GDP, G-proteins

There is considerable evidence that opioid receptors are coupled to G-proteins and produce their effects through these proteins (see Refs. 96, 97 for reviews of opioid receptors and G-proteins). The structure of cloned opioid receptors is consistent with their belonging to this receptor superfamily (see below). G-pro-teins are heterotrimers, consisting of a, jS, and y subunits, which bind guanine nucleotides to their a-subunit and catalyze the hydrolysis of GTP to GDP. G-proteins mediate the interac-... 

G protein A protein that binds GTP and hydrolyzes it to GDP. G proteins are molecular switches that change their structural state depending on whether GTP or GDP is bound. There are two classes of G proteins, the small G proteins such as Ras and the heterotrimeric G proteins such as the G protein that couples adenyl cyclase to the epinephrine receptor, providing the link between the... 

G proteins are named based on their ability to bind guanine nucleotides. G proteins are membrane proteins that in the inactive state bind guanosine diphosphate (GDP). G proteins are required for activation of adenylate cyclase by / -adrenergic agonists via interactions with receptor systems that activate or inhibit adenylate cyclase. Of the several known G proteins the two best characterized are Gs, a family of G proteins involved in stimulation of adenylate cyclase, and Gi, a closely related family involved in responses that inhibit adenylate cyclase. Both types of G proteins interact with other receptors as well and with target proteins other than adenylate cyclase. 

Step 2). When a GEF protein (gnanine nucleotide exchange factor) binds to a G-protein, it increases the rate of GTP exchange for a bound GDP, and therefore activates the G-protein (see Fig. 9.13, step 3). GDI proteins (GDP dissociation inhibitor) bind to the GDP-G protein complex and inhibit dissociation of GDP, thereby keeping the G protein inactive. 

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