What is a hormone biology

These hydrophilic (and mostly large) hormone molecules bind to receptors on the surface of "target" cells; that is, cells able to respond to the presence of the hormone. These receptors are transmembrane proteins . Binding of the hormone to its receptor initiates a sequence of intracellular signals that may

  • alter the behavior of the cell (such as by opening or closing membrane channels ) or
  • stimulate (or repress) gene expression in the nucleus by turning on (or off) the promoters and enhancers of the genes.
This is the sequence of events:
  • The hormone binds to a site on the extracellular portion of the receptor.
    • The receptors are transmembrane proteins that pass through the plasma membrane 7 times, with their N-terminal exposed at the exterior of the cell and their C-terminal projecting into the cytoplasm.
  • Binding of the hormone to the receptor
    • activates a G protein associated with the cytoplasmic C-terminal.
    • This initiates the production of a "second messenger" . The most common of these are
      • cyclic AMP , ( cAMP ) which is produced by adenylyl cyclase from ATP (shown here), and
      • inositol 1,4,5-trisphosphate ( IP 3 )
    • The second messenger, in turn, initiates a series of intracellular events (shown here as short arrows) such as
      • phosphorylation and activation of enzymes;
      • release of Ca 2+ into the cytosol from stores within the endoplasmic reticulum.
    • In the case of cAMP, these enzymatic changes activate the transcription factor CREB ( c AMP r esponse e lement b inding protein).
    • Once bound to its response element 5' TGACGTCA 3' in the promoters of genes that are able to respond to the hormone, activated CREB turns on gene transcription .
    • The cell begins to produce the appropriate gene products in response to the hormonal signal it had received at its surface.
Steroid Hormones Steroid hormones, being hydrophobic molecules, diffuse freely into all cells. However, their "target" cells contain cytoplasmic and/or nuclear proteins that serve as receptors of the hormone. The hormone binds to the receptor and the complex binds to hormone response elements — stretches of DNA within the promoters of genes responsive to the hormone. The hormone/receptor complex acts as a transcription factor turning target genes "on" (or "off").

What is a hormone biology

what is a hormone biology


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