signal transduction2

Example for cell surface receptor hormone signal transduction
Hypoglycemia lead to release of glucagon hormone ,after that glucagon bind to target cell surface receptors leading to activation of cAMP mediated cascade which activate Glycogen Phosphorylase which in turn liberate glucose from glycogen. The termination of the effect of the hormonal action by phosphorylation is effected by the action of protein Phosphatases. Glycogen phosphorylase becomes inactive in the dephosphorylated state; when cyclic AMP level falls, the regulatory subunit is dephosphorylated and protein phosphatase becomes active, which in turn hydrolyzes phosphate group from the enzyme. Protein kinases as well as protein phosphatases are involved in the action of different hormones.


hormonal signal transduction through cell surface receptors

Hormones with Intracellular Receptors
The hormones in this group include the steroid hormones and thyroid hormones. They diffuse through the plasma membrane and bind to the receptors in the cytoplasm .The hormone receptor (HR) complex is formed in the cytoplasm. The complex is then translocate to the nucleus. In the nucleus, the HR binds to the hormone response elements (HRE) or steroid response elements (SRE).The SRE acts as an enhancer element and when stimulated by the hormone, would increase the transcriptional activity. The newly formed mRNA is translated to specific protein, which brings about the metabolic effects. Steroid hormones influence gene expression, so that the rate of transcription is increased. The stability of mRNA is also increased. This would lead to induction of protein synthesis. Steroid receptors have been found to enhance initiation of transcription by formation of complexes at promoters.


hormonal signal transduction through intracellular receptors