Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Chapter I. BIOMOLECULES AND CELLULAR STRUCTURES

CHAPTER 5. LIPIDS. BIOMEMBRANES

5.2. STRUCTURE AND FUNCTIONS OF LIPIDS

Depending on the Chemical Structure of the components released upon complete Hydrolysis, Lipids are classified into the following groups:

1. Simple Lipids.

1.1. Acylglycerols.

1.2. Sterides.

1.3. Cerides (Waxes).

2. Complex Lipids.

2.1. Phospholipids.

2.1.1. Glycerophospholipids.

2.1.2. Sphingophospholipids.

2.2. Glycolipids.

2.2.1. Glycosylglycerols.

2.2.2. Glycosphingolipids.

1. Simple lipids — lipids that yield an alcohol and Fatty acids upon hydrolysis.

1.1. Acylglycerols (acylglycerides) — lipids that are esters of glycerol and fatty acids. Acylglycerols (specifically triacylglycerols, or triglycerides) are commonly known as neutral fats. Triglycerides are the main component of adipocytes in Human and Animal adipose tissue, serving as the storage molecular form of Higher Fatty Acids—the most energy-dense metabolic fuel. Natural triglycerides are mixed lipids, meaning their structure incorporates residues of various fatty acids.

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1.2. Sterides — lipids that are esters of cyclic alcohols, sterols, and fatty acids. Sterols are 3-hydroxy derivatives of the hydrocarbon steran (cyclopentanoperhydrophenanthrene). The most widespread sterol in animal organisms is Cholesterol, which Functions as a structural lipid in Plasma Membranes and serves as a precursor in the Synthesis of Other sterols and their derivatives (Steroids).

Biologically important sterols include Steroid Hormones OF the adrenal cortex, male and Sex Hormones, Vitamin D Group and their derivatives, and Bile acids.

1.3. Cerides (waxes) — simple lipids that are esters of higher Fatty Acids and high-molecular-weight alcohols, notably cetyl (С₁₆Н₃₃ОН) and myricyl (С₃₀Н₆₁ОН) alcohols. Animal-derived waxes include beeswax, spermaceti, and lanolin, which are used in pharmacy for the preparation of ointments and creams, as well as in the manufacturing of cosmetics.

2. Complex lipids — lipids that release an alcohol (glycerol, sphingosine, Inositol) upon hydrolysis, as well as phosphate, amino compounds, and CARBOHYDRATES. Complex lipids are polar, amphiphilic compounds, and most of them perform structural functions as components of Introduction/36.html">Biological Membranes.

2.1. Phospholipids.

A large group of complex lipids consists of phospholipids, which are subdivided into glycerophospholipids and sphingophospholipids depending on the alcohol moiety in their structure.

Phospholipids include:

Glycerophospholipids (phosphoglycerides) are esters of glycerol and higher fatty acids, which are derivatives of phosphatidic acid esterified with amino alcohols such as Choline, ethanolamine (colamine), and the oxyamino acid Serine.

The phosphodiester bond in glycerophospholipids is formed by the hydroxyl groups of choline (phosphatidylcholines, or lecithins), ethanolamine (phosphatidylethanolamines, or cephalins), or serine (phosphatidylserines).

Natural glycerophospholipids contain unsaturated fatty acid residues at the central (2nd, β-) position and saturated fatty acid residues at the terminal (1st, α-) position.

Sphingophospholipids are complex esters of the polyhydric amino alcohol sphingosine and higher fatty acids, which may also contain residues of choline and phosphoric acid. The N-acyl derivatives of sphingosine and Fatty acids are called ceramides:

Sphingophospholipids are phosphoric esters of ceramides and amino alcohols, namely choline, ethanolamine, and serine. Human and higher animal neural tissue contains sphingomyelins (N-acyl-sphingenyl-phosphocholines):

2.2. Glycolipids are compounds in which the lipid moiety is covalently linked to a carbohydrate moiety. Glycolipids are complex esters of higher fatty acids and glycerol or sphingosine, and they contain a carbohydrate component (specifically, glucose, galactose, and their derivatives, or an oligosaccharide group).

