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

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Abstract
Factor B (a component of the alternative pathway of complement) is believed to contain the proteolytic site of the complex enzymes C3 [complement component 3] convertase (C3bB) and C5 convertase (C3bn.hivin.B). Conflicting results were obtained in regard to the inactivation of these enzymes by diisopropyl phophorofluoridate but it has been suggested that activated Factor B (Factor .hivin.B) is a serine protease with the active site in Bb, a COOH-terminal fragment with a MW of .apprx. 60,000. 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. Positioning of a further 170 residues out of approximately 290 residues in 2 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. Factor B may be a novel type of serine protease with a catalytic chain of MW twice that of proteases previously studied and probably with a different activation mechanism.