Steroid hormones vs Peptide hormones

Compare and contrast the mechanisms of action for a steroid hormone and a peptide hormone (basics)

  • Steroid hormones are lipid soluble hormones, which allows them to cross the lipid membrane and penetrate the cell.
  • After penetrating the plasma membrane, steroids bind to specific receptor proteins in the nucleus or cytoplasm to form steroid-receptor complexes.
  • Binding of the steroid activates the receptor such that the steroid-receptor complexes accumulate in the nucleus where they bind to specific regulatory DNA sequences called Hormone response elements adjacent to specific hormone sensitive genes.
  • Binding of the steroid-receptor complexes enhances or supressed the rate of transcription of the adjacent gene, leading to different levels of the corresponding mRNA.
  • Translation of the mRNA leads to increased or decreased levels of the steroid-regulated gene product, producing the specific target cells response to the hormone.
  • The receptor domain for a steroid contains 3 regions: a hormone-binding region, DNA binding domain and a transcription activation domain.
  • Steroid hormones have a slow action as they affect the transcription of genes.

 

 

  • Peptide hormones are not lipid soluble and therefore cannot penetrate the cell membrane.
  • This means that they must bind to receptors on the surface of the cell membrane.
  • A lot of these hormones interact with a receptor that is associated with a G-protein, or tyrosine kinase associated receptors.
  • GPCR mechanism: Example of glucagon- Glucagon is a peptide hormone.
  • When glucagon binds to its receptor of a cell surface, the stimulatory G protein, Gs, is activated.
  • Gs then activates another membrane protein, Adenylate Cyclase, which produces cAMP from ATP.
  • cAMP or cyclic adenosine monophosphate is an intracellular secondary messenger that acts to notify the cell that the first messenger has bound.
  • cAMP allosterically activates Protein Kinase A which phosphorylates proteins in the cell.
  • This phosphorylation modulates their activity by either activating or inactivating them.
  • This mechanism is a quick response.
  • Glucagon causes the activation of phosphorylase kinase and inactivation of glycogen synthase by phosphorylation in some cell types.
  • Other GPCR’s include:
  • Guanlylate Cyclase which leads to the activation of cGMP and then PKG.
  • Phospholipase C which hydrolyses PIP2 to IP3 and DAG. DAG activates PKC and IP3 causes the release of ca2+ from the ER, activating Ca2+/calmodulin-dependent protein kinase.
  • Another protein.peptide hormone that has an opposing action to glucagon is insulin- which works via a different mechanism.
  • Insulin binds to a receptor that is associated with a tyrosine specific kinase. These receptors have an extracellular and intracellular region.
  • When insulin binds to the receptor it causes autophosphorylation of the tyrosine residues inside the cell (intracellular region). This leads to the activation of a phosphoprotein phosphatase which removes the phosphate groups from proteins in the cell, modulating their activity.
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