|
 PARTICIPATION OF FACTOR VIIa IN THE EXTRINSIC FACTOR
Xase COMPLEX The components of the extrinsic factor Xase enzyme
complex are illustrated. Factor VIIa and its cofactor tissue factor (TF) assemble on a
negatively charged membrane surface in a calcium dependent manner to form an enzyme
complex which proteolytically converts factor X to factor Xa. The complex is referred to
as the extrinsic factor Xase complex, because the tissue factor component is presumably
derived from a source "extrinsic" to the plasma environment, which is accessible
following vascular injury.
Human
factor VII is a single chain plasma glycoprotein, and is a zymogen in its native form (1-4). Proteolytic activation of factor VII, yields the enzyme factor VIIa, which when bound to the integral membrane protein tissue factor, forms an enzyme complex that proteolytically converts
factor X to factor
Xa. This enzyme complex is best known as the extrinsic factor Xase (pronounced ten-ase) complex, since by virtue of its tissue factor component, it is composed of protein normally extrinsic to the plasma environment.
The conversion of factor VII to factor VIIa is catalyzed by a number of proteases including
thrombin, factor IXa,
factor Xa, factor
XIa, and factor XIIa. Rapid activation also takes place when factor VII is combined with tissue factor in the presence of calcium (5-9). This latter event is consistent with autocatalysis, and is likely initiated by a small amount of pre-existing factor VIIa. The activation reaction results in cleavage of the peptide bond between arginine 152 and isoleucine 153.
The resulting factor VIIa consists of an NH2-derived light chain (Mr=20,000), and a COOH-terminal derived heavy chain (Mr= 30,000), which remain associated through a single disulfide bond (cys 135 - cys 262). The light chain contains the membrane binding "Gla domain", while the heavy chain contains the catalytic domain.
Unlike other serine proteases, factor VIIa alone is not readily inhibited by the
antithrombin III/heparin complex. However, in the presence of tissue factor, antithrombin III/heparin exhibits significant inhibition of factor VIIa (6).
Human factor VIIa is prepared from affinity purified factor VII, and purified by ion exchange chromatography. The purified protein is supplied in 50% (vol/vol)
glycerol/H2O and should be stored at -20°C. The purity is determined by SDS-PAGE analysis, and activity is measured in a factor VII clotting assay.
Properties of
Factor VIIa
| Localization: |
Plasma |
| Mode of action: |
Enzyme component of the extrinsic factor X activating complex; also activates factor
IX, thus by-passing the contact activation system. |
| Molecular weight: |
50,000 (human) (2) |
| Extinction coefficient: |
|
| Specific Activity: |
approximately 16,000 units/mg** |
| Structure: |
2 subunits; NH2-terminal derived light chain
(Mr=20,000), COOH-terminal
derived heavy chain (Mr=30,000), NH2-terminal
gla-domain, two EGF domains |
| Percent carbohydrate: |
13%*** (10) |
| Post-translational modifications: |
one
β-hydroxyaspartate (11), ten gla residues (12) |
* inferred from the zymogen, factor VII
** determined from single stage clotting assay
*** based upon analysis of bovine factor VII
|
Catalog
Number |
Description |
|
HCVIIA-0031 |
Human Factor VIIa (Compliment
fluorogenic substrate(s): HTI Catalog # SN-17a
and SN-17c) |
PRICING
INQUIRE
ABOUT THIS PRODUCT
References
1. Broze, G.J. and Majerus, P.W., Methods Enzymol., 80, 228 (1981).
2. Bajaj, S.P., et al., J. Biol. Chem., 256, 253 (1981).
3. Williams, E.B., et al., J. Biol. Chem., 264, 7536 (1989).
4. Kisiel, W. and McMullen, B.A., Thrombosis Res., 22, 375 (1981).
5. Radcliffe, R. and Nemerson, Y., J. Biol. Chem., 250, 388 (1975).
6. Lawson, J.H.,et al., J. Biol. Chem., 268, 767 (1993).
7. Kisiel, W., et al., Biochemistry, 16, 4189 (1977).
8. Radcliffe, R., et al., Blood, 50, 611 (1977).
9. Seligsohn, U., et al., J. Clin. Invest., 64, 1056 (1979).
10. Kisiel, W., et al., Biochemistry, 14, 4928 (1975).
11. McMullen, B.A., et al., BBRC, 115, 8 (1983).
12. Discipio, R.G., et al., Biochemistry, 16, 698 (1977).
|
