Factor B which is part of the alternate pathway of the complement system is cleaved by factor D into 2 fragments: Ba and Bb. Bb, a serine protease, then combines with complement factor 3b to generate the C3 or C5 convertase. Cleavage of Arg-|-Ser bond in complement component C3 alpha-chain to yield C3a and C3b, and Arg-|-Xaa bond in complement component C5 alpha-chain to yield C5a and C5b. Monomer (By similarity). Part of the C3-convertase enzyme complex comprised of Complement C3 beta chain (C3b) and Complement factor B Bb fragment (Bb) and CFP (By similarity). Interacts to C3b; this interaction is dependent on the presence of Mg2+ (By similarity). Interacts to CFP; this interaction contributes to the stabilization of the active C3-convertase enzyme complex (By similarity). Belongs to the peptidase S1 family.