Hundreds of distinct steroids are found in plants, animals, and fungi. All steroids are made in cells either from the sterols lanosterol (animals and fungi) or cycloartenol (plants). Both, lanosterol and cycloartenol, are derived from the cyclization of the triterpene squalene.[3]
Sterols are special forms of steroids, with a hydroxyl group at the atom C-3 and a skeleton derived from cholestane.[2] Cholesterol is one of the best known sterols.
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Some of the common categories of steroids:
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 |
|---|---|---|
| Cholstanes | cholesterol | 27 |
| Cholanes | cholic acid | 24 |
| Pregnanes | progesterone | 21 |
| Androstanes | testosterone | 19 |
| Estranes | estradiol | 18 |
Steroids include estrogen, cortisol, progesterone, and testosterone. Estrogen and progesterone are made primarily in the ovary and in the placenta during pregnancy, and testosterone in the testes. Testosterone is also converted into estrogen to regulate the supply of each, in the bodies of both females and males. Certain neurons and glia in the central nervous system (CNS) express the enzymes that are required for the local synthesis of pregnane neurosteroids, either de novo or from peripherally-derived sources. The rate-limiting step of steroid synthesis is the conversion of cholesterol to pregnenolone, which occurs inside the mitochondrion.[4]
Steroid biosynthesis is an anabolic metabolic pathway that produces steroids from simple precursors. This pathway is carried out in different ways in animals than in 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.
It starts in the mevalonate pathway in humans, with Acetyl-CoA as building blocks, which form DMAPP and IPP[5]. In following steps, DMAPP and IPP form lanosterol, the first steroid. Further modification belongs to the succeeding 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.
A number of drugs target the mevalonate pathway:
In plants and bacteria, the non-mevalonate pathway uses pyruvate and glyceraldehyde 3-phosphate as substrates.[6][7]
Isopentenyl pyrophosphate and dimethylallyl pyrophosphate donate isoprene units, which are assemblied and modificated to form terpenes and isoprenoids[7], which are a large class of lipids that include the carotenoids, and form the largest class of plant natural products.[8]
Here, the isoprene units are joined together to make squalene and then folded up and formed into a set of rings to make lanosterol.[9] Lanosterol can then be converted into other steroids such as cholesterol and ergosterol.[10][9]
Steroidogenesis is the process wherein desired forms of steroids are generated by transformation of other steroids ( The formation of steroids; commonly referring to the biological synthesis of steroid hormones, but not to the production of such compounds in a chemical laboratory). The pathways of steroidogenesis can differ from organism to organism, but the pathways of human steroidogenesis are shown in the figure.
Products of steroidogenesis include:
Steroids are mainly oxidized by cytochrome P450 oxidase enzymes, such as CYP3A4. These reactions introduce oxygen into the steroid ring and allows the structure to be broken up by other enzymes, to form bile acids as final products.[11] These bile acids can then be eliminated through secretion from the liver in the bile.[12] The expression of this oxidase gene can be upregulated by the steroid sensor PXR when there is a high blood concentration of steroids.[13]
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