Sphingomyelin
The first part of the word “sphingo” indicates that the molecule contains, instead of glycerol, a dihydric unsaturated alcohol - sphingosine. The most widespread representative of this group of compounds in the body is sphingomyelin. Sphingomyelin is found in the membranes of plant and animal cells; Nervous tissue, and in particular the brain, is especially rich in sphingophospholipids.A characteristic feature of phospholipids is their diphilicity, that is, the ability to dissolve both in an aqueous environment and in neutral lipids. This is due to the presence of pronounced polar properties in phospholipids. At pH 7.0, their phosphate group always carries a negative charge. Nitrogen-containing groups in the composition of phosphatidylcholine (choline) and phosphatidylethanolamine (ethanolamine) at pH 7.0 carry a positive charge. Thus, at pH 7.0, these glycerophospholipids are bipolar zwitterions and their net charge is zero. The serine residue in the phosphatidylserine molecule contains an α-amino group and a carboxyl group. Therefore, at pH 7.0, the phosphatidylserine molecule has two negatively and one positively charged groups and carries a total negative charge.
At the same time, fatty acid radicals in phospholipids do not have an electrical charge in an aqueous environment and thus determine the hydrophobicity of part of the phospholipid molecule. The presence of polarity due to the charge of polar groups determines hydrophilicity. Therefore, at the oil-water interface, phospholipids are arranged in such a way that polar groups are in the aqueous phase and non-polar groups are in the oil phase. Due to this, in an aqueous environment they form a bimolecular layer, and upon reaching a certain critical concentration - micelles.]
This is the basis for the participation of phospholipids in the construction of biological membranes.
Ultrasound treatment of a diphilic lipid in an aqueous medium leads to the formation of liposomes. A liposome is a closed lipid bilayer, inside which is part of the aqueous environment. Liposomes are used in the clinic and cosmetology as unique containers and carriers of drugs, nutrients to certain organs and for a combined effect on the skin.
The functional role of phospholipids is not limited to their participation in the construction of biomembranes. Thus, they are regulators of enzyme activity. For example, phosphatidylcholine, phosphatidylserine, sphingomyelin activate or inhibit the activity of enzymes that catalyze blood clotting processes. The regulatory function of lipids lies in the fact that a number of hormones (sex hormones, adrenal hormones) are derivatives of lipids. In addition, phospholipids
They perform a detergent function in the intestines and gall bladder. They are an important structural component of bile, along with free cholesterol and bile acids. A change in the ratio of any of these components leads to the precipitation and formation of gallstones. Phospholipids are also an important component of mixed micelles that are formed during lipid digestion.
It is a source of arachidonic acid, a precursor of eicosanoids.
They are sources of secondary messengers - diacylglycerol and inositol triphosphate, as mentioned above
Provide attachment of proteins to the membrane. Some extracellular proteins are attached to the outside of the plasma membrane by forming covalent bonds with phosphatidylinositol. Examples of such proteins are enzymes: alkaline phosphatase, lipoprotein lipase, cholinesterase.
Take part in the formation of transport forms of other lipids
Can perform an energetic function
They are a component of lung surfactant (see below)
2.1.2 Sphingolipids (sphingophospholipids)
Sphingomyelins. These are the most common sphingolipids. They are mainly found in the membranes of animal and plant cells. Nervous tissue is especially rich in them. Sphingomyelins are also found in the tissues of the kidneys, liver and other organs. Upon hydrolysis, sphingomyelins form one molecule of fatty acid, one molecule of the dihydric unsaturated alcohol sphingosine, one molecule of nitrogenous base and one molecule of phosphoric acid. The general formula of sphingomyelins can be represented as follows:
The general plan for the construction of the sphingomyelin molecule in a certain respect resembles the structure of glycerophospholipids. The sphingomyelin molecule contains a polar “head”, which carries both positive (choline residue) and negative (phosphoric acid residue) charges and two non-polar “tails” (long aliphatic chain of sphingosine and fatty acid acyl radical).
2.2 Glycolipids (glycosphingolipids)
Glycolipids are widely present in tissues, especially in nervous tissue, in particular in the brain. The main form of glycolipids in animal tissues are glycosphingolipids. The latter contain ceramide, consisting of sphingosine alcohol and a fatty acid residue, and one or more sugar residues.
