FUNDAMENTALS OF BIOCHEMISTRY. READER - G. A. Sevryukova - 2018

CHAPTER 4. BIOCHEMISTRY OF FATS: STRUCTURE, CLASSIFICATION, FUNCTIONS

4.1. Structural Features of Lipids (Fats)

Lipids are Water-insoluble, oily, or greasy substances that can be extracted from Cells using Solvents such as ether or chloroform. There are several classes of lipids, each performing specific biological Functions. Lipids play a vital role in Cell architecture and functioning.

Fatty acids are long-chain organic acids containing from 4 to 24 carbon atoms. They contain a single carboxyl group and a long non-polar hydrocarbon "tail," which renders most lipids insoluble in water (Fig. 32).

Class="center">Fig. 32. Stearic acid

The long hydrocarbon chain making up the molecular tail can be fully saturated—meaning it contains only single bonds—or unsaturated, containing one or more double bonds. Double bonds in fatty acids are never conjugated; a methylene group is always positioned between them.

Common fatty acids are insoluble in water, but in dilute NaOH or KOH solutions, they can form micelles, converting into so-called soaps—salts of fatty acids.

Na⁺ and K⁺ soaps are capable of emulsifying water-insoluble oils and fats. In this process, the hydrocarbon tails of the soap embed into fat droplets, while the polar heads interact with water. Thus, soaps form a hydrophilic shell around fat droplets, creating a finely dispersed mixture known as micelles (Fig. 33).

Ca²⁺ and Mg²⁺ fatty acid soaps dissolve very poorly and therefore do not emulsify fats. This explains The formation of white flakes when bath soap, consisting mainly of K⁺ soaps, is dissolved in hard water containing Ca²⁺ and Mg²⁺ salts.

Fig. 33. Formation of micelles around fat droplets

4.2. Main Classes and Representatives of Lipids (Fats)

Among lipids, neutral fats (triacylglycerols) are the most widespread. They are esters formed by glycerol and three fatty acid molecules (Fig. 34)

Fig. 34. Structure of a neutral fat — triacylglycerol

Triacylglycerols containing only saturated fatty acid residues have a solid consistency at room Temperature (e.g., tallow), whereas those containing three unsaturated acids are liquid (e.g., olive oil). It is worth noting that the shorter the fatty acid chain, the lower the melting point. When exposed to air, triacylglycerols containing unsaturated acids undergo autoxidation; oxygen reacts with fatty acid residues at the double bond sites, leading to the formation of compounds that give fats an unpleasant, rancid taste.

The primary function of triacylglycerols is lipid storage, as they serve as "fuel" for most organisms. Triacylglycerols are significantly better suited for energy storage than Glycogen: first, they can accumulate in very large quantities in an unhydrated form, and second, per unit weight, they store twice as much energy as CARBOHYDRATES. In the body, triacylglycerols are stored as fat droplets within adipocytes. In some animals (such as seals, walruses, and penguins), subcutaneous triacylglycerol deposits function as an energy reserve and thermal insulator, protecting the Organism from extreme cold.

Membrane Lipids include Phospholipids, which serve as Structural components of membranes and are never stored in large amounts. Phosphoglycerides play The Role of the principal phospholipid component in membranes. All phosphoglyceride molecules possess two non-polar hydrophobic tails and a polar hydrophilic HEAD (Fig. 35).

Fig. 35. Structure of phosphoglycerides

The Cell membrane is a structure composed of a lipid bilayer permeated by Proteins (Fig. 36). The Lipid Bilayer consists primarily of phospholipid molecules and acts as a barrier to water penetration. Large Globular proteins are distributed throughout the membrane. These are classified into peripheral proteins, semi-integral proteins, and integral proteins, which span the entire cell membrane. Most peripheral proteins function as Enzymes, while integral proteins form pores and channels, thereby facilitating the Transport of substances to which the lipid bilayer is impermeable.

Fig. 36. STRUCTURE OF THE cell membrane

The second major class of membrane lipids is Sphingolipids. There are three subclasses of sphingolipids: sphingomyelins, cerebrosides, and gangliosides. Unlike cerebrosides and gangliosides, sphingomyelins contain phosphocholine in their polar head group.

Sphingomyelins are found in myelin sheaths, providing insulation for nerve fibers. This ensures that electrical impulses travel strictly along a specific fiber without jumping to adjacent nerve fibers.

Cerebrosides are localized primarily in Brain cells, where they play a role in transmitting nerve impulses between Neurons. Gangliosides serve as essential structural Components of the receptor domains in cell membranes.

All the lipids discussed above belong to the category of saponifiable lipids; that is, when heated with alkalis, they undergo Hydrolysis, and their fatty acid residues form soaps. However, Cells also contain nonsaponifiable lipids, which lack fatty acid residues and therefore cannot form soaps. Nonsaponifiable lipids include Steroids, represented by Cholesterol (Fig. 37).

Fig. 37. Structure of the cholesterol molecule

Cholesterol is the primary structural component of cell membranes. Residing within the cell membrane alongside phospholipids and proteins, cholesterol ensures selective membrane permeability and exerts a regulatory influence on The activity of membrane-bound enzymes.

Cholesterol is synthesized from two molecules of acetyl-coenzyme A and acts as a precursor for the Synthesis of Other steroids, such as Bile acids and Steroid Hormones.

In the Skin, cholesterol—or more precisely, its oxidation product, 7-dehydrocholesterol—is converted into vitamin D₃ upon exposure to UV rays. Thus, the physiological functions of cholesterol are diverse.

Prostaglandins are present in all Organs and Tissues, exerting potent pharmacological effects on numerous physiological functions. They regulate renal hemodynamics, smooth Muscle contractility, gastric secretion, and lipid as well as Water-Salt METABOLISM.

The precursors for all prostaglandins are polyunsaturated fatty acids, notably arachidonic acid. Released from biological membrane phosphoglycerides by the action of phospholipases, arachidonic acid gives rise to prostaglandins and Leukotrienes.

Prostaglandins can induce physiological imbalances in the body. For instance, an overproduction of thromboxane A promotes platelet aggregation, thereby contributing to thrombosis, whereas an overproduction of prostacyclin relaxes vascular smooth muscle fibers, leading to platelet disaggregation and a disruption of cardiovascular Homeostasis.

Aspirin (acetylsalicylic acid) blocks the action of prostaglandin synthetase, thereby preventing the formation of thromboxane and prostacyclin.

The primary BIOLOGICAL EFFECTS OF leukotrienes are associated with inflammatory processes, allergic responses, and immune reactions.

Vitamin E blocks the action of lipoxygenase and, consequently, prevents the synthesis of leukotrienes.

Waxes are esters formed from Higher Fatty Acids and higher monohydric or dihydric alcohols containing from 16 to 22 carbon atoms. Their general formulas are shown in Fig. 38, where R, R', and R" represent potential radicals.

Fig. 38. General formula of wax

Waxes can be a component of the sebum coating skin, fur, and feathers. In plants, waxes account for 80% of all lipids forming the protective film On the surface of leaves and fruits. Waxes are also known to be normal metabolites in certain microorganisms. Natural waxes (such as beeswax, spermaceti, and lanolin) typically contain, In addition to the aforementioned esters, varying amounts of free fatty acids, alcohols, and Hydrocarbons with 21–35 carbon atoms.