Recombinant blood products and therapeutic enzymes - Pharmaceutical Biotechnology: Concepts and Aapplications - page 336

Xa

Prothrombin

S S

NH 2

Thrombin

S

S

NH 2

Figure 12.5 Proteolytic cleavage of prothrombin by factor Xa, yielding active thrombin. Although pro-
thrombin is a single-chain glycoprotein, thrombin consists of two polypeptides linked by what was originally
the prothrombin intrachain disulfi de bond. The smaller thrombin polypeptide fragment consists of 49 amino
acid residues, and the large polypeptide chain contains 259 amino acids. The N-terminal fragment released
from prothrombin contains 274 amino acid residues. Activation of prothrombin by Xa does not occur in free
solution, but at the site of vascular damage

cleaves prothrombin at two sites (arg 274 -thr 275 and arg 323 -ile 324 ), yielding active thrombin and an
inactive polypeptide fragment, as depicted in Figure 12.5.
Fibrinogen (factor I) is a large (340 kDa) glycoprotein consisting of two identical tri-polypeptide
units, α, β and γ. Its overall structural composition may thus be represented as (α β γ) 2 .
The N-terminal regions of the fi brinogen chains are rich in charged amino acids, which, via

charge repulsion, play an important role in preventing aggregation of individual fi brinogen molecules.
Thrombin, which catalyses the proteolytic activation of fi brinogen, hydrolyses these N-terminal pep-
tides. This renders individual fi brin molecules more conducive to aggregation, therefore promoting
soft clot formation. The soft clot is stabilized by the subsequent introduction of covalent cross-linkages
between individual participating fi brin molecules. This reaction is catalysed by factor XIIIa.

α and β

12.2.3 Clotting disorders

Genetic defects characterized by (a) lack of expression or (b) an altered amino acid sequence of
any clotting factor can have serious clinical consequences. In order to promote effective clotting,
both intrinsic and extrinsic coagulation pathways must be functional, and the inhibition of even
one of these pathways will result in severely retarded coagulation ability. The result is usually
occurrence of spontaneous bruising and prolonged haemorrhage, which can be fatal. With the ex-
ception of tissue factor and Ca 2 , defects in all other clotting factors have been characterized. Up
to 90 per cent of these, however, relate to a defi ciency in factor VIII, and much of the remainder is
due to a defi ciency in factor IX.