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A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings joined to one another. Examples of steroids include the dietary lipid cholesterol, the sex hormones estradiol and testosterone,:10–19 bile acids, and drugs such as the anti-inflammatory agent dexamethasone.
The steroid core is composed of seventeen carbon atoms bonded together in the form of four fused rings: three cyclohexane rings (designated as rings A, B and C in the figure to the right) and one cyclopentane ring (the D ring). Individual steroids vary, first and primarily, by the oxidation state of the carbon atoms in the rings and by the chains and functional groups attached to this four-ring system; second, steroids can vary more markedly via changes to the ring structure (e.g., via ring scissions that produce secosteroids like vitamin D3, see below). 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).
Hundreds of distinct steroids are found in animals, fungi, plants, and elsewhere. All natural steroids are made in living cells, either from the sterol lanosterol (animals and fungi, see examples) or from cycloartenol (plants). Both lanosterol and cycloartenol are derived via cyclization of the triterpenoid squalene.
As IUPAC guidance notes (and is explained more fully following the quote),
"Steroids are compounds possessing the skeleton of cyclopenta[a]phenanthrene or a skeleton derived therefrom by one or more bond scissions or ring expansions or contractions. Methyl groups are normally present at C-10 and C-13. An alkyl side chain may also be present at C-17. Sterols are steroids carrying a hydroxyl group at C-3 and most of the skeleton of cholestane. Additional carbon atoms may be present in the side chain."
Gonane (see opening image, above) is the simplest possible steroid and is composed of seventeen carbon atoms in carbon-carbon bonds that form four fused rings in a defined three-dimensional shape. The three cyclohexane rings (designated as rings A, B, and C in the figures above form the skeleton of a perhydro- derivative of phenanthrene. The D-ring has a cyclopentane structure; hence, though it is uncommon, per IUPAC steroids can also be named as various hydro-derivatives of cyclopenta[a]phenanthrene. When the two methyl groups and 8 carbon side chain (at C-17, as shown for cholesterol) are present, the steroid is said to have a cholestane framework. The following are some examples of steroid structures:
In addition to the ring scissions (cleavages), and expansions and contractions (cleavage and reclosing to a larger or smaller rings) noted in the IUPAC definition—all variations in the carbon-carbon bond framework—steroids can also vary:
For instance, sterols such as cholesterol and lanosterol have an hydroxyl group attached at position C-3, while testosterone and progesterone have a carbonyl (oxo substituent) at C-3; of these examples, lanosterol alone has two methyl groups at C-4, and cholesterol with a C-5 to C-6 double bond differs from testosterone and progesterone, which have a C-4 to C-5 double bond.
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|This documentation needs attention from an expert in Chemistry. The specific problem is: categories and details of each steroid structural type are incomplete, leading to distorted representation of scope of compound class. (May 2014)|
The animal steroids include compounds of vertebrate and insect origin, in the latter case including ecdysteroids such as ecdysterone, which is involved in the control of molting in some species. Vertebrate examples include the steroid hormones and cholesterol, the latter of which is a structural component of cell membranes that is involved in determining the fluidity of cell membranes and is a principal constituent of plaques implicated in atherosclerosis. The steroid hormones include:
In the Prokarya, pathways exist both for producing the tetracyclic steroid framework (e.g., in myxobacteria)—where its origin from eukaryotes is conjectured —as well as the more common pentacyclic triterpinoid hopanoid framework.
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It is also possible to classify steroids based upon their chemical composition. One example of how MeSH performs this classification is available at the Wikipedia MeSH catalog. Examples from this classification include:
|Class||Examples||Number of carbon atoms|
Secosteroids (L. seco, "to cut") are a subclass of steroidal compounds resulting, biosynthetically or conceptually, via scission (cleavage) of parent steroid rings, generally one of the four. Major secosteroid subclasses are defined by the steroid carbon atoms where this scission has taken place. For instance, the prototypical secosteroid cholecalciferol, vitamin D3 (shown), is in the important 9,10-secosteroid subclass, derived via cleavage between carbon atoms C-9 and C-10 of the steroid B-ring (similarly 5,6-secosteroids, 13,14-steroids, etc.).
Norsteroids (nor-, L. norma, from "normal" in chemistry, indicating carbon removal) and homosteroids (homo-, Gk. homos for same, indicating carbon addition) are two structural subclasses of steroids formed via biosynthetic or bench chemistry steps, in the former case involving enzymic ring expansion/contraction reactions, and in the latter accomplished similarly (biomimetically) or, more often, through ring closures of acyclic precursors with more or fewer ring atoms than in the parent steroid framework. These two classes represent further unique classes of steroids with important biological activities and societal impacts; the effect of these chemical operations on the ring structures is such that recognition of the parent tetracyclic ring system can be challenging.
