What is Steroids?
Steroids comprise a group of cyclic organic compounds whose most common characteristic is an arrangement of seventeen carbon atoms in a four-ring structure, where the rings are three composed of 6-carbons (rings A, B, and C) followed by one with 5-carbons (ring D). Further common features are an 8-carbon side chain attached to a carbon on ring D, and two or more methyl groups at the points where adjacent rings are "fused". Hundreds of distinct steroids are found in animals, fungi, plants, and elsewhere, and specific steroids underlie proper structure and function in many biological processes. Their core tetracyclic ring structure is synthesized in each organism by biochemical pathways that involve cyclization of a thirty-carbon chain, squalene, into an intermediate, either lanosterol or cycloartenol. From such intermediates, organisms then derive critical steroids such as cholesterol, the sex hormones estradiol and testosterone, and bile acids. Based on such structures, synthetic and medicinal chemists synthesize novel steroids for use as drugs, e.g. the anti-inflammatory agent dexamethasone.
As noted, the three cyclohexane rings of steroids are designated as rings A, B and C, and the one cyclopentane ring is designated as ring D. Individual steroids vary, first and primarily, by the oxidation state of the carbon atoms of the rings and by the chains and functional groups attached to this four-ring system. Sterols are a particularly important form of steroids, with sterols having a cholestane-derived framework and an hydroxyl group at the C-3 ring position being the most prominent (e.g., as in cholesterol, shown at right). In addition, steroids can vary more markedly via changes to the ring structure (e.g., via ring scissions that produce secosteroids such as vitamin D3, see below).