21.6: Types of Hormones

Hormones can be classified into three main types based on their chemical structures: steroids, peptides, and amines. Their actions are mediated by the specific receptors they bind to on target cells.

Steroid hormones are derived from cholesterol and are lipophilic in nature. This allows them to readily traverse the lipid-rich cell membrane to bind to their intracellular receptors in the cytoplasm or nucleus. Once bound, the cytoplasmic hormone-receptor complex translocates to the nucleus. Here, it binds to regulatory sequences on the DNA to alter gene expression.

Peptide hormones are made up of chains of amino acids and are hydrophilic. Hence, they are unable to diffuse across the cell membrane. Instead, they bind to extracellular receptors present on the surface of target cells. Such binding triggers a series of signaling reactions within the cell to ultimately carry out the specific functions of the hormone.

Amine hormones are derived from a single amino acid, either tyrosine or tryptophan. This class of hormones is unique because they share their mechanism of action with both steroid as well as peptide hormones. For example, although epinephrine and thyroxine are both derived from the amino acid tyrosine, they mediate their effects through diverse mechanisms. Epinephrine binds to G-protein coupled receptors present on the surface of the plasma membrane, which initiates a signaling cascade that activates second messengers in the cytoplasm to produce a cell-specific response.

In contrast, the thyroid hormone thyroxine (T4) is converted to its active form triiodothyronine (T3) and transported across the plasma membrane. Within the cell, thyroid hormone receptors are present in a complex with nuclear DNA. The thyroid hormone binds to this hormone-DNA complex to alter gene expression.

Hormones are signaling molecules that are carried via the bloodstream to various parts of the body. They act on target cells and organs to regulate numerous biological processes. Hormones are classified into three main types, based on their chemical structures: steroids, amines and peptides.

Steroid hormones are derived from cholesterol. Like cholesterol, they contain four fused carbon rings with different side chains attached. Steroid hormones include estradiol, released from ovaries, female reproductive organs, and testosterone, released from testes, male reproductive organs.

The second class, amine hormones, are synthesized from a single amino acid, either tyrosine or tryptophan. The hormone epinephrine, derived from the amino acid tyrosine, triggers the fight or flight response during a stressful situation, such as a predator attack. The hormone melatonin, derived from tryptophan, regulates the internal biological clock.

Melatonin levels control the sleep-wake cycle, with low levels stimulating wakefulness during the day and high levels causing drowsiness at night. Lastly, peptide hormones consist of a chain of amino acids. The pancreas produces the peptide hormone insulin.

After a meal, insulin is released by the increase in blood glucose which stimulates cells to absorb excess glucose. Steroid hormones are lipophilic, or lipid soluble, and easily diffuse across the plasma membrane. Amine and peptide hormones are hydrophilic, or lipid insoluble, and cannot cross the plasma membrane of cells, therefore, these hormones bind to surface receptors expressed on their target cells to elicit a response.