title: Hydrophobic effect and hydrogen bonds account for the improved activity of a complement inhibitor, compstatin.
abstract: Tryptophans at positions 4 and 7 of compstatin, a peptide complement inhibitor, are crucial for its interaction with C3. However, the nature of their involvement has not been studied to date. Here we investigate the molecular forces involved in the C3-compstatin interactions, mediated by aromatic residues, by incorporating in these two positions various tryptophan analogues (5-methyltryptophan, 5-fluorotryptophan, 1-methyltryptophan, and 2-naphthylalanine) and assessing the resulting peptides for activity by enzyme-linked immunosorbent assay (ELISA) and binding by isothermal titration calorimetry (ITC). Of all the compstatin analogues, peptides containing 1-methyltryptophan at position 4 exhibited the highest binding affinity (Kd = 15 nM) and activity (IC50 = 0.205 microM), followed by a peptide containing 5-fluorotryptophan at position 7. Our observations suggest that hydrophobic interactions involving residues at position 4 and the hydrogen bond initiated by the indole nitrogen are primarily responsible and crucial for the increase in activity. These findings have important implications for the design of clinically useful complement inhibitors.
authors.affiliation: Protein Chemistry Laboratory, Department of Pathology & Laboratory Medicine, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
authors.forename: Madan
authors.initials: M
authors.lastname: Katragadda
authors.forename: Paola
authors.initials: P
authors.lastname: Magotti
authors.forename: Georgia
authors.initials: G
authors.lastname: Sfyroera
authors.forename: John D
authors.initials: JD
authors.lastname: Lambris
mesh_terms: D000409:Alanine / Q000031:analogs & derivatives / Q000138:chemical synthesis / Q000737:chemistry; D002151:Calorimetry; D003176:Complement C3 / Q000037:antagonists & inhibitors* / Q000737:chemistry*; D004797:Enzyme-Linked Immunosorbent Assay; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D008958:Models, Molecular; D009281:Naphthalenes / Q000138:chemical synthesis / Q000737:chemistry; D010456:Peptides, Cyclic / Q000138:chemical synthesis / Q000737:chemistry*; D011485:Protein Binding; D055672:Static Electricity; D013329:Structure-Activity Relationship; D014364:Tryptophan / Q000031:analogs & derivatives / Q000138:chemical synthesis / Q000737:chemistry
chemical_list: D003176:Complement C3; D009281:Naphthalenes; D010456:Peptides, Cyclic; C111828:compstatin; C004182:5-methyltryptophan; C026927:5-fluorotryptophan; D014364:Tryptophan; D000409:Alanine; C513002:2-naphthylalanine