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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