Cooh

S ils 1

Exterior

S ils 1

Exterior

Cytoplasm

Site 2

Figure 2 Model for the binding of C5a to its receptor. As shown in (B), the interaction takes place at distinct sites. The first, designated site 1, is between the N-terminus, and possibly the second extracellular loop of the receptor, and the core of C5a. Specifically, primary interaction is thought to be between several of the aspartic acid residues in the N-terminus and Arg40 and possibly Arg37 and His15 of C5a. The second interaction site, designated site 2, is between the C-terminus of C5a and the interhelical region of the receptor. The primary interaction involves Arg74 and Lys68 of C5a. The initial site of productive contact is believed to take place at site 1, as depicted in (A). The contact at site 1 effectively raises the local concentration of C5a and thereby promotes the more difficult interaction at site 2. (Reproduced from Siciliano et al (1994).)

Site 2

Cytoplasm

Figure 2 Model for the binding of C5a to its receptor. As shown in (B), the interaction takes place at distinct sites. The first, designated site 1, is between the N-terminus, and possibly the second extracellular loop of the receptor, and the core of C5a. Specifically, primary interaction is thought to be between several of the aspartic acid residues in the N-terminus and Arg40 and possibly Arg37 and His15 of C5a. The second interaction site, designated site 2, is between the C-terminus of C5a and the interhelical region of the receptor. The primary interaction involves Arg74 and Lys68 of C5a. The initial site of productive contact is believed to take place at site 1, as depicted in (A). The contact at site 1 effectively raises the local concentration of C5a and thereby promotes the more difficult interaction at site 2. (Reproduced from Siciliano et al (1994).)

terminus to interact with a second, ill-defined, binding site in a transmembrane segment. Using synthetic C5a hexapeptide analogs and mutant receptors, an interaction of the peptide C-terminal arginine with Arg206 in the fifth transmembrane helix of the receptor has been proposed as a mechanism to activate this second binding site. It should be noted, however, that C5adcsarg, which has no C-terminal arginine, is a potent chemoattractant, but is rather ineffective in activating many other cell functions, indicating that different types of ligand-receptor interactions exist which lead to distinct signaling events.

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