title: Structural and circular-dichroism studies on the interaction between human C1-esterase inhibitor and C1s.
abstract: The reaction between complement factor C1s and C1-esterase inhibitor has been investigated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, N-terminal amino acid analysis and c.d. studies. It is confirmed that a very stable stoichiometric 1:1 complex with a molecular weight of about 180000 is formed, involving the light chain of C1s. On the sodium dodecyl sulphate/polyacrylamide gels a small peptide with a molecular weight of about 5000 can be seen, which may be released from the C-terminal portion of the inhibitor moiety in a manner analogous to that occurring in other similar proteinase-inhibitor reactions. By N-terminal amino acid analysis, a newly formed threonine residue is found in the complex, suggesting that the inhibitor peptide chain is cleaved in the complex between C1s and C1-esterase inhibitor. The stabilizing bond may therefore be an ester bond. C.d. studies of the native C1-esterase inhibitor indicated the presence of about 38% alpha-helix, about 24% beta-structure and about 38% unordered structure. By gradual cleavage of the disulphide bridges under non-denaturating conditions, gradual changes in the c.d. spectra occurred, suggesting loss of ordered secondary structures. The c.d. spectra of the complex between C1s and C1-esterase inhibitor indicate that tryptophan residues are affected by the complex-formation.
mesh_terms: D000596:Amino Acids / Q000032:analysis; D000641:Ammonia / Q000494:pharmacology; D002942:Circular Dichroism; D003166:Complement Activating Enzymes / Q000037:antagonists & inhibitors*; D003174:Complement C1 Inactivator Proteins / Q000494:pharmacology*; D003173:Complement C1s; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D046911:Macromolecular Substances; D010446:Peptide Fragments / Q000302:isolation & purification; D011487:Protein Conformation
chemical_list: D000596:Amino Acids; D003174:Complement C1 Inactivator Proteins; D046911:Macromolecular Substances; D010446:Peptide Fragments; D000641:Ammonia; D003166:Complement Activating Enzymes; D003173:Complement C1s