Membrane Receptors

All the receptors except the nuclear receptors are membrane receptors and comprise G-protein coupled receptors (GPCRs), ligand gated-ion channel receptors (LGCRs), voltage-gated-ion channel receptors (VGCRs), tyrosine kinase receptors, tyrosine phosphatase receptors, and hematopoietic cytokine receptors.

1. G-Protein Coupled Receptor Superfamily

G-protein coupled receptors (GPCRs) are the largest receptor superfamily with several drugs developed against GPCRs. Members of GPCRs exhibit a common structural motif consisting of seven stretches of hydrophobic amino acid residues that span the membrane and different stretches of amino acids that form extracellular and intracellular loops (Fig. 3). GPCRs are receptors with seven transmembrane spanning regions and transduce the binding of extracellular ligands into intracellular signaling events through GTP-regulatory proteins (G-proteins) [7]. The high-resolution crystal structure of bacteriorhodpsin suggests that the transmembrane (TM) core though consists of polar residues; only a limited number

Figure 3 Schematic representation of the general structure of GPCR with trimeric G-protein. GPCRs have seven transmembrane (TM) spanning helices with extracellular N-terminal segment, which is variable in length. The TMs are connected by three extracellular loops (exoloop) and three intracellular loops (cytoloop) and the C-terminal intracellular segment. The ligand receptor interactions are different for different classes of GPCRs. In the inactive state the G-protein is a trimeric complex with GDP bound to the a-subunit.

Figure 3 Schematic representation of the general structure of GPCR with trimeric G-protein. GPCRs have seven transmembrane (TM) spanning helices with extracellular N-terminal segment, which is variable in length. The TMs are connected by three extracellular loops (exoloop) and three intracellular loops (cytoloop) and the C-terminal intracellular segment. The ligand receptor interactions are different for different classes of GPCRs. In the inactive state the G-protein is a trimeric complex with GDP bound to the a-subunit.

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