steroid steroid a steroid (named after the steroid cholesterol which was first described in gall stones from ancient greek chole- 'bile' and stereos 'solid') is a biologically active organic compound with four rings arranged in a specific molecular configuration. steroids have two principal biological functions: as important components of cell membranes that alter membrane fluidity; and as signaling molecules. hundreds of steroids are found in plants, animals and fungi. all steroids are manufactured in cells from the sterols lanosterol (opisthokonts) or cycloartenol (plants). lanosterol and cycloartenol are derived from the cyclization of the triterpene squalene. this article is about the family of polycyclic chemical compounds. for the drugs, also used as performance-enhancing substances, see anabolic steroid. for the scientific journal, see steroids (journal). for the death grips ep, see steroids (crouching tiger hidden gabber megamix). structure of 24-ethyl-lanostane, a hypothetical steroid with 32 carbon atoms. its core ring system (abcd), composed of 17 carbon atoms, is shown with iupac-approved ring lettering and atom numbering.: 1785f the steroid core structure is typically composed of seventeen carbon atoms, bonded in four "fused" rings: three six-member cyclohexane rings (rings a, b and c in the first illustration) and one five-member cyclopentane ring (the d ring). steroids vary by the functional groups attached to this four-ring core and by the oxidation state of the rings. sterols are forms of steroids with a hydroxy group at position three and a skeleton derived from cholestane.: 1785f steroids can also be more radically modified, such as by changes to the ring structure, for example, cutting one of the rings. cutting ring b produces secosteroids one of which is vitamin d3. examples include anabolic steroids, the lipid cholesterol, the sex hormones estradiol and testosterone,: 10–19 and the anti-inflammatory drug dexamethasone. space-filling representationball-and-stick representation5α-dihydroprogesterone (5α-dhp), a steroid. the shape of the four rings of most steroids is illustrated (carbon atoms in black, oxygens in red and hydrogens in grey). the nonpolar "slab" of hydrocarbon in the middle (grey, black) and the polar groups at opposing ends (red) are common features of natural steroids. 5α-dhp is an endogenous steroid hormone and a biosynthetic intermediate. nomenclature gonane, the simplest steroid, consisting only of the common steroid nucleus steroid 5α and 5β stereoisomers: 1786f gonane, also known as steran or cyclopentanoperhydrophenanthrene, the simplest steroid and the nucleus of all steroids and sterols, is composed of seventeen carbon atoms in carbon-carbon bonds forming four fused rings in a three-dimensional shape. the three cyclohexane rings (a, b, and c in the first illustration) form the skeleton of a perhydro derivative of phenanthrene. the d ring has a cyclopentane structure. when the two methyl groups and eight carbon side chains (at c-17, as shown for cholesterol) are present, the steroid is said to have a cholestane framework. the two common 5α and 5β stereoisomeric forms of steroids exist because of differences in the side of the largely planar ring system where the hydrogen (h) atom at carbon-5 is attached, which results in a change in steroid a-ring conformation. isomerisation at the c-21 side chain produces a parallel series of compounds, referred to as isosteroids. examples of steroid structures are: testosterone, the principal male sex hormone and an anabolic steroid cholic acid, a bile acid, showing the carboxylic acid and additional hydroxyl groups often present dexamethasone, a synthetic corticosteroid drug lanosterol, the biosynthetic precursor to animal steroids. the number of carbons (30) indicates its triterpenoid classification. progesterone, a steroid hormone involved in the female menstrual cycle, pregnancy, and embryogenesis medrogestone, a synthetic drug with effects similar to progesterone β-sitosterol, a plant or phytosterol, with a fully branched hydrocarbon side chain at c-17 and an hydroxyl group at c-3 in addition to the ring scissions (cleavages), expansions and contractions (cleavage and reclosing to a larger or smaller rings)—all variations in the carbon-carbon bond framework—steroids can also vary: in the bond orders within the rings, in the number of methyl groups attached to the ring (and, when present, on the prominent side chain at c17), in the functional groups attached to the rings and side chain, and in the configuration of groups attached to the rings and chain.: 2–9 for instance, sterols such as