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Hormone Biochemistry

  • There are three fundamentally different biochemical classes of hormones. Each class is synthesized differently and each class has distinct modes of action.
Peptide Hormones
  • Biochemical Nature:
    • Peptide hormones are simply small proteins composed of a linear amino acid polymer.
  • Synthesis:
    • Peptide hormones are synthesized like any other protein using ribosomees and are subsequently secreted out of the cell. Prior to secretion the hormones are stored in secretory vesicles and their secretion into the blood stream is usually highly regulated.
  • Mode of Action
    • Because peptide hormones are proteins, they cannot cross the plasma membranes of cells. Therefore, they act by binding and activating specific membrane receptors on target cells which then initiate signal transduction cascades.
Steroid Hormones
  • Biochemical Nature:
    • Steroid hormones are small hydrophobic molecules made of several fused carbon rings.
  • Synthesis:
    • Steroid hormones are generally derived from chemical modification of cholesterol. Because they are small, hydrophobic molecules which can easily cross plasma membranes, they cannot be stored in the synthesizing organ; therefore, their synthetic process is the typical mode of regulation.
  • Mode of Action:
    • Steroid Hormones easily cross target cell plasma membranes and subsequently bind and activate specific Nuclear Receptor cytosolic proteins. Nuclear Receptors are ligand-dependent transcription factors which are normally inactive but upon binding specific steroid hormones initiate transcription of a wide variety of genes which carry out the biological effect of the hormone.
Amine Hormones
  • Biochemical Nature:
    • Amine hormones are small molecules which may or may not be hydrophobic.
  • Synthesis:
    • Amine hormones are derived from chemical alteration of the amino acid tyrosine. They are stored in multiple ways and their release from storage depots is highly regulated.
  • Mode of Action:
    • Hydrophobic amine hormones like Thyroid Hormone can cross the plasma membrane and bind and activate specific nuclear receptors similar to steroid hormones.
    • Hydrophilic amine hormones like epinephrine and norepinephrine cannot cross the plasma membrane and so thus bind and activate specific membrane receptors which carry out signal transduction cascades.