Peptide Hormones
Based on their chemical structure, hormones can be classified into two
groups: steroid hormones and peptide hormones. In both cases, the hormone must bind to a protein
receptor on the target cell to affect that cell. The
types of receptors they affect are different, however. Peptide hormones are
large, hydrophilic, charged, and cannot diffuse across the plasma membrane. As a result, the
receptors they bind to are located on the cell surface. When a peptide hormone binds to its
receptor on the surface of a target cell, it activates the receptor and causes it to transmit a
signal into the cellular interior. The nature of this signal can be to turn on a protein kinase
that phosphorylates certain proteins and changes their activity, or to release second messengers
in the cell, such as calcium or cyclic AMP, that amplify the signal and alter many different
cellular activities. This form of indirect signaling by a hormone is called a signal transduction
cascade because of the amplification by downstream signaling factors.
Peptide hormones can be small peptides such as
ADH, with just a few amino acid residues, or large complex polypeptides like insulin. Peptide
hormones are often produced as large inactive precursors, or pro-hormones, that are cleaved by
proteases into smaller active peptides before they are released. Since peptide hormones are
secreted proteins, they are synthesized on the rough ER in the cell, then packaged and processed
in the Golgi before they are delivered to the plasma membrane for secretion. Since hormone
signaling is usually regulated, the release of hormones is usually regulated. The hormones are
stored in secretory vesicles in the cytoplasm, waiting for the signal that fuses the vesicle with
the plasma membrane, dumping the hormones into the extracellular fluid and blood and transporting
them by the circulatory system to distant target tissues in the body.
This animation (Audio - Important) describes
peptide hormones.
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