CHA PTER 36, FIG URE 6 Cofactor proteins, factor V and factor VIII. Factor V and factor VIII coagulant (not
the von Willebrand factor carrier of factor VIII) contain six distinct structural domains. The two A domains, A, and
A2 at the N-terminal end of the polypeptide chain, are separated from the A3 domain by a highly glycosylated B
domain. The two C domains are at the C-terminal end of the molecule. The A domain sequences are homologous to
the A domains of ceruloplasmin. Both factor Va and factor Villa act as catalysts in the activation of prothrombin and
factor X, respectively. Activation sites are indicated by green arrows; inactivation sites by red arrows. In factor Va,
complete inactivation requires cleavage of Arg306.
CHA PTER 36, FIG URE 7 A group of structurally similar protein inhibitors of the serine proteinases known as
SERPINS (SERine Proteinase INhibitors). The structure shown is human antithrombin. The reference SERPIN,
proteinase inhibitor or
j-antitrypsin contains -30%
helix (9 helices) and 40% sheet (5 |3 sheets). Other members
of the SERPIN family contain both additional helices and |3 sheets. The reactive center loop of antithrombin, residues
378-396, contains the reactive site residues Arg393 and Ser394. Upon reaction with the target proteinase or after cleav-
age by the target proteinase (a reaction that inactivates the inhibitor without inactivating the proteinase), the reactive
center loop folds between the S3 and S5 sheets.
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