Hormones can be divided into three classes based on their chemical structure: lipid-derived, amino acid-derived, and peptide hormones (which includes peptides and proteins). One of the keys, distinguishing features of lipid-derived hormones is that they can diffuse across plasma membranes, whereas the amino acid-derived and peptide hormones cannot.
There are three major types of hormones that are lipid hormones, amino acid-derived hormones, and peptide hormones
1. Most lipid hormones are derived from cholesterol, so they are structurally similar to it. The primary class of lipid hormones in humans is the steroid hormones. Examples of steroid hormones include estradiol, which is an estrogen, or female sex hormone, and testosterone, which is an androgen, or male sex hormone. These two hormones are released by the female and male reproductive organs, respectively. Other steroid hormones include aldosterone and cortisol, which are released by the adrenal glands along with some other types of androgens. Steroid hormones are insoluble in water; transport proteins carry them in the blood. As a result, they remain in circulation longer than peptide hormones.
2. The amino acid-derived hormones are relatively small molecules derived from the amino acids tyrosine and tryptophan. Examples of amino acid-derived hormones include epinephrine and norepinephrine, which are synthesized in the medulla of the adrenal glands, and thyroxine, which is produced by the thyroid gland. The pineal gland in the brain makes and secretes melatonin, which regulates sleep cycles.
3. The structure of peptide hormones is that of a polypeptide chain (chain of amino acids). The peptide hormones family includes molecules that are short polypeptide chains, such as antidiuretic hormone (ADH) and oxytocin produced in the brain and released into the blood in the posterior pituitary gland. This class also includes small proteins, such as growth hormones produced by the pituitary, and large glycoproteins, such as follicle-stimulating hormone produced by the pituitary.
Hormones can also be classified by their function. Glucocorticoids, for example, are a set of steroid hormones that are synthesized in the adrenal cortex and regulate glucose metabolism. Cortisol is the most important glucocorticoid hormone.
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Key Points
• Most lipid hormones are steroid hormones, which are usually ketones or alcohols and are insoluble in water.
• Steroid hormones include estradiol, testosterone, aldosterone, and cortisol. A specific type of steroid hormones are called glucocorticoids, the most important of which is cortisol.
• Amino acid-derived hormones are derived from tyrosine and tryptophan. These include epinephrine and norepinephrine (produced by the adrenal medulla).
• Amino acid-derived hormones also include thyroxine (produced by the thyroid gland) and melatonin (produced by the pineal gland).
• Peptide hormones consist of a polypeptide chain; they include molecules such as oxytocin (short polypeptide chain) or growth hormones ( proteins ).
• Amino acid-derived hormones and protein hormones are water-soluble and insoluble in lipids
Key Terms
oxytocin: a hormone that stimulates contractions during labor, and then the production of milk
epinephrine: (adrenaline) an amino acid-derived hormone secreted by the adrenal gland in response to stress
estrogen: any of a group of steroids (lipid-hormones) that are secreted by the ovaries and function as female sex hormones
cholesterol: a waxy, fat-like substance that’s found in all the cells in your body
testosterone: the primary male sex hormone
thyroxine: a hormone the thyroid gland secretes into the bloodstream to control metabolism
antidiuretic hormone: a hormone secreted by the hypothalamus to control water levels in the body
glucocorticoids: a set of steroid hormones involved in glucose metabolism, the most important of which is cortisol