Factor B (Bb) Protein

Centrally important to the alternative pathway is the zymogen Factor B. Factor B is a 93 kD protein which is converted to an active proteolytic enzyme via a two step reaction involving C3b and Factor D. Factor B binds to C3b (or hydrolyzed C3) either in fluid phase or on a membrane or other surface. In the second step, Factor D cleaves the bound Factor B to yield Ba (33 kD) and Bb (60KD) The resulting bimolecular complex is the alternative pathway convertase and is capable of cleaving additional C3 or, together with a second C3b molecule, C5.