Any of a group of lipids derived
from a saturated compound called cyclopentanoper-hydrophenanthrene.
Steroids have a basic molecular structure of 15 carbon atoms arranged in
four rings and bounded by up to 28 hydrogen atoms. Among the most important
steroid derivatives are the steroid alcohols, or sterols,
such as cholesterol. Other steroids
include the bile acids, which aid digestion
of fats in the intestine; the sex hormones (androgens
and estrogens); and the corticosteroid
hormones, produced by the adrenal cortex. Vitamin
D is also based on the steroid structure. Synthetic steroids are widely
used in medicine.
|The steroid (cyclopentanoperhydrophenanthrene) nucleus:
(A) structure and labeling of the carbon atoms; (B) conformational
structure: hydrogen atoms are to be found at the end of each unlabeled
spur bond. These bonds point either above the plane of the molecule
(solid lines) or below (broken lines), while the hydrogen atoms are
either equatorial (roughly in the plane of the carbon skeleton) or
axial (lying above or below the skeletal plane); (C) Beta-sitosterol,
a typical plant steroid. Side chains and substituent atoms (in this
case the hydroxyl groupon carbon atom 3) are either alpha- or beta-,
depending on whether their bonding points either below or above the
plane of the carbon skeleton.