ABSTRACT
Factors B and D are serine proteases participating in the activation of the alternative pathway. Factor D is the only single-domain serine protease of the complement system and the only activator of factor B in blood. Factor B is a modular protein consisting of three domains including a carboxyl terminal serine protease domain, which provides the catalytic subunit of the alternative pathway C3/C5 convertase. Factor D is a 24.4-kDa, single polypeptide chain serine protease endowed with unique structural and functional properties. Factor B, the second serine protease of the alternative pathway, is a 93-kDa, single polypeptide chain protein with a total of 739 amino acid residues. Both enzymes, factors D and B, participating in the activation of the alternative pathway appear to belong to the subgroup of inducible serine proteases. Both express very low reactivity against synthetic substrates but high catalytic efficiency against their natural substrates, C3b-bound factor B and C3/C5, respectively.
