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Terms in this set (4)

A.The cAMP cycle
The binding of a transmitter to certain receptors activates the stimulatory G protein (Gs), consisting of αs-, β-, and γ-subunits. When activated, the αs-subunit exchanges its bound guanosine diphosphate (GDP) for guanosine triphosphate (GTP), causing αs to dissociate from the βγ complex. Next, αs associates with an intracellular domain of adenylyl cyclase, thereby stimulating the enzyme to produce cAMP from adenosine triphosphate (ATP). The hydrolysis of GTP to GDP and inorganic phosphate (Pi ) leads to dissociation of αs from the cyclase and its reassociation with the βγ complex. The cyclase then stops producing the second messenger. As transmitter dissociates from the receptor, the three subunits of the G protein reassociate, and the guanine nucleotide-binding site on the α-subunit is occupied by GDP

B.Activation of protein Kinase A
Four cAMP molecules bind to the two regulatory subunits of protein kinase A (PKA), liberating the two catalytic subunits, which are then free to phosphorylate specific substrate proteins on certain serine or threonine residues, thereby regulating protein function to produce a given cellular response. Two kinds of enzymes regulate this pathway. Phosphodiesterase's convert cAMP to adenosine monophosphate (which is inactive), and protein phosphatases remove phosphate groups (P) from the substrate proteins, releasing inorganic phosphate, Pi . Phosphatase activity is, in turn, decreased by the protein inhibitor-1 (not shown), when it is phosphorylated by PKA