Product Description

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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Complement Technology的C1是级联反应中的第一个补体成分，称为补体经典途径。C1与抗原结合的抗体（免疫复合物）结合并被其激活，从而产生引发级联反应的蛋白酶。C1实际上是钙依赖性复合物中结合在一起的三种不同蛋白质（C1q，C1r和C1s）的非共价复合物。C1q通过其六个臂中的两个或多个与IgG或IgM的Fc结构域结合。多臂与免疫复合物的结合被认为会引入压力，从而导致复合物中的两个C1r蛋白（蛋白酶酶原）自身激活，从而产生两种活性的C1r丝氨酸蛋白酶（Morikis，D.和Lambris，JD（2005））。 。这些激活的C1r亚基裂解并激活复合物中的两个C1s蛋白酶酶原。活化的C1裂解补体成分C4，释放出C4a，并启动C4b与活化表面的共价连接。活化的C1也切割C2，并且C2的较大片段结合至表面附着的C4b，形成C4b，C2a，其为经典途径的C3 / C5转化酶。