A steroid is an organic compound with four fused rings arranged in a specific molecular configuration.
The lens, or crystalline lens, is a transparent biconvex structure in most land vertebrate eyes. Along with the cornea, aqueous and vitreous humours it refracts light, focusing it onto the retina. In many land animals the shape of the lens can be altered, effectively changing the focal length of the eye, enabling them to focus on objects at various distances. This adjustment of the lens is known as accommodation. In many fully aquatic vertebrates such as fish other methods of accommodation are used such as changing the lens's position relative to the retina rather than changing lens shape. Accommodation is analogous to the focusing of a photographic camera via changing its lenses. In land vertebrates the lens is flatter on its anterior side than on its posterior side, while in fish the lens is often close to spherical.
In anatomy, a crystallin is a water-soluble structural protein found in the lens and the cornea of the eye accounting for the transparency of the structure. It has also been identified in other places such as the heart, and in aggressive breast cancer tumors. Since it has been shown that lens injury may promote nerve regeneration, crystallin has been an area of neural research. So far, it has been demonstrated that crystallin β b2 (crybb2) may be a neurite-promoting factor.
Squalene is an organic compound. It is a triterpene with the formula C30H50. It is a colourless oil, although impure samples appear yellow. It was originally obtained from shark liver oil (hence its name, as Squalus is a genus of sharks). An estimated 12% of bodily squalene in humans is found in sebum. Squalene has a role in topical skin lubrication and protection.
Hopanoids are a diverse subclass of triterpenoids with the same hydrocarbon skeleton as the compound hopane. This group of pentacyclic molecules therefore refers to simple hopenes, hopanols and hopanes, but also to extensively functionalized derivatives such as bacteriohopanepolyols (BHPs) and hopanoids covalently attached to lipid A.
Cycloartenol is an important triterpenoid often found in plants. It belongs to the sterol class of steroids. It is the starting point for the synthesis of almost all plant steroids, making them chemically distinct from the steroids of fungi and animals, which are, instead, produced from lanosterol.
Steranes constitute a class of tetracyclic triterpanes derived from steroids or sterols via diagenetic and catagenetic degradation, such as hydrogenation. They are found in sediments and sedimentary rocks in nature. Steranes are derivatives of gonane, the steroid nucleus which is also called "cyclopentanoperhydrophenanthrene". They have an androstane skeleton with a side chain at the C-17 carbon. The sterane structure constitutes the core of all sterols. Steranes are widely used as biomarkers for the presence of eukaryotes in past ecosystems because steroids are nearly exclusively produced by eukaryotes. In particular, cholestanes are diagenetic products of cholesterol in animals, while stigmastanes are diagenetic products of stigmasterols in algae and land plants. However, some bacteria are now known to produce sterols and it is inferred that the ultimate origin of sterol biosynthesis is in bacteria. Sterols are produced via protosterols that are direct cyclization compounds of squalene by the catalysis of oxidosqualene cyclase. All known sterols in eukaryotes are enzymatically extensively modified from protosterols, while organisms that only produce protosterols are not known. The oldest record of modified steranes are in sedimentary rocks deposited ca. 720–820 million years ago. In contrast, diagenetic products of protosterols are widely distributed in older Proterozoic rocks and imply the presence of extinct proto-eukaryotes and/or sterol-producing bacteria before the evolution of crown-group eukaryotes.
β-sitosterol (beta-sitosterol) is one of several phytosterols with chemical structures similar to that of cholesterol. It is a white, waxy powder with a characteristic odor, and is one of the components of the food additive E499. Phytosterols are hydrophobic and soluble in alcohols.
Triterpenes are a class of terpenes composed of six isoprene units with the molecular formula C30H48; they may also be thought of as consisting of three terpene units. Animals, plants and fungi all produce triterpenes, including squalene, the precursor to all steroids.
(S)-2,3-Oxidosqualene ((S)-2,3-epoxysqualene) is an intermediate in the synthesis of the cell membrane sterol precursors lanosterol and cycloartenol, as well as saponins. It is formed when squalene is oxidized by the enzyme squalene monooxygenase. 2,3-Oxidosqualene is the substrate of various oxidosqualene cyclases, including lanosterol synthase, which produces lanosterol, a precursor to cholesterol.
Squalene monooxygenase is a eukaryotic enzyme that uses NADPH and diatomic oxygen to oxidize squalene to 2,3-oxidosqualene. Squalene epoxidase catalyzes the first oxygenation step in sterol biosynthesis and is thought to be one of the rate-limiting enzymes in this pathway. In humans, squalene epoxidase is encoded by the SQLE gene. Several eukaryote genomes lack a squalene monooxygenase encoding gene, but instead encode an alternative squalene epoxidase that performs the same task.
Lanosterol synthase (EC 5.4.99.7) is an oxidosqualene cyclase (OSC) enzyme that converts (S)-2,3-oxidosqualene to a protosterol cation and finally to lanosterol. Lanosterol is a key four-ringed intermediate in cholesterol biosynthesis. In humans, lanosterol synthase is encoded by the LSS gene.
Lanosterol 14α-demethylase (CYP51A1) is the animal version of a cytochrome P450 enzyme that is involved in the conversion of lanosterol to 4,4-dimethylcholesta-8(9),14,24-trien-3β-ol. The cytochrome P450 isoenzymes are a conserved group of proteins that serve as key players in the metabolism of organic substances and the biosynthesis of important steroids, lipids, and vitamins in eukaryotes. As a member of this family, lanosterol 14α-demethylase is responsible for an essential step in the biosynthesis of sterols. In particular, this protein catalyzes the removal of the C-14α-methyl group from lanosterol. This demethylation step is regarded as the initial checkpoint in the transformation of lanosterol to other sterols that are widely used within the cell.
Prenyltransferases (PTs) are a class of enzymes that transfer allylic prenyl groups to acceptor molecules. Prenyl transferases commonly refer to isoprenyl diphosphate syntheses (IPPSs). Prenyltransferases are a functional category and include several enzyme groups that are evolutionarily independent.
In enzymology, a sterol 14-demethylase (EC 1.14.13.70) is an enzyme of the Cytochrome P450 (CYP) superfamily. It is any member of the CYP51 family. It catalyzes a chemical reaction such as:
In enzymology, a cycloartenol synthase is an enzyme that catalyzes the chemical reaction
Squalene-hopene cyclase (SHC) (EC 5.4.99.17) or hopan-22-ol hydro-lyase is an enzyme in the terpene cyclase/mutase family. It catalyzes the interconversion of squalene into a pentacyclic triterpenes, hopene and hopanol. This enzyme catalyses the following chemical reactions.
Parkeol is a relatively uncommon sterol secondary metabolite found mostly in plants, particularly noted in Butyrospermum parkii. It can be synthesized as a minor product by several oxidosqualene cyclase enzymes, and is the sole product of the enzyme parkeol synthase.
Oxidosqualene cyclases (OSC) are enzymes involved in cyclization reactions of 2,3-oxidosqualene to form sterols or triterpenes.
Pierre Benveniste, born on 22 December 1937 in Neuilly-sur-Seine, is a French researcher in plant biochemistry and professor at the University of Strasbourg.
