Sterol
Encyclopedia
Sterols, also known as steroid alcohols, are a subgroup of the steroid
s and an important class of organic molecules. They occur naturally in plant
s, animal
s, and fungi, with the most familiar type of animal sterol being cholesterol
. Cholesterol is vital to cellular function, and a precursor to fat-soluble vitamin
s and steroid hormones
.
s and sterols of animals are called zoosterols. Important zoosterols are cholesterol
; notable phytosterols include campesterol
, sitosterol, and stigmasterol
. Ergosterol
is a sterol present in the cell membrane
of fungi, where it serves a role similar to cholesterol in animal cells.
organisms. For example, cholesterol forms part of the cellular membrane in animals, where it affects the cell membrane's fluidity and serves as secondary messenger in developmental signaling. In humans and other animals, corticosteroids, such as cortisol
act as signaling compounds in cellular communication and general metabolism.
s, more commonly known as plant sterols, have been shown in clinical trials to block cholesterol absorption sites in the human intestine, thus helping to reduce cholesterol in humans. They are currently approved by the U.S. Food and Drug Administration
for use as a food additive; however, there is some concern that they may block absorption, not only of cholesterol, but of other important nutrients as well. At present the American Heart Association
has recommended that supplemental plant sterols be taken only by those diagnosed with elevated cholesterol, and has particularly recommended that they not be taken by pregnant women or nursing mothers.
s with a hydroxyl group at the 3-position of the A-ring. They are amphipathic lipids
synthesized from acetyl-coenzyme A via the HMG-CoA reductase
pathway. The overall molecule is quite flat. The hydroxyl group on the A ring is polar
. The rest of the aliphatic chain is non-polar
.
Steroid
A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...
s and an important class of organic molecules. They occur naturally in plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s, animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s, and fungi, with the most familiar type of animal sterol being cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
. Cholesterol is vital to cellular function, and a precursor to fat-soluble vitamin
Vitamin
A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...
s and steroid hormones
Steroid hormone
A steroid hormone is a steroid that acts as a hormone. Steroid hormones can be grouped into five groups by the receptors to which they bind: glucocorticoids, mineralocorticoids, androgens, estrogens, and progestogens...
.
Types of sterols
Sterols of plants are called phytosterolPhytosterol
Phytosterols, which encompass plant sterols and stanols, are steroid compounds similar to cholesterol which occur in plants and vary only in carbon side chains and/or presence or absence of a double bond. Stanols are saturated sterols, having no double bonds in the sterol ring structure. More than...
s and sterols of animals are called zoosterols. Important zoosterols are cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
; notable phytosterols include campesterol
Campesterol
Campesterol is a phytosterol whose chemical structure similar to that of cholesterol. Many vegetables, fruits, nuts and seeds contain campesterol, but in low concentrations. Banana, pomegranate, pepper, coffee, grapefruit, cucumber, onion, oat, potato and lemon grass are few examples of common...
, sitosterol, and stigmasterol
Stigmasterol
Stigmasterol is one of a group of plant sterols, or phytosterols, that include beta-sitosterol, campesterol, ergosterol , brassicasterol, delta-7-stigmasterol and delta-7-avenasterol, that are chemically similar to animal cholesterol...
. Ergosterol
Ergosterol
Ergosterol is a sterol found in fungi, and named for ergot, a common name for the members of the fungal genus Claviceps from which ergosterol was first isolated. Ergosterol does not occur in plant or animal cells...
is a sterol present in the cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
of fungi, where it serves a role similar to cholesterol in animal cells.
Role in biochemistry
Sterols and related compounds play essential roles in the physiology of eukaryoticEukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
organisms. For example, cholesterol forms part of the cellular membrane in animals, where it affects the cell membrane's fluidity and serves as secondary messenger in developmental signaling. In humans and other animals, corticosteroids, such as cortisol
Cortisol
Cortisol is a steroid hormone, more specifically a glucocorticoid, produced by the adrenal gland. It is released in response to stress and a low level of blood glucocorticoids. Its primary functions are to increase blood sugar through gluconeogenesis; suppress the immune system; and aid in fat,...
act as signaling compounds in cellular communication and general metabolism.
As a nutritional supplement
PhytosterolPhytosterol
Phytosterols, which encompass plant sterols and stanols, are steroid compounds similar to cholesterol which occur in plants and vary only in carbon side chains and/or presence or absence of a double bond. Stanols are saturated sterols, having no double bonds in the sterol ring structure. More than...
s, more commonly known as plant sterols, have been shown in clinical trials to block cholesterol absorption sites in the human intestine, thus helping to reduce cholesterol in humans. They are currently approved by the U.S. Food and Drug Administration
Food and Drug Administration
The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...
for use as a food additive; however, there is some concern that they may block absorption, not only of cholesterol, but of other important nutrients as well. At present the American Heart Association
American Heart Association
The American Heart Association is a non-profit organization in the United States that fosters appropriate cardiac care in an effort to reduce disability and deaths caused by cardiovascular disease and stroke. It is headquartered in Dallas, Texas...
has recommended that supplemental plant sterols be taken only by those diagnosed with elevated cholesterol, and has particularly recommended that they not be taken by pregnant women or nursing mothers.
Chemical classification and structure
Sterols are a subgroup of steroidSteroid
A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...
s with a hydroxyl group at the 3-position of the A-ring. They are amphipathic lipids
Amphipathic lipids
Amphipathic lipids are molecules that are mostly lipid-like in structure, but at one end have a region that is polar or ionic . The hydrophilic region is usually referred to as the head group, and the lipid portion is known as the tail. Cell membranes typically consist of three separate classes...
synthesized from acetyl-coenzyme A via the HMG-CoA reductase
HMG-CoA reductase
HMG-CoA reductase is the rate-controlling enzyme of the mevalonate pathway, the metabolic pathway that produces cholesterol and other isoprenoids...
pathway. The overall molecule is quite flat. The hydroxyl group on the A ring is polar
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
. The rest of the aliphatic chain is non-polar
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
.
External links
- Sterols Cyberlipid.org