Fig. 2.

Schematic of the human complement anaphylatoxin receptor structure. A schematic structure of the human C3a and C5 receptors is shown. Both receptors are G protein-coupled, 7-membrane-spanning family members. Each transmembrane domain is numbered. The C3aR is unique among GPCRs due to the unusually large extracellular loop between transmembrane domains 4 and 5. The as yet unidentified C4a receptor is shown as partially transparent in the center panel. Shown below the C3aR and C5aR are the relative binding affinities for their known ligands. * The affinity of C5a for both C5aR is in the low nanomolar range. ** The affinity for C5a desArg for C5aR1 is in the low nanomolar range, while the affinity for C5aR2 is in the micromolar range.