Steroid

(CC) Image: David E. Volk
Structure and nomenclature of cholesterol. All other steroids nomenclature is based on cholesterol. By convention, substituents pointing up, like C-18 and C-19 are called $\beta$ while those pointing down are called $\alpha$.

Steroids, or steroid hormones are powerful hormones with drastic effects, both good and bad, when artificially introduced into living systems. Athletes have often taken anobolic steroids to improve muscle growth and athletic performance. Glucocorticoids play a role in inflamation, and estrogens have been linked to cancer. Testosterone and estrogen influence sexual traits (maleness/femaleness). All steroid hormones are naturally synthesized from cholesterol. The five major classes of steroids are: progestagens, glucocorticoids, mineralcorticoids, androgens and estrogens. The steroid hormones activate gene expression by binding to enhancer proteins, called steroid receptors.

cholesterol (C27)

Cholesterol is the precursor from which all steroid hormones are synthesized. Because of this, all steroid numbering and nomenclature follow that of cholesterol. Some cholesterol derivatives have a proton added to the C-5 carbon. If the H-5 proton is alpha-oriented (points down) then rings A and B are fused in a trans conformation, but in the beta orientation, the rings are fused in a cis conformation. All steroids with an H5 are in the 5$\alpha$ orientation, while bile salts derived from cholesterol have the 5$\beta$ orientation.

pregnenolone (C21)

(CC) Image: David E. Volk
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All steroid hormones have 21 or fewer carbons, although their precursor cholesterol has 27 carbon atoms. pregnenolone is the first steroid derived from cholesterol. It is synthesized through an intermediate, 20$\alpha$,22$\beta$-dihydroxy-cholesterol, which is subsequently oxidized at C-20 to form a ketone with cleavage of carbons 22-27.