Biologic activity of synthetic analogues of C5a anaphylatoxin.

C5a is an inflammatory mediator that evokes a variety of immune effector functions including chemotaxis, cell activation, spasmogenesis, and immune modulation. It is well established that the effector site in C5a is located in the C‐terminal region, although other regions in C5a also contribute to receptor interaction. We have examined the N‐terminal region (NTR) of human C5a by replacing selected residues in the NTR with glycine via site‐directed mutagenesis. Mutants of rC5a were expressed as fusion proteins, and rC5a was isolated after factor Xa cleavage. The potency of the mutants was evaluated by measuring both neutrophil chemotaxis and degranulation (β‐glucuronidase release). Mutants that contained the single residue substitutions Ile‐6 → Gly or Tyr‐13 → Gly were reduced in potency to 4–30% compared with wild‐type rC5a. Other single‐site glycine substitutions at positions Leu‐2, Ala‐10, Lys‐4, Lys‐5, Glu‐7, Glu‐8, and Lys‐14 showed little effect on C5a potency. The double mutant, Ile‐6 → Gly/Tyr‐13 → Gly, was reduced in potency to <0.2%, which correlated with a correspondingly low binding affinity for neutrophil C5a receptors. Circular dichroism studies revealed a 40% reduction in α‐helical content for the double mutant, suggesting that the NTR contributes stabilizing interactions that maintain local secondary or tertiary structure of C5a important for receptor interaction. We conclude that the N‐terminal region in C5a is involved in receptor binding either through direct interaction with the receptor or by stabilizing a binding site elsewhere in the intact C5a molecule.

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Solution structure of Compstatin, a potent complement inhibitor

Morikis

et al. 1998

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The third component of complement, C3, plays a central role in activation of the classical, alternative, and lectin pathways of complement activation. Recently, we have identified a 13-residue cyclic peptide (named Compstatin) that specifically binds to C3 and inhibits complement activation. To investigate the topology and the contribution of each critical residue to the binding of Compstatin to C3, we have now determined the solution structure using 2D NMR techniques; we have also synthesized substitution analogues and used these to study the structure-function relationships involved. Finally, we have generated an ensemble of a family of solution structures of the peptide with a hybrid distance geometry-restrained simulated-annealing methodology, using distance, dihedral angle, and 3JNH.H,-~o~pling constant restraints. The Compstatin structure contained a type I p-turn comprising the segment GlnS-Asp6-Trp7-Glys. Preference for packing of the hydrophobic side chains of Val3, Val4, and Trp' was observed. The generated structure was also analyzed for consistency using NMR parameters such as NOE connectivity patterns, 3JNH.H,-~o~pling constants, and chemical shifts. Analysis of Ala substitution analogues suggested that Val3, G l d , Asp6, Trp7, and Glys contribute significantly to the inhibitory activity of the peptide. Substitution of Gly' caused a 100-fold decrease in inhibitory potency. In contrast, substitution of Val4, His', His", and Arg" resulted in minimal change in the activity. These findings indicate that specific side-chain interactions and the &turn are critical for preservation of the conformational stability of Compstatin and they might be significant for maintaining the functional activity of Compstatin.The complement system is the first line of immunological defense against foreign pathogens (Muller-Eberhard, 1992). Its activation through the classical, alternative, or lectin pathways leads to the generation of anaphylatoxic peptides C3a and C5a and formation of the C5b-9 membrane attack complex. Complement component C3 plays a central role in activation of all three pathways. Activation of C3 by complement pathway C3 convertases and its subsequent attachment to target surface leads to assembly of the membrane attack complex and ultimately to damage or lysis of the target cells (Frank & Fries, 1991). C3 is unique in that it possesses Reprint requests to:

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Biologic activity of synthetic analogues of C5a anaphylatoxin.

Cited by 66 publications

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Stabilization of an isolated helical capping box in solution by hydrophobic interactions: Evidence from the NMR study of bioactive peptides from the C-terminus of human C5a anaphylatoxin

Stabilization of an isolated helical capping box in solution by hydrophobic interactions: Evidence from the NMR study of bioactive peptides from the C-terminus of human C5a anaphylatoxin

Site‐specific mutations in the N‐terminal region of human C5a that affect interactions of C5a with the neutrophil C5a receptor

Solution structure of Compstatin, a potent complement inhibitor

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