cholesterol and lanosterol have a hydroxyl group attached at position c-3, while testosterone and progesterone have a carbonyl (oxo substituent) at c-3; of these, 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). cholesterol, a prototypical animal sterol. this structural lipid and key steroid biosynthetic precursor.: 1785f 5α-cholestane, a common steroid core species distribution and function in eukaryotes, steroids are found in fungi, animals, and plants. fungal steroids fungal steroids include the ergosterols, which are involved in maintaining the integrity of the fungal cellular membrane. various antifungal drugs, such as amphotericin b and azole antifungals, utilize this information to kill pathogenic fungi. fungi can alter their ergosterol content (e.g. through loss of function mutations in the enzymes erg3 or erg6, inducing depletion of ergosterol, or mutations that decrease the ergosterol content) to develop resistance to drugs that target ergosterol. ergosterol is analogous to the cholesterol found in the cellular membranes of animals (including humans), or the phytosterols found in the cellular membranes of plants. all mushrooms contain large quantities of ergosterol, in the range of tens to hundreds of milligrams per 100 grams of dry weight. oxygen is necessary for the synthesis of ergosterol in fungi. ergosterol is responsible for the vitamin d content found in mushrooms; ergosterol is chemically converted into provitamin d2 by exposure to ultraviolet light. provitamin d2 spontaneously forms vitamin d2. however, not all fungi utilize ergosterol in their cellular membranes; for example, the pathogenic fungal species pneumocystis jirovecii does not, which has important clinical implications (given the mechanism of action of many antifungal drugs). using the fungus saccharomyces cerevisiae as an example, other major steroids include ergosta‐5,7,22,24(28)‐tetraen‐3β‐ol, zymosterol, and lanosterol. s. cerevisiae utilizes 5,6‐dihydroergosterol in place of ergosterol in its cell membrane. animal steroids animal steroids include compounds of vertebrate and insect origin, the latter including ecdysteroids such as ecdysterone (controlling molting in some species). vertebrate examples include the steroid hormones and cholesterol; the latter is a structural component of cell membranes that helps determine the fluidity of cell membranes and is a principal constituent of plaque (implicated in atherosclerosis). steroid hormones include: sex hormones, which influence sex differences and support reproduction. these include androgens, estrogens, and progestogens. corticosteroids, including most synthetic steroid drugs, with natural product classes the glucocorticoids (which regulate many aspects of metabolism and immune function) and the mineralocorticoids (which help maintain blood volume and control renal excretion of electrolytes) anabolic steroids, natural and synthetic, which interact with androgen receptors to increase muscle and bone synthesis. in popular use, the term "steroids" often refers to anabolic steroids. plant steroids plant steroids include steroidal alkaloids found in solanaceae and melanthiaceae (specially the genus veratrum), cardiac glycosides, the phytosterols and the brassinosteroids (which include several plant hormones). prokaryotes in prokaryotes, biosynthetic pathways exist for the tetracyclic steroid framework (e.g. in mycobacteria) – where its origin from eukaryotes is conjectured – and the more-common pentacyclic triterpinoid hopanoid framework. types by function the major classes of steroid hormones, with prominent members and examples of related functions, are: corticosteroids: glucocorticoids: cortisol, a glucocorticoid whose functions include immunosuppression mineralocorticoids: aldosterone, a mineralocorticoid that helps regulate blood pressure through water and electrolyte balance sex steroids: progestogens: progesterone, which regulates cyclical changes in the endometrium of the uterus and maintains a pregnancy androgens: testosterone, which contributes to the development and maintenance of male secondary sex characteristics estrogens: estradiol, which contributes to the development and maintenance of female secondary sex characteristics additional classes of steroids include: neurosteroids such as dhea and allopregnanolone bile acids such as taurocholic acid aminosteroid neuromuscular blocking agents (mainly synthetic) such as pancuronium bromide steroidal antiandrogens (mainly synthetic) such as cyproterone acetate steroidogenesis inhibitors (mainly exogenous) such as alfatradiol membrane sterols such as cholesterol, ergosterol, and