<prompt> True statement about the G-protein coupled receptor is:It binds to ligands at the cell surfaceAssociation of all the three subunits alpha, beta and gamma is essential for its actionG-protein acts as inhibitory and excitatory as alpha subunit exists in two formsGTP binds to G protein in resting state  <response>: A i.e. It binds to ligands at the cell surfac G protein coupled linked receptor GPCRs These cell surface receptors bind to the ligand at the cell surfaceQ. And have 7 trans membrane segments that loop in and out of the cell membraneQ. So the receptor is also ka seven transmembrane domain 7TM receptors, heptahelical or serpentine receptor. Pa of receptor that protudes into cytoplasm cytoplasmic tail of receptor is coupled to G protein that has 3 different subunits hetrotrimeric - a, y. This G protein has an ability to bind guanosine nucleotides. - In resting inactive state the a, 13, y subunits form a complex that binds GDP on a subunitQ. When receptor gets activated after ligand binding it causes this GDP bound trimeric G protein complex to associate with cytoplasmic tail of receptor and to exchange GDP by GTP. This exchange causes a subunit to dissociate from combined fl r subunits. This separated a subunit brings about biological effectQ. The 0 y subunits do not separate from each other and activates variety of effectors. So dissociation of subunits 1t action. When ligand is removed from receptor, the intrinsic GTP ase activity of a subunit inactivates itself by conveing its bound GTP into GDPQ. In the absence of hormone, the heterotrimeric a,13,y G protein complex is in an inactive GDP bound form anchored to plasma membrane through prenylated groups on 13 y subunits and by myristoylated groups on a-subunit , but not associated with the receoptor. Binding of hormone H to receptor causes conformational change of receptor and activation of G protein complex dt exchange of GDP with GTP on a subunit. Tadenyl cyclase, CaNa C1- channels for a-s - TIC channels for al and ao - T Phospholipase CIt for aq and phospholipase CI32 for an - T C GMP phosphodiesterases for at and CI channel for cti2 Depending on the coupling of ligand hormone to inhibitory G proteinGi or stimulatory G protein Gs receptors, these can either decrease or increase the activity of intracellular enzyme. so it is not dependent on a subunit. Mechanism of action: Ligand photon, protein, amine hormone, neurotransmitter binding -- conformation change activation of G protein -- exchange of GDP for GTP and dissociation between a- and 13 y subunits which interact with effectors. a - subunit act on Na, Ca, K- channels and phospholipse - C PLC. p y subunit acts on adenyl cyclase, phospholipase A2, DLC, Ca - ATPase etc. Adenyl cyclase - cAMP pathway activation i.e. adenyl cyclase activation lit T cAMP causes stimulatin of cAMP dependent pein kinase protein kinase A which phosphorylates the receptorQ. G protein coupled receptor kinase are protein kinase that phosphorylates only active GPCRs Phosphorylation of receptor 1t translocation arrest linking. GPCRs are found only in eukaryotes, including yeast, plants choanoflagellates. These are grouped in 6 classes: A rhodopsin like; B secretin receptor family; C pheromone metabotropic glutamate; D Fungal mating pheromone receptor; E cyclic AMP receptor; and F frizzled or smoothened. 