It acts by cleaving prothrombin in two places (an Arg-Thr and then an Arg-Ile bond), which yields the active thrombin.

Defects in protein Z lead to increased factor Xa activity and a propensity for thrombosis.

To date, 191 crystal structures of factor Xa with various inhibitors have been deposited in the protein data bank.

The S4 sub-pocket has three ligand binding domains: the "hydrophobic box", the "cationic hole" and the water site.

Factor Xa inhibitors generally bind in an L-shaped conformation, where one group of the ligand occupies the anionic S1 pocket lined by residues Asp189, Ser195, and Tyr228, and another group of the ligand occupies the aromatic S4 pocket lined by residues Tyr99, Phe174, and Trp215.

Inborn deficiency of factor X is very rare (1:1,000,000), and may present with epistaxis (nosebleeds), hemarthrosis (bleeding into joints) and gastrointestinal blood loss.

Apart from congenital deficiency, low factor X levels may occur occasionally in a number of disease states.

Deficiency of vitamin K or antagonism by warfarin (or similar medication) leads to the production of an inactive factor X.

Factor X is an enzyme, a serine endopeptidase, which plays a key role at several stages of the coagulation system.

Factor Xa is the prime component of the prothrombinase complex which converts large amounts of prothrombin—the "thrombin burst".

Factor Xa also plays a role in other biological processes that are not directly related to coagulation, like wound healing, tissue remodelling, inflammation, angiogenesis and atherosclerosis.

Recently a new series of specific, direct acting inhibitors of Factor Xa has been developed.

At that time, those investigators could not know that the human genetic defect they had identified would be found in the previously characterized enzyme called thrombokinase.

Thrombokinase was the name coined by Paul Morawitz in 1904 to describe the substance that converted prothrombin to thrombin and caused blood to clot.

[20] In 1947, J Haskell Milstone isolated a proenzyme from bovine plasma which, when activated, converted prothrombin to thrombin.

Its activity was greatly enhanced by addition of calcium, other serum factors, and tissue extracts,[22] which represented the thromboplastins that promoted the conversion of prothrombin to thrombin by their interaction with thrombokinase.

In 1964 Milstone summarized his work and that of others: “There are many chemical reactions which are so slow that they would not be of physiological use if they were not accelerated by enzymes.