Hormones are also grouped according to chemical structure. Structures dictate if the hormone prefers to be surrounded by water or fat (water or fat soluble), which determines:
- if the hormone travels in the blood alone or attached to a protein
- if the hormone will bind to receptor sites outside or inside of the target cell (fat soluble can bind both whereas water soluble hormones usually bind on the outside) and
- how the hormone is broken down (metabolized).
Three general structures are known.
- Steroid hormones are fat-soluble molecules made from cholesterol. Among these are the three major sex hormones groups: estrogens, androgens and progesterones. Males and females make all three, just in different amounts. Steroids pass into a cell’s nucleus, bind to specific receptors and genes and trigger the cell to make proteins.
- Amino acid derivatives, such as epinephrine, are water-soluble molecules derived from amino acids (the building blocks of protein). These hormones are stored in endocrine cells until needed. They act by binding to protein receptors on the outside surface of the cell. The binding alerts a second messenger molecule inside the cell that activates enzymes and other cellular proteins or influences gene expression.
- Insulin, growth hormone, prolactin and other water-soluble polypeptide hormones consist of long chains of amino acids, from several to 200 amino acids long. They are stored in endocrine cells until needed to regulate such processes as metabolism, lactation, growth and reproduction.