Opioid receptors (OpR) are a Subfamily A4 G protein-coupled receptors with opioids as ligands[1]. OpR types are classified according to the ligands which bind to them.
- The μ-opioid receptor binds morphine. For more details on μ-opioid receptor see
The binding of an opioid induces a in the μ-opioid receptor that activates an inhibitory G-protein (Gαi/o). This results in the dissociation of the G-protein complex. The Gα subunit then inhibits adenylyl cyclase. The Gβγ subunit acts to inhibit Ca2+ channels and activate K+ channels. .
The κ-opioid receptor is a . The extracellular side is home to the proteins primary . These 2 units will span the length for the cell membrane to form the basis of the receptor molecule. , where helices I (in light blue) and helices VIII (in dark blue). This area will make up the basis for the intermembrane surface area. A distinguishing feature that separates the κ-opioid receptor from other receptors, is the large β-hairpin, , located near the main active site of the protein. It is believed that its function is to cap the active site of the receptor. Although in general, this protein is primarily composed of α-helices, not β-sheets (Compare to ). This evidence reinforces the idea that this protein is a transmembrane protein rather than one found inside the cytosol. In general transmembrane protein are composed almost entirely of α-helices (or β-sheet arranged in special fashion called a β-barrel), in order to have maximum stability inside the membrane. Interesting feature of the κ-opioid receptor is the formed by Cys131 and Cys210 which is conserved across all opioid receptors. of κ-opioid receptor. The human κ-opioid receptor ligand binding pocket displays a unique combination of key characteristics both shared with and distinct from those in the chemokine and aminergic receptor families.
- The δ-opioid receptor binds enkephalins. For details see Delta opioid receptor.
- The Nociceptin/orphanin FQ opioid receptor binds the heptadecapeptide orphanin[2].
You can check out the in the window on the right. It shows the mu opioid receptor bound to a peptide ligand and a G protein . The G protein ("G" because it binds to GTP) consists of three parts A , B , and C ).
