title: Amino acid sequence of human D of the alternative complement pathway.
abstract: The primary structure of human D, the serine protease activating the C3 convertase of the alternative complement pathway, has been deduced by sequencing peptides derived from various chemical (CNBr and o-iodosobenzoic acid) and enzymatic (trypsin, lysine protease, Staphylococcus aureus V8 protease, and chymotrypsin) cleavages. Carboxypeptidase A was also used to confirm the COOH-terminal sequence. The peptides were purified by high-pressure liquid chromatography. The proposed sequence of human D contains 222 amino acids and has a calculated molecular weight of 23 748. It exhibits a high degree of homology with other serine proteases, especially around the NH2-terminus as well as the three residues corresponding to the active-site His-57, Asp-102, and Ser-195 (chymotrypsinogen numbering). This sequence homology is highest (40%) with plasmin, intermediate (35%) with pancreatic serine proteases, such as elastase, trypsin, chymotrypsin, and kallikrein, and least (30%) with the serum enzymes thrombin and factor X. D, however, exhibits only minimal amino acid homology with the other sequenced complement serine proteases, Clr (25%) and Bb (20%). The substitution of a basic lysine for a neutral amino acid three residues NH2-terminal to the active-site serine as well as a small serine residue for a bulky aromatic amino acid at position 215 (chymotrypsinogen numbering) in the binding pocket may be important in determining the exquisite substrate specificity of D. The presence of His-40 which interacts with Asp-194 (chymotrypsinogen numbering) to stabilize other serine protease zymogens [Freer, S. T., Kraut, J., Robertus, J. D., Wright, H. T., & Xuong, N. H. (1970) Biochemistry 9, 1997] argues in favor of such a D precursor molecule.
mesh_terms: D000595:Amino Acid Sequence; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D003166:Complement Activating Enzymes / Q000302:isolation & purification; D003167:Complement Activation; D011416:Complement Factor D / Q000302:isolation & purification; D003170:Complement Pathway, Alternative; D003488:Cyanogen Bromide; D010450:Endopeptidases; D006801:Humans; D010446:Peptide Fragments / Q000032:analysis
chemical_list: D010446:Peptide Fragments; D003166:Complement Activating Enzymes; D010450:Endopeptidases; C493488:CFD protein, human; D011416:Complement Factor D; D003488:Cyanogen Bromide