Factor X is a vitamin K dependent, two-chain glycoprotein

zymogen (Mr = 59,000) synthesized in the liver that circulates in

plasma at a concentration of approximately 10 μg/mL. The NH2-

terminal light chain (Mr = 17,000) possesses the gammacarboxyglutamic

(gla) residues that enable factor X/Xa to bind to

phospholipids/membranes in a calcium dependent manner. The

COOH-terminal heavy chain (Mr = 42,000) possesses the latent

catalytic domain.

Activation to factor Xa occurs by interaction with the intrinsic factor

Xase complex (Factor VIIa/IXa/Ca+2/phospholipid) or the extrinsic

factor Xase complex (Factor VIIa/tissue factor/

Ca+2/phospholipid). Both complexes cleave the molecule at

Arg52-lle53, releasing an activation peptide from the heavy chain,

resulting in factor Xa as a two-chain molecule where the light

chain remains with a Mr of 17,000 and the heavy chain has been

reduced to a Mr of 29,000.

Factor Xa provides the enzymatic activity of the Prothrombinase

complex (Factor Xa/Factor Va/ Ca+2/phospholipid) which converts

prothrombin to thrombin. While factor Xa can convert prothrombin

to thrombin alone, its activity is greatly enhanced when a part of

the complex. Its activity may be inhibited by inactivation of the

factor Va cofactor or directly by a natural inhibitor such as

antithrombin III.