C5a receptors

The C5a receptor was cloned independently by two groups several years ago, and was shown to be a G

Model C3a peptide

Model C3a peptide

Arg - Gin - His Ala -Arg - Ala -Ser- His-Leu-Gly-Leu-Ala-Aig-COOH

Gly - L^s- L^s-T^r- Ang - Ala - Ser • l¿s-Leu - Gly - Leu - Ala ■ Arg-COOH

C3a/C3a 57-77 - Receptor interactions

C3a/C3a 57-77 - Receptor interactions

Model C3a peptide - receptor interactions

Figure 1 (Upper) The helical region of the C-terminal end of C3a is shown with the hydrophobic patch outlined (dashed line) including residues Tyr59, Ile60 and Leu63. This patch is believed to interact with a hydrophobic sub-binding site of the C3a receptor. The superpotent synthetic analog of C3a has two trypto-phanyl residues substituted for the hydrophobic patch on the natural factor. The residues that are underlined are nonessential replacements that improve solubility of the analog. (Lower) The schematic model illustrates how the natural factor C3a and the synthetic analog C3a 57-77 interacts with the receptor. The hyopthesis is that the model Trp-containing superpotent peptide mimics C3a by interacting with the receptor in a similar manner. Interaction at the hydrophobic sub-binding site cooperatively enhances binding of the essential Leu-Gly-Leu-Ala-Arg effector site. If binding of Trp-Trp at the hydrophobic site is greater than the Tyr-lle-Leu patch, it could explain the superpotency of the analog peptide.

protein coupled seven-transmembrane receptor of 350 residues in length with high (34%) homology to the fMLP receptor. This has allowed a detailed molecular characterization of ligand-receptor interaction using monoclonal antibodies and mutants of both C5a and its receptor. Furthermore, based on weak binding of the C-terminal octapeptide of C5a to the C5a receptor, several peptide analogs with reasonable affinity and agonistic, partial agonistic or antagonistic properties were developed. These studies led to a two-binding model of ligand-receptor interaction, as first proposed by MS Springer (Figure 2). One involves the core of C5a (with participation of Lysl9 and Lys20) interacting with the extracellular N-terminus (in particular with several aspartic acid residues in this region), presumably resulting in a conformational change of C5a, which allows its C-

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