Fatty acyl group is the main component of lipid with complex structure, and it is one of the most basic classifications of biological lipid.
Fatty acyl groups include fatty acids and fatty acyl groups. Fatty acids and fatty acyl groups have a series of repeated methylene groups, which endows these lipids with hydrophobicity.
A fatty acid whose main chain consists of repeated methylene sequences with carboxyl groups at the end.
Classification of fatty acids: Generally speaking, fatty acids are classified according to their carbon chain length and saturation.
According to the length of carbon chain, fatty acids can be divided into short-chain fatty acids (SCFA, carbon number less than 6), medium-chain fatty acids (MCFA, carbon number between 6 ~ 12), long-chain fatty acids (LCFA, carbon number between 13 ~ 18) and ultra-long-chain fatty acids (VLCFA, carbon number between 6 ~18).
According to hydrocarbon chain saturation, fatty acids are divided into saturated fatty acids and unsaturated fatty acids. Among them, unsaturated fatty acids are divided into monounsaturated fatty acids and polyunsaturated fatty acids.
Common fatty acids are stearic acid, palmitic acid, oleic acid, linoleic acid, arachidonic acid and cyclic acid.
B fatty acyl, also known as fatty acyl compounds, refers to fatty acids and lipids containing fatty acid residues. Fatty acyl groups are various molecules obtained by fatty acid synthesis reaction of acetyl-CoA, malonyl-CoA and methylmalonic acid monoacyl-CoA.
Common fatty acyl groups are: fatty alcohol, fatty aldehyde, fatty ester, fatty amide, fatty nitrile, fatty ether, alkane, S- oxidized alkane and glycosyl fatty acyl group.
Glyceryl ester is the product of esterification of hydroxyl groups on glycerol with fatty acids. Esterification products of glycerol and fatty acids (except glycerophosphate).
Glyceryl esters are classified according to the number of esterified hydroxyl groups of glycerol. One hydroxyl group in glycerol is esterified into monoglyceride, two hydroxyl groups are esterified into diglyceride and three hydroxyl groups are esterified into triglyceride. Among them, triglycerides are the most common in nature.
In eukaryotic cells, triglycerides form tiny oil droplets in water medium, which are used as storage of metabolic fuel. These specialized cells in vertebrates are called adipocytes. Triglycerides are also stored in the seeds of many plants, providing energy and precursors for seed germination. Because of the long carbon chain and high reduction, triglycerides store more energy than sugar (twice), and the hydrophobicity of triglycerides ensures that there is no need to transport additional hydrate weight during transportation. In some animals, glyceride stored under the skin is not only a kind of energy, but also can keep the body warm at extremely low temperature. Seals, walruses, penguins and warm-blooded polar animals are all covered with very rich glycerides. Hibernating animals (such as bears) need to accumulate a lot of fat before hibernation. Fat can not only store energy, but also keep warm.
The amount of oleophospholipids esterified by hydroxyl groups on glycerol is divided into glycerol phosphate and lysoglycerol phosphate.
When two molecules of fatty acids are linked to the hydroxyl groups on C 1 and C2 of glycerol through ester bonds, one molecule of phosphate group with strong hydrophilicity is linked to the -OH of C3, forming the simplest glycerophosphate, namely phosphatidic acid. Phospholipids in eukaryotes and bacteria are in the sn-3 position of glycerol, while phospholipids in archaea are in the sn- 1 position of glycerol.
In phosphatidic acid, H+ on phosphate group is replaced by other groups to form other complex glycerophosphates. Such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylglycerol, etc.
In glycerophosphate, 1 or 2 hydroxyl groups of glycerol are generally linked to fatty acids in the form of ester bonds, but there are also alkyl groups or 1Z- alkenyl groups, namely alkyl ether phospholipids and alkenyl ether phospholipids (acetal phospholipids).
LysoGP: Monoglyceride is used as the mother nucleus, and the hydroxyl at sn-3 position is esterified with phosphoric acid, or phosphoric acid is combined with other substances containing hydroxyl to form ester.
Sphingolipid is a complex group of compounds with the same sphingosine base. C 1, C2 and C3 of sphingosine molecules have functional groups -OH, -NH2 and -OH, which are similar to the three hydroxyl groups of glycerol in glycerophosphate. It is synthesized from serine and acyl coenzyme A with long fatty chain, and then converted into ceramide, sphingolipid, sphingolipid and other compounds. The sphingolipids of mammals are mainly sphingomyelins, and those of insects are mainly ethanolamine phosphate ceramide. There are phytin and sphingolipid containing mannose in fungi.
