The LIPID MAPS Lipid Classification System proposed by The Lipid Maps Program funded by The National Institutes of Health (NIH) in 2003 classifies LIPID into eight major groups.
- Fatty acids (FA)
Fatty acids are the simplest type of fat and are also a component of many more complex fats. Fatty acids contain a long aliphatic hydrocarbon chain with a carboxyl group at one end. The general formula of straight-chain saturated fatty acids is C(n)H(2n+1)COOH.
Glycerides are the esterification products of the hydroxyl groups of glycerol and fatty acids. Glycerides are classified according to the number of esterified hydroxyl groups of glycerol. One hydroxyl group in glycerol is esterified as monoglyceride, two hydroxyl groups are esterified as diglyceride, and three hydroxyl groups are esterified as triglyceride. Among them, vegetable oils and animal fats are mainly triglycerides.
Glycerophospholipids (phospholipids) are lipids containing phosphoric acid and are the main type of lipid present in cell membranes. Phospholipids in eukaryotes and bacteria have their polar heads connected to the sn-3 position of glycerol, while phospholipids in archaea have their polar heads connected to the sn-1 position of glycerol. The basic structure of glycerophospholipid is phosphatidic acid and a substituent group connected to phosphoric acid. Due to the different substituent groups, it can be divided into many categories, such as choline, cardiolipin, inositol and so on.
Sphingoid molecules have a common sphingoid base skeleton, which is synthesized from serine and long aliphatic acyl-CoA and then converted into ceramide, sphingoid, and glycosphingolipids. Among them, ceramide (Cer) is a common sphingosine derivative with a fatty acid linked to an amide group. The sphingolipids in mammals are mainly sphingomyelin, while the sphingomyelin in insects is mainly phosphoethanolamine ceramide. Glycosphingolipids are compounds in which sphingolipids and sugars are linked by glycosidic bonds, such as cerebrosides with simple structures and more complex gangliosides.
Sterol esters are a kind of cyclopentane polyhydrophenanthrene derivatives formed by three hexane rings and one cyclopentane. Except for the lack of bacteria, it is widely present in the cells and tissues of animals and plants. Sterols mainly include cholesterol and its derivatives. Sterols include hormones and signaling molecules that play important roles in the body, such as estrogen, androgens, testosterone, androgens, progesterone, glucocorticoids and mineralocorticoids, and are also precursors of many vitamin D. Other sterols include bile acids and their conjugate bases.Other sterols include bile acids and their conjugate bases. These substances are derivatives of mammals after oxidizing cholesterol, and are mainly produced in the liver. Sterols in plants are called phytosterols, such as β-sitosterol, stigmasterol and rapeseed sterol. The main sterol in fungal cell membranes is ergosterol.
- Pregnenolone lipids (PR)
Pregnenolone lipids are synthesized from isopentenyl diphosphate and dimethylallyl diphosphate. Carotenoids are important simple isoprenoids and precursors of vitamin A. Another important class of molecules is quinone and hydroquinone. Vitamin E, Vitamin K and Coenzyme Q10 also belong to this category.
- Saccharolipids (SL)
The term saccharolipids is formed by combining sugar and lipid. Saccharolipids are chemical compounds belonging to the general category of glycolipids. Saccharolipids refer to lipid compounds containing glycosyl ligands, which are a class of amphiphilic molecules that exist widely in organisms. Saccharolipids are also divided into two categories: glycosylglycerols and glycosphingolipids. Glycosphingolipids are divided into neutral glycosphingolipids and acid glycosphingolipids.
- Polyketides (PK)
Polyketones are composed of subunits of acetyl and propionyl-CoA, including a large number of secondary metabolites and natural products of animals, plants, bacteria, fungi, and marine organisms, with very different structures. Most of the main structure is a ring, and its main structure can be glycosylated, methylated, hydroxylated, oxidized and so on. Polyketones or their derivatives include many commonly used antibacterial and anticancer drugs, such as erythromycin, tetracycline antibiotics, and antitumor epimycin.