Lipids are an important class of biomolecules in the organism, with a wide variety of properties such as insoluble in water and soluble in non-polar organic solvents. Lipids are involved in regulating many life activities and abnormalities in lipid metabolism may cause many diseases such as obesity, atherosclerosis, and diabetes. Lipidomic analysis is used to elucidate lipid metabolism by studying the composition, structure, and quantification of lipids in biological samples, and can be used to search for biomarkers and to study the mechanisms of lipid molecules at the aggregate level in various life phenomena. The National Institutes of Health (NIH) has broadly classified lipids into eight major groups.
Fatty acid lipidomics
Fatty acids are the simplest type of lipid and are also components of many more complex lipids. Fatty acid lipidomics molecules contain a long aliphatic hydrocarbon chain with a carboxyl group at one end. The general formula for a straight-chain saturated fatty acid is C(n)H(2n+1)COOH.
Glycerol ester lipidomics
Glycerol esters are the products of esterification of hydroxyl groups of glycerol with fatty acids. Glycerol esters are classified according to the number of hydroxyl groups of glycerol that are esterified. One hydroxyl group of glycerol is esterified as monoglyceride, two hydroxyl groups are esterified as diglyceride, and three hydroxyl groups are esterified as triglyceride. Among vegetable oils and animal fats, triglycerides are predominant, but they are broken down into monoglycerides, diacylglycerides and free fatty acids because of the naturally occurring enzymes.
Glycerophospholipids, commonly referred to as phospholipids, are lipids containing phosphoric acid, the main type of lipid found in cell membranes. 2 molecules of fatty acids are linked to the hydroxyl groups on C1 and C2 of glycerol by ester bonds, and a molecule of hydrophilic, highly polar phosphate group is linked to the -OH of C3 to form the simplest glycerophospholipids, phosphatidic acid. Phospholipids in eukaryotes and bacteria have their polar head attached to the sn-3 position of glycerol, while phospholipids in archaea have their polar head attached to the sn-1 position of glycerol. The basic structure of glycerophospholipids is phosphatidic acid and a substituent group attached to phosphate, which can be divided into many groups due to different substituents, such as choline, cardiolipin, inositol, etc.
Sphingolipid lipidomics have a common sphingosine backbone and are converted into ceramides, phosphosphingolipids, glycosphingolipids, etc. by the ab initio synthesis of serine and long fat chains of acyl coenzyme A. Among them, ceramide Cer is a common sphingosine derivative with a fatty acid attached to an amide group. Most of the fatty acids are saturated or monounsaturated, with a carbon chain length of about 16 to 26 carbons. Sphingolipids in mammals are mainly sphingolipids, whereas in insects they are mainly phosphoethanolamine ceramides. Glycosphingolipids are compounds in which sphingolipids and sugars are linked by glycosidic bonds, such as cerebrosides, which are simple in structure, and gangliosides, which are more complex.
Solid alcohol lipids are a class of cyclopentane polyhydrophenanthrene derivatives formed by three hexane rings and one cyclopentane, which are widely found in cells and tissues of plants and animals, except for bacteria. Solid alcohols mainly include cholesterol and its derivatives, and sterols all have the same four-ring structure and contain hormones and signaling molecules that have important roles in the body. Other sterols include bile acids and their conjugate bases, which are derivatives of oxidized cholesterol in mammals and are mainly produced in the liver. Sterols in plants are called phytosterols, such as beta-glutensterols, soy sterols and canola sterols. The main sterol in fungal cell membrane is ergosterol.
Pregnenolone lipids lipidomics
It is synthesized from five-carbon isopentenyl diphosphate and dimethyl allyl diphosphate. Carotenoids are important simple isoprenoids and precursors of vitamin A. Another important class of molecules are quinones and hydroquinones. Vitamin E, vitamin K and coenzyme Q10 also belong to this group.
A general term for substances formed by the combination of sugars and lipids. Glycolipids are lipid compounds that contain glycosyl ligands. It is a class of amphiphilic molecules that are widely found in living organisms. Glycolipids are also divided into two major groups: glycosylglycerols and glycosphingolipids. Glycosphingolipids are divided into neutral glycosphingolipids and acidic glycosphingolipids. Glycosphingolipids are also important components of cell surface antigens, and some normal cells have significant changes in surface glycosphingolipid composition after carcinogenesis.
Polyketides consist of subunits of acetyl and propionyl coenzyme A, which include a large number of secondary metabolites and natural products of animals, plants, bacteria, fungi and marine organisms, and are very different in structure. Most of the primary structures are rings, and their primary structures can be glycosylated, methylated, hydroxylated, oxidized, etc. Polyketide lipidomics molecules or their derivatives contain many commonly used antibacterial and anticancer agents, such as erythromycin, tetracycline antibiotics and anti-tumor epi-mycin.