Complement Factor B, Human, Native

Molecular Biology / MB-Complement

-70°C

Supplied as a liquid in 10mM sodium phosphate, 145mM sodium chloride, pH 7.2. No preservative added.

Complement factor B (fB) is purified from normal human serum. Complement factor B is a glycosylated protein composed of a single 93, 000 Da polypeptide chain. It is an essential component of the alternative pathway of complement activation and is found in plasma at ~200ug/mL. In the presence of Mg++ factor B binds to C3b and the C3b, B complex can be activated by factor D, a serine protease that circulates as an active trypsin-like serine protease. Cleavage of factor B by factor D causes the release of the Ba fragment (33, 000 D) and leaves the 60, 000 Bb fragment bound to C3b. This Bb subunit is a serine protease. C3b, Bb is called a C3 and a C5 convertase because it converts both of these proteins to their active forms by cleaving off the small peptides C3a and C5a, respectively. Another role for factor B is in the initiation of the alternative pathway. Continuous conversion of native C3 to a C3b-like form is a result of spontaneous hydrolysis of the thioester in C3. ThisC3(H2O) binds factor B in a Mg++ stabilized complex. Factor B in the C3(H2O), B complex can be activated by factor D releasing Ba. During alternative pathway initiation, fluid phase C3(H2O), Bb cleaves C3 producing metastable C3b which can attach to carbohydrates on cell surfaces and on plasma proteins. If this C3b attaches to a host cell or protein it is rapidly inactivated by a variety of mechanisms due to the actions of factor H, CR1, MCP, and factor I. C3b that attaches to a foreign target lacking these regulators remains active long enough to bind factor B and form C3b, Bb as described above. This is the cell surface-bound C3/C5 convertase of the alternative pathway of complement. C3b, Bb is an unstable trypsin-like serine protease with a half-life of ~90 seconds in the absence of factors that accelerate decay (factor H, DAF, and CR1). The proteolytic site is in the C-terminal domain of the Bb subunit. A unique feature of the alternative pathway is the ability of C3b, Bb to amplify itself on the surface of a complement-activating target particle. This enzyme cleaves C3 producing metastable C3b which can attach to the cell near the initial C3b. Each C3b deposited can bind factor B and form another C3/C5 convertase and deposit more C3b in an expanding ring of attached proteins. C3b, Bb with a second C3b nearby becomes a more efficient C5 convertase and it cleaves C5 releasing C5a and depositing C5b-9 complexes in the bilipid layer of the target cell. This amplification mechanism of the alternative pathway can deposit 2, 000, 000 C3b molecules on a yeast cell or 30, 000 C3b on a bacterium 10-15min after they come in contact with blood. These numbers represent a monolayer of covalently attachedopsonins (C3b, iC3b and C3d) which are ligands for phagocytic immune cells. The numbers of C3b and C5b-9 deposited far exceed those produced by the classical or lectin pathway due to the factor B-containing convertase and its ability to amplify itself and spread across the surface of a target.

Source:
Complement factor B, from normal human serum

Activity:
~90% versus normal human serum standard

Functional Activity:
~1200 Factor Bh50 U/mg
50% of Factor B in NHS on a mg/mg basis

Extinction Coefficient:
A280nm=1.27 at 1.0mg/ml

Country of Origin: USA

Storage and Stability:
Aliquot to avoid repeated freezing and thawing and store at -70°C. For maximum recovery of product, centrifuge the original vial after thawing and prior to removing the cap.