Steroid

A steroid is a member of a structural family based on a characteristic fused-ring carbon framework, usually three six-membered rings and one five-membered ring. Cholesterol, bile-derived compounds and many signalling molecules share variations of this skeleton.

Substituents, oxidation states and especially ring-junction stereochemistry change molecular shape and function. Thus “steroid” is a structural classification, not a synonym for one hormone or one biological effect. Removing or modifying a side chain can preserve the underlying steroid framework while changing biochemical behaviour substantially.

Androstenedione is a 19-carbon steroid with ketone groups at carbon positions 3 and 17 and a double bond between positions 4 and 5. The structural name androst-4-ene-3,17-dione encodes these features. Its molecular formula is ${\mathrm{C}}_{19}{\mathrm{H}}_{26}{\mathrm{O}}_2$.^[ref]

Androstenedione is also a substrate in steroid hormone synthesis. Aromatase converts it to estrone, with formate among the products. The enzyme record lists a separate reaction converting testosterone to estradiol; the two starting steroids and their products should not be conflated.^[ref]

Carbon accounting gives a useful check on the first transformation: the substrate has 19 carbon atoms, whereas estrone has 18, and the remaining carbon appears in formate. That accounting describes only the carbon balance; the complete reaction also requires oxygen and reducing equivalents. Thus conversion to an estrogen involves a chemical rearrangement and oxidation process, rather than simply attaching a different biological label to the same steroid structure.^[ref]

A bile acid is a steroid-derived amphiphilic compound with a carboxyl-containing side chain. Its comparatively hydrophobic ring framework and polar groups help organize otherwise poorly water-soluble lipids in an aqueous environment.

Biological conjugation can convert the acid group into an amide, changing ionization and solubility behaviour. At relevant aqueous conditions, ionized forms are often called bile salts. The number and orientation of hydroxyl groups affect the balance between polar and nonpolar surfaces, so different bile acids need not have interchangeable physical properties.