The simplest glycosphingolipids are galactosylceramides and glucosylceramides.
Galactosylceramides are the main sphingolipids of the brain and other nervous tissues, but are also found in small quantities in many other tissues. Galactosylceramides contain a hexose (usually D-galactose), which is linked by an ester bond to the hydroxyl group of the amino alcohol sphingosine. In addition, galactosylceramide contains a fatty acid. Most often it is lignoceric, nervonic or cerebronic acid, i.e. fatty acids with 24 carbon atoms. There are sulfogalactosylceramides, which differ from galactosylceramides by having a sulfuric acid residue attached to the third carbon atom of the hexose. In the mammalian brain, sulfogalactosylceramides are mainly found in the white matter, and their levels in the brain are much lower than those of galactosylceramides.
Glucosylceramides are simple glycosphingolipids, present in tissues other than the nervous tissue, mainly glucosylceramides. They are present in small quantities in brain tissue. Unlike galactosylceramides, they have a glucose residue instead of a galactose residue.
More complex glycosphingolipids are gangliosides, formed from glycosylceramides. Gangliosides additionally contain one or more sialic acid molecules. In human tissues, the dominant sialic acid is neuraminic acid. In addition, instead of a glucose residue, they often contain a complex oligosaccharide. Gangliosides are found in large quantities in nervous tissue. They apparently perform receptor and other functions. One of the simplest gangliosides is gametoside, isolated from the stroma of erythrocytes. It contains ceramide, one molecule of glucose, one molecule of N-acetylneuraminic acid.
2.3 Steroids
All lipids considered are usually called saponified, since their alkaline hydrolysis produces soaps. However, there are lipids that do not hydrolyze to release fatty acids. These lipids include steroids. Steroids are compounds widespread in nature. They are often found in association with fats. They can be separated from the fat by saponification (they end up in the unsaponifiable fraction). All steroids in their structure have a core formed by hydrogenated phenanthrene (rings A, B and C) and cyclopentane (ring D).
Phenanthrene Perhydrophenanthrene The general structural basis of steroids.
Steroids include, for example, hormones of the adrenal cortex, bile acids, vitamins D, cardiac glycosides and other compounds. In the human body, an important place among steroids is occupied by sterols (sterols), i.e. steroid alcohols. The main representative of sterols is cholesterol (cholesterol).
Due to the complex structure and asymmetry of the molecule, steroids have many potential stereoisomers. Each of the six-carbon rings (rings A, B and C) of the steroid core can take on two different spatial conformations - the “chair” or “boat” conformation.
In natural steroids, including cholesterol, all rings are in the shape of a “chair”, which is a more stable conformation. In turn, relative to each other, the rings can be in cis or trans positions.
Cholesterol. As noted, among steroids there is a group of compounds called sterols (sterols). Sterols are characterized by the presence of a hydroxyl group at position 3, as well as a side chain at position 17. In the most important representative of sterols, cholesterol, all rings are in the trans position; in addition, it has a double bond between the 5th and 6th carbon atoms. Therefore, cholesterol is an unsaturated alcohol. The ring structure of cholesterol is characterized by significant rigidity, while the side chain is relatively flexible. So, cholesterol contains an alcohol hydroxyl group at C-3 and a branched aliphatic chain of 8 carbon atoms at C-17. The chemical name for cholesterol is 3-hydroxy-5,6-cholestin. The hydroxyl group at C-3 can be esterified with a higher fatty acid, resulting in the formation of cholesterol esters (cholesterides).
Cholesterol is the source of formation of bile acids and steroid hormones in the body of mammals. The physiological functions of cholesterol are diverse.
