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 approximately 200 µg/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 Da) 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 (Morikis, D. and Lambris, J.D. (2005); Morley, B.J. and Walport, M.J. (2000)).
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. This C3(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 approximately 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 (Morley, B.J. and Walport, M.J. (2000)).
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