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C5. Protein Kinase G (PKG)

Cascade of events:  A transmembrane receptor OR a cytoplasmic protein  WITH LIGAND-DEPENDENT ENZYMATIC ACTIVITY (guanylate cyclase) binds an extracellular chemical signal (for the transmembrane receptor) or an intracellular ligand (for the cytoplasmic protein receptor), causing a conformational change in the receptor.  The bound receptor is now a guanylate cyclase which converts GTP into cyclic GMP. This second messenger, like cAMP which binds to and activates PKA,  binds to and activates thecGMP-dependent protein kinase (PKG).  An example of a hormone which works through a transmembrane receptor is atrial naturetic factor, which binds to a cell-surface receptor whose shape change is communicated to the intracellular receptor domain through the single transmembrane helix, leading to the activation of the intracelluar guanylate cyclase domain of the protein.   Likewise, nitric oxide, NO, a gaseous ligand formed from Arg, can diffuse through the cell membrane, where it binds to the heme of a cytoplasmic receptor/guanylate cyclase which form cGMP, leading to activation of PKG. (Steroid hormones also enter the cell where they interact with a cytoplasmic receptor which often acts as steroid hormone-dependent transcription factor. )

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