Mechanisms of Hormone Action
Objective 2
List the major chemical classes of hormones of the body. Relate the mechanism of hormone action to the biochemical nature of the hormone.
Water-Soluble vs Lipid-Soluble Hormones
Hormones can be divided into two major groups based on their chemical structure. The first group includes hormones derived from amino acids. This group includes amines, peptides, and proteins. These are water-soluble hormones. Examples of water-soluble hormones include epinephrine, norepinephrine, antidiuretic hormone, and follicle-stimulating hormone. This is not a total list but just examples. We will be learning more about these hormones as we proceed through this unit.
The second group are lipid-soluble hormones, which include steroids. Examples of these hormones include testosterone, estrogen, and cortisol. While thyroid hormones have large amounts of the amino acid tyrosine, they also have a benzene ring, which makes them lipid-soluble.
The mechanism of action the hormone depends on its water solubility. This is very important to keep in mind when determining how the hormone will act on a target cell. In Unit 4 and Unit 13 we saw how charged, polar molecules cannot diffuse directly through the cell membrane. On the other hand, small, non-polar molecules can diffuse directly through the cell membrane. These lipid-soluble hormones then bind directly to intracellular receptors.
Water-Soluble Hormones: Mechanism of Action
Water-soluble hormones cannot diffuse through the cell membrane but must stimulate a cellular change by a more complex mechanism. They must bind to membrane receptors and initiate a change in the cell indirectly via a G protein-coupled receptor (GPCR).
Here are the steps required for a water-soluble soluble hormone to initiate a cellular change:
- Hormone (first messenger) binds to a receptor on the cell membrane.
- By forming a receptor-hormone complex, a membrane protein called G protein is activated.
- The activation of the G protein will then activate the enzyme adenylate cyclase.
- Adenylate cyclase converts ATP into cyclic adenosine monophosphate (cAMP).
- cAMP acts as a second messenger by activating one or more protein kinases.
- Kinases phosphorylate (add a phosphate to) proteins which either activates or deactivates the protein (hormones can be excitatory or inhibitory). There are many different type of kinases, so the effects can be numerous.
- To limit the duration of the response, unless new hormone binds to the receptors, an enzyme called phosphodiesterase inactivates cAMP.
Lipid-Soluble Hormones: Mechanism of Action
The action of the lipid-soluble hormones is the easiest to understand. Again, because they are lipid-soluble, they can diffuse through the lipid bilayer of the cell membrane. This gives them the opportunity to bind directly to intracellular receptors. The mechanism by which a lipid-soluble hormone works is as follows:
- The hormone diffuses through the cell membrane.
- Hormone can bind