Combinations of these ring alterations are also possible and are known in nature. For instance, ewes that graze on corn lily ingest cyclopamine (shown) and veratramine, two of a sub-family of steroids where the C- and D-rings are contracted and expanded, respectively, via a biosynthetic migration of the original C-13 atom. Ingestion of these C-nor-D-homosteroids result in birth defects in progeny lambs: cyclopia in the case of cyclopamine and leg deformity with veratramine. A further C-nor-D-homosteroid, nakiterpiosin, is excreted by Okinawan cyanobacteriosponges, Terpios hoshinota, leading to coral mortality from black coral disease. Nakiterpiosin-type steroids are active against the Smoothened - Hedgehog pathway that is hyperactive in various cancers; Merck chemists established that C-13 atom migration could be achieved by "bench chemistry", and this biomimetic synthesis allows medicinal chemistry to proceed on this steroidal anticancer hypothesis.
Similar to lipids, steroids represent highly concentrated energy stores. However, steroids are not typically used as sources of energy. In mammals, they are normally metabolized and excreted.
A number of drugs target the mevalonate pathway:
Steroid biosynthesis is an anabolic metabolic pathway that produces steroids from simple precursors. A unique biosynthetic pathway is followed in animals compared to many other organisms, making the pathway a common target for antibiotics and other anti-infective drugs. In addition, steroid metabolism in humans is the target of cholesterol-lowering drugs such as statins.
In humans and other animals, the biosynthesis of steroids follows the mevalonate pathway that uses acetyl-CoA as building-blocks to form dimethylallyl pyrophosphate (DMAPP) and isopentenyl pyrophosphate (IPP). In subsequent steps, DMAPP and IPP are joined to form geranyl pyrophosphate (GPP), which in turn is used to synthesize the steroid lanosterol. Further modifications of lanosterol into other steroids are classified steroidogenesis transformations.
DMAPP and IPP in turn donate isoprene units, which are assembled and modified to form terpenes and isoprenoids, which are a large class of lipids that include the carotenoids, and form the largest class of plant natural products.
Here, the isoprene units are joined together to make squalene and then folded up and formed into a set of rings to make lanosterol. Lanosterol can then be converted into other steroids such as cholesterol and ergosterol.
Steroidogenesis is the biological process by which steroids are generated from cholesterol and transformed into other steroids. The pathways of steroidogenesis differ between different species – as an example the pathways of human steroidogenesis are shown in this figure below: Following is a list of the major classes of steroid hormones and some prominent members, with examples of major related functions:
Locations of human steroidogenesis:
Several key enzymes can be activated through DNA transcriptional regulation on activation of SREBP (Sterol Regulatory Element-Binding Protein-1 and -2). This intracellular sensor detects low cholesterol levels and stimulates endogenous production by the HMG-CoA reductase pathway, as well as increasing lipoprotein uptake by up-regulating the LDL receptor. Regulation of this pathway is also achieved by controlling the rate of translation of the mRNA, degradation of reductase and phosphorylation.
Steroids are oxidized mainly by cytochrome P450 oxidase enzymes, such as CYP3A4. These reactions introduce oxygen into the steroid ring and allow the structure to be broken up by other enzymes, to form bile acids as final products. These bile acids can then be eliminated through secretion from the liver in the bile. The expression of this oxidase gene can be upregulated by the steroid sensor PXR when there is a high blood concentration of steroids.
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The isolation of steroids refers, depending on context, either to the isolation of the considerable quantities of pure chemical matter required for chemical structure elucidation, derivitzation/degradation chemistry, biological testing, and other research needs (generally milligrams to grams, but historically, often more), or to the isolation of "analytical quantities" of the substance of interest, where the focus is on identification and quantitation of the substance (e.g., in biological tissue or fluid), and where the amount isolated depends on the analytical method applied (but is generally always sub-microgram in scale).[page needed] The methods of isolation applied toward achieving these two distinct scales of product are likewise distinct, but generally involve extraction, precipitation, adsorptions, chromatography, and sometimes crystallizations. In both cases, the isolated substance is purified to chemical homogeneity, i.e., specific combined separation and analytical methods such as LC-MS methods are chosen to be "orthogonal"—achieving their separations based on distinct modes of interaction between substance and isolating matrix—with the goal being detection of only a single species present in the purportedly pure sample. The expression structure determination refers to methods that are applied to determine the chemical structure of an isolated, pure steroid, a process that involves an array of chemical and physical methods that have changed markedly over the history of steroid research, but that have included NMR and small molecule crystallography.:10–19 Methods of analysis include samplings of both of these prior areas, but especially analytical methods aimed at determining if a steroid is present in an analytical mixture, and determining its quantity in that medium.
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Phytosterols, for instance, mixtures of soybean sterols, can be used as starting materials and converted into two kinds of steroid hormone intermediates through microbial transformation. Microbial catabolism of phytosterol sidechains yields either C-19 steroids, a precursor to most steroid hormones including sex hormones, or C-22 steroids, a precursor to adrenocortical hormones.
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The chemical conversion of sapogenins to steroids—e.g., via the Marker degradation—is a method of partial synthesis that is a long-established alternative to microbial transformation of phytosterols to steroids, and underpinned Syntex efforts using the Mexican barbasco trade (harvesting and marketing large tubers of wild-growing plants, e.g., yams) to produce early synthetic steroids.
A number of Nobel Prizes have been awarded for research involving steroids. These prizes include: