Binding of cAMP by an R subunit of PKA occurs in a cooperative fashion; i.e., binding of the first cAMP molecule to CNB-B lowers the Kd for binding of the second cAMP to CNB-A.

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Switching mechanism of monomeric G proteins 

“Off” state, Removal of the γ phosphate by GTPase-catalyzed hydrolysis causes switch I and switch II to relax into a different conformation

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Monomeric G proteins, including Ras and various Ras-like proteins, do not directly bind to receptors but act as intermediate proteins in signal transduction pathways.

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Monomeric G proteins are evolutionarily ancient, as evidenced by their widespread functions examples include: 

eIF2

EF1α and EF2

Ran

Sar

Rab

Rho, Rac, and Cdc42

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All G protein–coupled receptors contain seven transmembrane α-helical regions (H1–H7). In many GPCRs a short α helix H8 lies parallel to the membrane surface, and a covalently bound palmitic acid group

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Three principal classes of G PCR that bind their ligand in different ways: