Definition: A steroid is a terpenoid lipid characterized by a carbon skeleton with four fused rings, generally arranged in a 6-6-6-5 fashion.
Steroids vary by the functional groups attached to these rings and the oxidation state of the rings. Hundreds of distinct steroids are found in plants, animals, and fungi.
All steroids are made in cells either from the sterol lanosterol (animals and fungi) or the sterol cycloartenol (plants). Both sterols are derived from the cyclization of the triterpene squalene.
Structure
The steroids contain the same fused four-ring core structure that the sterol lipids. They have different biological roles as hormones and signaling molecules.
The C18 steroids include the estrogen family whereas the C19 steroids comprise the androgens such as testosterone and androsterone. The C21 subclass includes the progestogens as well as the glucocorticoids and mineralocorticoids.
The secosteroids, comprising various forms of vitamin D, are characterized by cleavage of the B ring of the core structure.
Other examples of steroids are the bile acids (steroid acids) and their conjugates, which in mammals are oxidized derivatives of cholesterol and are synthesized in the liver.
See also
sterol