Complement factor P (properdin) is purified from normal human serum. Factor P is a 53,000 dalton cationic protein that circulates in blood in the form of dimers, trimers and tetramers. It binds rapidly to C3b on surfaces where complement has begun to activate. Properdin binds most avidly to C3b,Bb the alternative pathway C3/C5 convertase, but also binds to C3b < C3b,B < C3b,Bb. Its primary function is to stabilize the C3b,Bb complex allowing increased alternative pathway activation (Pangburn, M.K., (1988); Nolan, K.F. and Reid, K.B.M. (1993)). Properdin enhances formation of the alternative pathway C3 convertase by increasing binding of factor B to P,C3b complexes. Thus, properdin is an accelerator (positive regulator) of complement activation. Properdin has recently been proposed to be able to also initiate activation of the alternative pathway by binding to the target surface and initiating C3/C5 convertase formation (Kemper C. and Hourcade D.E. (2008)).
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