood coagulation and haemostasis

Table 24.1 The coagulation factors.
Factor number Descriptive name Active form
I Fibrinogen Fibrin subunit
II Prothrombin Serine protease
III Tissue factor Receptor/cofactor *

V Labile factor Cofactor
VII Proconvertin Serine protease
VIII Antihaemophilic factor Cofactor
IX Christmas factor Serine protease
X Stuart – Prower factor Serine protease
XI Plasma thromboplastin antecedent Serine protease
XII Hageman (contact) factor Serine protease
XIII Fibrin - stabilizing factor Transglutaminase
Prekallikrein (Fletcher factor) Serine protease
HMWK (Fitzgerald factor) Cofactor *

HMWK, high molecular weight kininogen.
* Active without proteolytic modifi cation.

Active serine site
of IXa attacks here
Light chain
S-S S-S
Reaction
peptide
Heavy chain Active MW ≈ 14 000
Se Se
Factor X Factor Xa
MW ≈ 60 000

Figure 24.9 Serine (Se) protease activity. This example shows the activation of the serine site of factor X by

factor IX.
2 +
extrinsic Xase (VIIa, TF, PL, Ca ), intrinsic Xase amplifi cation when micromolar concentrations are
2 +
(IXa, VIIIa, PL, Ca ) generating FXa and pro- produced, i.e. a million - fold higher concentration
2 +
thrombinase complex (Xa, Va, PL, Ca ) generating than produced during the initiation phase.
thrombin. The generation of thrombin following

vascular injury occurs in two waves of very diff erent
Initiation
magnitude with different functions. During the ini-

tiation phase small amounts are generated (picomo- Coagulation is initiated by the interaction of the
lar concentrations) which prepares the coagulation membrane bound tissue factor (TF), exposed and
cascade for the second larger thrombin burst in the activated by an enzyme protein disulphide isomer-