BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 13. STRUCTURE AND PROPERTIES OF LIPIDS

13.2. Neutral lipids

These include derivatives of glycerol, Higher Fatty acids, alcohols, and aldehydes with the general formula

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Fats (mono-, di-, and triacylglycerols) are esters of glycerol and monocarboxylic aliphatic acids. They are simple, widespread Lipids in nature that form The basis of plant oils and animal adipose tissue. Specialized Cells of adipose tissue, adipocytes, can accumulate significant amounts of triacylglycerols in the form of finely dispersed fat droplets that fill the entire volume of The Cell. These non-polar, hydrophobic compounds interact with other molecules exclusively via Structure/103.html">Van der Waals forces. Mono- and diglyceride molecules are more polar due to the presence of one or two hydroxyl groups:

Triacylglycerols containing identical fatty acid residues are called simple (tripalmitate, trioleate, etc.). If triacylglycerols contain different acyl radicals, they are called mixed, for example, palmitoleinostearate.

Depending on The Nature of the fatty acid residues, triacylglycerols are classified as saturated and unsaturated. Most natural fats are mixtures of simple and mixed, saturated and unsaturated triacylglycerols. The melting point of fats depends on The ratio of saturated and unsaturated fatty acid radicals in their molecules. Thus, unsaturated triacylglycerols have a significantly lower melting point than saturated ones. The presence of short-chain fatty acid residues within the molecule also lowers the melting point of triacylglycerols. Therefore, at room Temperature, saturated fats are in a solid state (mutton fat), while those containing short-chain fatty acid residues (C₄-C₁₀) are in a semi-solid consistency (butter), and unsaturated triacylglycerols are in a liquid state (sunflower oil).

Fats are insoluble in Water, yet readily soluble in non-polar organic Solvents. They undergo Hydrolysis upon boiling with acids and bases, or enzymatically via the action of triacylglycerol lipase. The hydrolysis of fats in NaOH and KOH solutions, known as saponification, leads to The formation of a mixture of K⁺ or Na⁺ salts of Fatty Acids and glycerol:

Unsaturated fats containing polyenic fatty acid residues eventually undergo oxidation via enzymatic or autocatalytic pathways involving atmospheric oxygen and light. The interaction between unsaturated fatty acid radicals and oxygen proceeds via chain reactions, initially forming peroxides, which act as catalysts in subsequent oxidation reactions to yield epoxides, methyl alkyl ketones, aldehydes, and organic acids According to the following scheme:

Products of Lipid Peroxidation drastically alter the chemical and organoleptic (taste) properties of fats, thereby diminishing their nutritional value. In cells, the Oxidation of Unsaturated fats is almost entirely blocked due to the presence of Vitamins A and E, selenium, and other antioxidants.

The primary function of fats in organisms is the storage of metabolic energy. This is a much more efficient form of energy deposition than starch and Glycogen. Firstly, per unit mass, triacylglycerols store twice as much energy as glucose; secondly, fats can be deposited in significantly larger quantities than glycogen. For instance, human adipose tissue can store 10-11 kg of fat, which is sufficient to meet the energy demands of a 70 kg individual during 40 days of starvation. For comparison, the glycogen reserves in The Liver and Muscles (~350 g) of a person of such mass under conditions of total starvation last for only one day.

Neutral lipids also include neutral plasmalogens. Natural plasmalogens are compounds featuring an ether bond, whose hydrophobic components are higher fatty acids and higher fatty aldehydes (1-alkenyl ether lipids) with the general formula:

Neutral lipids with an ether bond include alkyl neutral lipids, whose hydrophobic component in their structure is represented by higher fatty acids and alcohols, most commonly 1-O-alkyl-2,3-diacylglycerols with the general formula:

Cholesterol esters also belong to the class of neutral lipids. Cholesterol and its esters belong to the group of Steroids, whose structural core is based on the condensed ring system of cyclopentanoperhydrophenanthrene:

In the animal Organism, cholesterol Functions as a structural component of cell membranes and as a precursor for the synthesis of Steroid Hormones, vitamin D, and Bile acids. Complex esters of cholesterol with higher fatty acids, alcohols, and aldehydes, known as cholesterides, serve as forms for the TRANSPORT AND STORAGE of cholesterol within cells.

Among simple and complex cholesterol esters, the majority (about 70%) are accounted for by esters of higher fatty acids.

Neutral Glycolipids, which are widely distributed in nature within animal and plant Tissues, are represented mainly by mono- and digalactosylglycerols: