Partial sequence of human complement component factor B: novel type of serine protease

Factor B (a component of the alternative pathway of complement) is believed to contain the proteolytic site of the complex enzymes C3 convertase (C3bB) and C5 convertase (C3bnB). Conflicting results have been obtained in regard to the inactivation of these enzymes by diisopropyl phosphorofluoridate but it has been suggested that activated Factor B (Factor B) is a serine protease with the active site in Bb, a COOH-terminal fragment of approximately 60,000 molecular weight. Partial amino acid sequence studies of Bb derived from human Factor B have shown that the NH2-terminal 40 residues have no homology with NH2-terminal sequences of other serine proteases. However, positioning of a further 170 residues out of approximately 290 residues in two continuous CNBr fragments from the COOH terminus has shown that there is a strong homology of sequence in this section. The active site residues histidine, aspartic acid, and serine all are present in positions corresponding with those of typical serine proteases. It is suggested that Factor B is a novel type of serine protease with a catalytic chain of molecular weight twice that of proteases previously studied and probably with a different activation mechanism.