Glycosylglycerols (glycosylglycerides) are glycolipids that are esters of glycerol.

Glycosphingolipids are glycolipids that are esters of N-acylsphingosines (ceramides). This class of glycolipids (or Sphingolipids) is of great biological significance due to the widespread presence of glycosphingolipids in Biomembranes, particularly in neural tissue; furthermore, there is A number of hereditary disorders associated with impaired METABOLISM of this lipid class.

Depending on The structure of the carbohydrate portion of the molecule, glycosphingolipids are divided into several classes: cerebrosides, gangliosides, sulfatides, and globosides:

a) cerebrosides are ceramide monohexosides; the most common representatives are galactocerebrosides and glucocerebrosides:

Galactocerebrosides are natural cerebrosides found in the membranes of Brain Neurons; an abnormal accumulation of glucocerebrosides in the brain is observed in Gaucher disease, which is accompanied by delayed mental development and severe neurological disorders.

b) sulfatides are sulfated derivatives of cerebrosides, the most common representative being galactocerebroside sulfate (3'-sulfogalactocerebroside).

A genetic defect leading to an elevated content of galactocerebroside sulfate in neural tissue occurs in the hereditary disease known as metachromatic leukodystrophy, which manifests as severe psychiatric disorders.

c) globosides are oligosaccharide derivatives (oligohexosides) of ceramides.

The oligosaccharide residue of globosides most frequently contains galactose, glucose, or N-acetylgalactosamine. Examples of globosides include the dighexoside lactosylceramide of Erythrocyte membranes and the trihexoside galactosyllactosylceramide.

d) gangliosides are glycosphingolipids that contain, In addition to galactose, glucose, and N-hexosamines in their oligosaccharide chain, one or more residues of the nine-carbon monosaccharide neuraminic or N-acetylneuraminic (sialic) acid.

A special nomenclature is used to designate individual gangliosides. According to this system, a ganglioside molecule is denoted by the letter G (ganglioside) followed by subscripts consisting of letters that indicate the number of sialic acid residues (M—mono-; D—di-; T—tri; Q—quatro) and conventional numerical designations:

where: Cer is ceramide; Glc is glucose; Gal is galactose; NeuAc is N-acetylneuraminic acid.

Gangliosides are most abundant in the membranes of ganglion neurons. Their abnormal accumulation in the brain resulting from a genetic defect in the Enzymes responsible for their metabolism (gangliosidoses) leads to severe neuropsychiatric disorders: G_M2-gangliosidosis (Tay-Sachs disease), G_M1-gangliosidosis, etc.

The classes of lipids considered above (simple and complex) are capable of hydrolysis. At the same time, lipids also include numerous Biomolecules that exhibit lipid physicochemical properties (Hydrophobicity, insolubility in polar Solvents) but cannot be hydrolyzed into simpler compounds. These include free fatty acids and biologically active derivatives of arachidonic acid (Eicosanoids), higher alcohols, Isoprenoids and their derivatives—particularly various steroids—as well as Fat-soluble Vitamins (A, D, E, K).

Lipoproteins are Complexes of Proteins with lipids of various chemical structures. The formation of lipoproteins involves non-covalent bonds and physicochemical interactions (hydrogen, ionic, Van der Waals, and hydrophobic interactions).

Of greatest biochemical significance are lipoproteins in which lipids are present in human Blood Plasma, as well as lipoproteins that serve as integral components of biological membranes.

Blood plasma lipoproteins are molecular complexes of various lipid classes (primarily triglycerides, cholesterol, and phosphoglycerides) with proteins, forming micellar structures in which lipids are solubilized (suspended) in plasma, which is physically and chemically an aqueous solution. The physiological function of these lipoproteins is the interorgan transport of lipids, classifying them as transport lipoproteins. The Chemical Composition and metabolism of human blood plasma transport lipoproteins will be discussed in detail below (Chapter 16).