various phytosterols toxins such as steroidal saponins and cardenolides/cardiac glycosides as well as the following class of secosteroids (open-ring steroids): vitamin d forms such as ergocalciferol, cholecalciferol, and calcitriol intact ring system steroids can be classified based on their chemical composition. one example of how mesh performs this classification is available at the wikipedia mesh catalog. examples of this classification include: cholecalciferol (vitamin d3), an example of a 9,10-secosteroid cyclopamine, an example of a complex c-nor-d-homosteroid class example number of carbon atoms cholestanes cholesterol 27 cholanes cholic acid 24 pregnanes progesterone 21 androstanes testosterone 19 estranes estradiol 18 in biology, it is common to name the above steroid classes by the number of carbon atoms present when referring to hormones: c18-steroids for the estranes (mostly estrogens), c19-steroids for the androstanes (mostly androgens), and c21-steroids for the pregnanes (mostly corticosteroids). the classification "17-ketosteroid" is also important in medicine. the gonane (steroid nucleus) is the parent 17-carbon tetracyclic hydrocarbon molecule with no alkyl sidechains. cleaved, contracted, and expanded rings secosteroids (latin seco, "to cut") are a subclass of steroidal compounds resulting, biosynthetically or conceptually, from 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 9,10-secosteroid subclass and derives from the cleavage of carbon atoms c-9 and c-10 of the steroid b-ring; 5,6-secosteroids and 13,14-steroids are similar. norsteroids (nor-, l. norma; "normal" in chemistry, indicating carbon removal) and homosteroids (homo-, greek homos; "same", indicating carbon addition) are structural subclasses of steroids formed from biosynthetic steps. the former involves enzymic ring expansion-contraction reactions, and the latter is accomplished (biomimetically) or (more frequently) through ring closures of acyclic precursors with more (or fewer) ring atoms than the parent steroid framework. combinations of these ring alterations are known in nature. for instance, ewes who 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 results in birth defects in lambs: cyclopia from cyclopamine and leg deformity from veratramine. a further c-nor-d-homosteroid (nakiterpiosin) is excreted by okinawan cyanobacteriosponges. e.g., terpios hoshinota, leading to coral mortality from black coral disease. nakiterpiosin-type steroids are active against the signaling pathway involving the smoothened and hedgehog proteins, a pathway which is hyperactive in a number of cancers. biological significance steroids and their metabolites often function as signalling molecules (the most notable examples are steroid hormones), and steroids and phospholipids are components of cell membranes. steroids such as cholesterol decrease membrane fluidity. similar to lipids, steroids are highly concentrated energy stores. however, they are not typically sources of energy; in mammals, they are normally metabolized and excreted. steroids play critical roles in a number of disorders, including malignancies like prostate cancer, where steroid production inside and outside the tumour promotes cancer cell aggressiveness. biosynthesis and metabolism simplification of the end of the steroid synthesis pathway, where the intermediates isopentenyl pyrophosphate (pp or ipp) and dimethylallyl pyrophosphate (dmapp) form geranyl pyrophosphate (gpp), squalene and lanosterol (the first steroid in the pathway) the hundreds of steroids found in animals, fungi, and plants are made from lanosterol (in animals and fungi; see examples above) or cycloartenol (in other eukaryotes). both lanosterol and cycloartenol derive from cyclization of the triterpenoid squalene. lanosterol and cycloartenol are sometimes called protosterols because they serve as the starting compounds for all other steroids. steroid biosynthesis is an anabolic pathway which 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-infection drugs. steroid metabolism in humans is also the target of cholesterol-lowering drugs, such as statins. in humans and other animals the biosynthesis of steroids follows the mevalonate pathway, which uses acetyl-coa as building blocks for dimethylallyl diphosphate (dmapp) and isopentenyl diphosphate (ipp). in subsequent steps dmapp and ipp conjugate to form farnesyl diphosphate (fpp), which further conjugates with each other to form the linear triterpenoid squalene. squalene biosynthesis is catalyzed by squalene synthase, which belongs to the squalene/phytoene synthase family. subsequent epoxidation and cyclization of