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Sphingolipids. They are mainly found in the membranes of animal and plant cells. Nervous tissue is especially rich in them. Sphingomyelins are also found in the tissue of the kidneys, liver and other organs. Upon hydrolysis, sphingomyelins form one molecule of fatty acid, one molecule of the dihydric unsaturated alcohol sphingosine, one molecule of a nitrogenous base (usually choline) and one molecule of phosphoric acid. The general formula of sphingomyelins can be represented as follows:
The general plan for the construction of the sphingomyelin molecule in a certain respect resembles the structure of glycerophospholipids. The sphingomyelin molecule contains a polar “head”, which carries both positive (choline residue) and negative (phosphoric acid residue) charges, and two non-polar “tails” (a long aliphatic chain of sphingosine and a fatty acid acyl radical). In some sphingomyelins, for example those isolated from the brain and spleen, instead of sphingosine, the alcohol dihydrosphingosine (reduced sphingosine) was found:
7.6 Steroids
All lipids considered are usually called saponified, since their alkaline hydrolysis produces soaps. However, there are lipids that do not hydrolyze to release fatty acids. These lipids include steroids. Steroids are compounds widespread in nature. They are often found in association with fats. They can be separated from the fat by saponification (they end up in the unsaponifiable fraction). All steroids in their structure have a core formed by hydrogenated phenanthrene (rings A, B and C) and cyclopentane (ring D) (Fig. 24):
Figure 24 - Generalized steroid core
Steroids include, for example, hormones of the adrenal cortex, bile acids, vitamins D, cardiac glycosides and other compounds. In the human body, sterols (sterols) occupy an important place among steroids, i.e. steroid alcohols. The main representative of sterols is cholesterol (cholesterol).
Due to the complex structure and asymmetry of the molecule, steroids have many potential stereoisomers. Each of the six-carbon rings (rings A, B and C) of the steroid core can take on two different spatial conformations - the “chair” or “boat” conformation.
Cholesterol is the source of formation in the body of mammals of bile acids, as well as steroid hormones (sex and corticoid). Cholesterol, or more precisely the product of its oxidation - 7-dehydrocholesterol, is converted into vitamin D 3 in the skin under the influence of UV rays. Thus, the physiological function of cholesterol is diverse.
Cholesterol is found in animal fats, but not in vegetable fats. Plants and yeast contain compounds similar in structure to cholesterol, including ergosterol.
Ergosterol is a precursor of vitamin D. After exposure of ergosterol to UV rays, it acquires the property of having an antirachitic effect (when the B ring opens).
Restoration of the double bond in the cholesterol molecule leads to the formation of coprosterol (coprostanol). Coprosterol is found in feces and is formed as a result of the restoration by bacteria of the intestinal microflora of the double bond in cholesterol between the C 5 and C 6 atoms. 
These sterols, unlike cholesterol, are very poorly absorbed in the intestines and therefore are found in human tissues in trace amounts.
8 Chemistry of carbohydrates
The term “carbohydrates” was first proposed by Professor of Dorpat (now Tartu) University K.G. Schmidt in 1844. At that time, it was assumed that all carbohydrates have the general formula C m (H 2 O) n, i.e. carbohydrate + water. Hence the name "carbohydrates". For example, glucose and fructose have the formula C(H2O)6, cane sugar (sucrose) C12(H2O)11, starch [C6(H2O)5]n, etc. Later it turned out that a number of compounds, which in their properties belong to the class of carbohydrates, contain hydrogen and oxygen in a slightly different proportion than indicated in the general formula (for example, deoxyribose C 5 H 10 O 4). In 1927, the International Commission for the Reform of Chemical Nomenclature proposed replacing the term “carbohydrates” with the term “glycides,” but the old name “carbohydrates” has taken root and is generally accepted.
The chemistry of carbohydrates occupies one of the leading places in the history of the development of organic chemistry. Cane sugar can be considered the first organic compound isolated in a chemically pure form. Produced in 1861 by A.M. Butlerov's synthesis (outside the body) of carbohydrates from formaldehyde was the first synthesis of representatives of one of the three main classes of substances (proteins, lipids, carbohydrates) that make up living organisms. The chemical structure of the simplest carbohydrates was elucidated at the end of the 19th century. as a result of fundamental research by E. Fischer. A significant contribution to the study of carbohydrates was made by domestic scientists A.A. Colley, P.P. Shorygin, N.K. Kochetkov and others. In the 20s of this century, the foundations of the structural chemistry of polysaccharides were laid by the work of the English researcher W. Haworth. From the second half of the 20th century. There is a rapid development of the chemistry and biochemistry of carbohydrates, due to their important biological significance.
