All known effects of cyclic AMP in eukaryotic cells results from

More than a thousand protein kinases are now known. Some impoant hormone responsive protein kinases are cAMPdependent kinases, epidermal growth factor- dependent tyrosine kinase, insulin-dependent tyrosine kinase. All the known effects of cAMP in eukaryotic cells result from activation of protein kinases, which are serine/threonine kinases.Ceain enzymes are activated by dephosphorylation Hepatic Protein Phosphatase-1 is a typical example where the enzyme itself is inhibited by phosphorylationof its regulatory subunit. When cyclic AMP level falls, the regulatory subunit is dephosphorylated and protein phosphatase becomes active, which in turn hydrolyzesphosphate group from the enzyme. Protein kinases, as well as protein phosphatases, are involved in the action of different hormones. The actions of cAMP in eukaryotic cells are multifaceted and these include:A) Activation of Protein kinase and phosphorylation of effector proteins like enzymes and ion channels. These enzymes may directly phosphorylate enzymes or secondary kinases that phosphorylate other enzymes. For example,PKA phosphorylates hormone-sensitive lipase thus activating it.Phosphorylase Kinase that phosphorylates glycogen phosphorylase.When ion channel proteins or transpoers are phosphorylated, the membrane potential is modified, thus regulating the influx of calcium.B) cAMP also has a long-lasting effect on gene expression. The translocation of the active PKA subunits to the nucleus induces phosphorylation of cAMP-regulated gene regulatory proteins or CREBs. These proteins will bind to cAMP-sensitive regulatory elements (CRE) on genes, thus controlling their expression.Ref: DM Vasudevan, Page no: 523