Steroid

A steroid is a type of organic molecule built around a specific four-ring structure, with the rings labeled A, B, C, and D. In living organisms, steroids mainly do two jobs: they help build cell membranes and control how fluid those membranes are, and they act as signaling molecules. Common examples include cholesterol, the sex hormones estradiol and testosterone, anabolic steroids, and the anti-inflammatory drug dexamethasone. These compounds appear in hundreds of forms across fungi, plants, and animals.

The core of a steroid is called gonane, or cyclopentanoperhydrophenanthrene. This nucleus usually has seventeen carbon atoms arranged in four fused rings—three six-membered cyclohexane rings (A, B, and C) and one five-membered cyclopentane ring (D). What makes one steroid different from another are the functional groups attached to this core and the oxidation state of the rings themselves. Sterols are a subgroup of steroids that have a hydroxyl group at position three and a skeleton based on cholestane. More radical changes can occur, like cutting one of the rings—for instance, breaking ring B creates secosteroids, one of which is vitamin D3.

Steroids get their name from cholesterol, which was first found in gallstones; the word comes from the Greek *chole* (bile) and *stereos* (solid). The gonane nucleus is the foundation for all steroids and sterols. When two methyl groups and an eight-carbon side chain at C-17 are present, the steroid has a cholestane framework. Two common stereoisomeric forms, 5α and 5β, exist depending on which side of the planar ring system the hydrogen at carbon-5 attaches, which changes the shape of the A-ring. Isomerization at the C-21 side chain yields a parallel series called isosteroids.

Beyond ring cuts, expansions, and contractions, steroids can also differ in the bond orders within the rings, the number of methyl groups attached (including on the side chain at C-17), the functional groups on the rings and side chain, and the configuration of those groups. For example, sterols like cholesterol and lanosterol have a hydroxyl group at C-3, while testosterone and progesterone have a carbonyl (oxo) group there. Only lanosterol has two methyl groups at C-4. Cholesterol has a double bond between C-5 and C-6, whereas testosterone and progesterone have one between C-4 and C-5.

Almost all biologically relevant steroids can be seen as derivatives of a parent hydrocarbon skeleton. These parent structures have specific names, such as pregnane and androstane. Derivatives carry functional groups indicated by suffixes or prefixes with numbers showing their position on the nucleus. Many natural steroids have widely used trivial names, like progesterone, testosterone, or cortisol. These trivial names can also serve as bases for new names by adding prefixes only—for instance, 17α-hydroxyprogesterone has a hydroxy group at position 17 compared to progesterone.

The letters α and β denote absolute stereochemistry at chiral centers in steroids, a system distinct from the R/S convention used in general organic chemistry. In steroid drawings, α-bonds appear as dashed wedges (oriented toward the plane of the ring system), and β-bonds as solid wedges (oriented away). The name "11-deoxycortisol" is an example of a derived name that uses cortisol as a parent structure but lacks an oxygen atom (hence "deoxy") at position 11. The numbering of carbon atoms in the steroid nucleus follows a standard template.