role of sodium and potassium channels is to regulate membrane potential. The major role of G-protein-coupled receptors is to transmit signals into the cell. They are characterized by having seven transmembrane segments, and can respond to a wide variety of agonists, including photons,
amines, hormones, neurotransmitters, and proteins. Some agonists bind to the extracellular part of the receptor and others to the transmembrane region. The G-protein-coupled receptor is actually the GTP-binding protein because in the active state it binds to GTP (guanosine triphosphate). There are two types of G-proteins: the heteromeric G protein, consisting of three subunits, and the monomeric small G protein. Adenyl cyclase catalyzes the conversion of ATP to cGMP, which is an important second messenger in cells.
Signaling enzymes which are cell-surface receptors include serine/threonine kinases, guanylyl cyclase, and tyrosine kinase (Overview, 2005). Tyrosine kinase catalyzes the dephosphorylation of the tyrosine residue. Guanylyl cyclase catalyzes the synthesis of cGMP. Signaling enzymes anchored on the cytosolic face of the plasma membrane include the non-receptor tyrosine kinases and Ras. Intracellular signaling is carried out by cAMP, NF-kB, STAT and MAPK. The targets of cAMP include cAMP-dependent protein kinase (PKA), cyclic nucleotide- gated ion channels, and guanine exchanging factors Epac1 and Epac2 that regulate Rap1. The cAMP pathway may also ôcross talkö with other signalin
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