Ceramide is a common sphingosine base derivative, which is composed of fatty acids and amino groups of sphingosine, mostly saturated fatty acids or monounsaturated fatty acids, and the carbon chain length is 16-26. It is the structural matrix of all sphingolipids (sphingomyelin, sphingolipids).
Sphingolipid consists of a molecule of long-chain sphingosine, a molecule of long-chain fatty acid and a molecule of polar head, and the polar head is connected by phosphoric acid through ester bond. Sphingolipin also has a polar head and two hydrophobic tails, but this molecule does not contain glycerol.
Sterol lipids, also known as sterols, are classified according to their biological functions.
Sterol esters include sterols, steroids, schizosteroids, bile acids and derivatives, steroid conjugates, hopanes, etc.
Sterols include cholesterol, ergosterol, stigmasterol, C24 propyl sterol, etc.
Cholesterol and its derivatives have been widely studied in mammalian system. Together with glycerophosphate and sphingomyelin, they constitute an important part of membrane lipids. Cholesterol is composed of three six-carbon rings (A, B and C rings) and one five-carbon ring (D ring). This mother nucleus is cyclopentane-polyphenanthrene mother nucleus.
Steroids also contain the same fused tetracyclic nucleus structure, which has different biological functions from hormones and signal molecules. These are subdivided according to the number of carbon in the core skeleton. C 18 steroids include estrogen family, while C 19 steroids include androgens such as testosterone and androsterone. C2 1 subclass contains two carbon side chains at C 17 position, including progesterone, glucocorticoid and mineralocorticoid. Schizosteroids include various forms of vitamin D, which is characterized by the opening of the B ring in the main structure of sterols. Other sterols include bile acids and their conjugated bases. Bile acids exist in animal bile, and bile acids separated from human and bovine bile are mainly cholic acid. Cholic acid is an emulsifier of oil (C 17 side chain is hydrophilic), and its physiological function is to emulsify fat and promote its hydrolysis and absorption in intestine. Therefore, cholic acid is called "biological soap" and is produced in the liver.
Sterols in plants are called phytosterols, such as β-sitosterol, stigmasterol and rapeseed sterol.
Ergosterol mainly exists in the cell membrane of fungi, and it is a compound with cyclopentane polyphenylanthracene as the mother nucleus.
Isopentenol lipids are compounds with isopentenol sequence as the mother nucleus and are fused isoprene units, that is, terpenes. Repetitive units are connected by carbon-carbon bonds and are nonpolar. Two isoprene units are connected end to end to form monoterpene; Terpenoids containing 4, 6 and 8 isoprene units are called diterpenes, triterpenoids or tetraterpenes, respectively. There are also some compounds that can be regarded as derivatives of isopentenol lipids, which are closely related to terpenoids, but their structural formulas are not even multiples of five-carbon units, such as sandalwood, which lacks a carbon atom, and borneol and camphor that we introduced.
Another important class of isoprene alcohol lipid molecules are quinones and phenols, such as vitamin E, vitamin K and coenzyme Q 10. Isopentene lipid contains many substances with different structures, so it is difficult to name its system. Traditionally, the names used mostly come from the source of raw materials of compounds, which is more confusing.
Glycolipids are compounds in which fatty acids are directly linked to the sugar skeleton, forming a structure compatible with the bilayer lipid membrane. In glycolipids, the backbone of glycerol in glycerides and phospholipids is replaced by sugar, and the number of fatty acyl groups does not need to distinguish which bond, amide bond or ester bond. Glycolipids can appear as polysaccharides or phosphorylated derivatives.
Polyketone is formed by polymerization of acetyl and propionyl units catalyzed by polyketone synthase. It is mainly divided into aromatic polyketone compounds and composite polyketone compounds. One of the most diverse natural products in structure and function, a large part of them are cyclic molecules, and the skeleton can be further glycosylated, methylated, hydroxylated and oxidized.
The structural diversity of natural products produced by polyketide synthase is much greater, and many of them have the characteristics of lipids. Class Ⅰ polyketide synthases form bound macrolides, which are usually 14-40 atoms in size, while class Ⅱ and Ⅲ polyketide synthases produce complex aromatic ring systems.
The main chain of polyketone compounds is usually further modified by glycosylation, methylation, hydroxylation, oxidation and/or other processes. Some polyketones are linked to peptides synthesized by non-ribosomes to form hybrid scaffolds. Many commonly used antibacterial, antiparasitic and anticancer drugs are polyketone or its derivatives. These drugs include erythromycin, tetracycline, nystatin, avermectin and anti-tumor epothilone.
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