Steroids take their name from a waxy substance first found in gallstones
Testosterone, cortisol, cholesterol and the anti-inflammatory drug dexamethasone look nothing alike in what they do, yet all share the same skeleton of four fused carbon rings. The whole family is named after cholesterol, first described in gallstones, whose Greek roots mean solid bile. Snip one ring open and you get vitamin D.
The common core, called gonane, usually contains seventeen carbon atoms arranged as three six-sided rings and one five-sided ring, labelled A through D. What turns this frame into a hormone, a drug or a membrane component is the pattern of chemical groups hung on it and small shifts in its bonds. Cholesterol and testosterone, for example, differ partly in where a single double bond sits: between carbons 5 and 6 in cholesterol, between 4 and 5 in testosterone. Cutting ring B produces a class called secosteroids, which includes vitamin D3.
Every cell that makes steroids starts from a long molecule called squalene, which folds and closes into a sterol. Animals and fungi turn it into lanosterol and cholesterol, while plants use a different product, cycloartenol. From there, cells build hundreds of variants. Steroids serve two broad jobs: they adjust how fluid cell membranes are, and they act as chemical messengers, as sex hormones and stress hormones do.
Their membrane role may have shaped the history of life. Sterols help keep the membranes of complex cells stable and flexible, and researchers think they may have made it possible for an ancestral cell to engulf the bacterium that became the mitochondrion. Bacteria rarely make sterols at all, though a few do, including the methane-eating Methylococcus capsulatus and Gemmata obscuriglobus, which cannot survive without them. Most bacteria instead use hopanoids, related five-ring lipids.
Naming these molecules is a discipline of its own. Chemists mark groups pointing below the ring plane as alpha and those above as beta, a system separate from the usual conventions of organic chemistry. Since 1989 the official recommendation has been to say oxo rather than keto, because keto would count a ring carbon twice. A name like 11-deoxycortisol simply means cortisol missing its oxygen at carbon 11.
Source: Steroid
