title: Structural evidence that complement factor B constitutes a novel class of serine protease.
abstract: The 28,000-dalton COOH-terminal cyanogen bromide peptide of complement factor B was isolated disulfide bonded to a second polypeptide of Mr = 3,500. The amino acid sequence of the smaller peptide, CB2-3, and 51 of 55 NH2-terminal residues of the larger peptide, CB2-2, were determined on an automated sequenator. CB2-2 exhibited extensive homology in its primary structure to the known serine proteases and included the sequence, Ala-Ala-His-Cys, which is part of the active site of these enzymes. By contrast, CB2-3 demonstrated only limited sequence identity with the NH2 terminus of the serine proteases. Mild acid hydrolysis was employed to further cleave CB2-2 into fragments of Mr = 20,000 and 8,000. On analysis the 8,000-dalton peptide was observed to contain the active site serine sequence, Gly-Asp-Ser-Gly-Gly-Pro. The data, therefore, clearly document that factor B is also a serine protease, although its mechanism of activation differs from this class of proteolytic enzymes.
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D011415:Complement Factor B; D010450:Endopeptidases; D004792:Enzyme Precursors; D006801:Humans; D008970:Molecular Weight; D010446:Peptide Fragments / Q000032:analysis; D012697:Serine Endopeptidases; D013045:Species Specificity
chemical_list: D004792:Enzyme Precursors; D010446:Peptide Fragments; D010450:Endopeptidases; D012697:Serine Endopeptidases; D011415:Complement Factor B