Structural Biochemistry - A Study Guide - E. A. Bessolitsyna 2015

Neutral Fats, or Mono-, Di-, and Triacylglycerols

Neutral fats are the most widely distributed group of Lipids in nature. These compounds are esters of Fatty acids and glycerol. One, two, or three hydroxyl groups of glycerol can be esterified with fatty acids to form mono-, di-, and triacylglycerols, respectively. Although the terms "mono-, di-, and triglycerides" are frequently used to denote these glycerol esters, they are not chemically rigorous. The general formula of triacylglycerols, where R, Ŕ, and Ŕ́ represent identical or different fatty acid residues linked to glycerol via ester bonds (Figure 46).

Figure 46. Structural formula of a triacylglycerol (triglyceride)

The nomenclature of neutral fats is based on the names of their constituent fatty acids. For instance, tristearin contains three stearic acid residues per molecule, whereas oleodistearin contains one oleic and two stearic acid residues. Triacylglycerols containing identical fatty acid residues at all three positions are termed simple triacylglycerols; their names are derived directly from the corresponding fatty acid. For example, tristearoylglycerol, tripalmitoylglycerol, and trioleoylglycerol contain residues of stearic, palmitic, and oleic acids, respectively. Trivial names for these compounds—namely, tristearin, tripalmitin, and triolein—are more commonly used. Triacylglycerols containing two or three different fatty acid residues are called mixed triacylglycerols. Most natural fats, such as olive oil, butter, and other dietary fats, consist of complex mixtures of simple and mixed triacylglycerols comprising fatty acids that vary in both chain length and degree of saturation.

Neutral fats are weak amphiphiles and are considerably more hydrophobic than free fatty acids. This is because their ester bonds—like the free hydroxyl groups in mono- and diacylglycerols—are un-ionized and only weakly polar. Consequently, The properties of these substances are primarily governed by the presence of hydrophobic alkyl groups in their molecules. Fats are insoluble in Water yet readily soluble in nonpolar Solvents.

Triacylglycerols containing exclusively saturated fatty acid residues exist as solids at room Temperature. A prime example is tristearin, the principal component of beef tallow. Conversely, triacylglycerols containing three Unsaturated fatty acids (such as triolein, the major component of olive oil) are liquid at room temperature. Butter is a mixture of triacylglycerols, some of which incorporate relatively short-chain fatty acids; because shortening the fatty acid chain lowers its melting point, butter maintains a soft consistency at room temperature. The richer a fat is in short-chain and unsaturated fatty acid residues, the higher its solubility and the lower its melting point. Both saturation and increased chain length elevate the melting point; thus, at room temperature, tristearin is a solid (m.p. 71°C), whereas triolein (m.p. –17°C) and tributyrin (m.p. –75°C) are liquids.

Most animal fats predominantly contain varying proportions of palmitic, stearic, palmitoleic, oleic, and linoleic acid esters. Fats from different Tissues of the same Organism can differ significantly in composition. For example, human subcutaneous fat is richer in saturated fatty acids than Liver fat, which, in turn, contains a higher proportion of unsaturated fatty acids. Milk fat and butterfat contain the highest amounts of short-chain fatty acids compared to other fats. The subcutaneous fats of various mammals differ markedly in the degree of unsaturation of their fatty acid residues, as reflected by differences in their melting points: beef fat has a relatively high melting point, whereas lard melts at a much lower temperature.

Plant fats (oils) also exhibit great diversity in their fatty acid composition, varying in both hydrocarbon chain length and degree of unsaturation. Many of these exist as liquids at room temperature.

Hydrolysis and Saponification

Triacylglycerols undergo hydrolysis under acidic conditions to yield three molecules of Fatty acid and one molecule of glycerol (Figure 47).

This reaction proceeds quite slowly in boiling water, but is significantly accelerated by catalytic concentrations of H⁺. In animal and plant tissues, this reaction is catalyzed by Enzymes known as esterases or, more specifically, lipases.

The Cleavage of neutral fats by alkalis (OH^-) is termed saponification (Figure 47). The resulting carboxylate ions (R-COO^-) react with cations to form soaps. Potassium hydroxide yields liquid soap, whereas sodium hydroxide produces hard soap. The reaction is irreversible; carboxylate ions do not recombine with the hydroxyl groups of glycerol. Saponification is facilitated by the fact that the initial batches of formed soap act as detergents. The final products—soaps and glycerol—are soluble in water and insoluble in nonpolar solvents such as ether. This property is frequently utilized in Lipid Chemistry to fractionate mixtures of substances. The saponifiable fraction is defined as the portion of total lipids that becomes water-soluble and ether-insoluble following Treatment with hot alkali. Thus, neutral fats are saponifiable.

Figure 47. Hydrolysis and saponification of triacylglycerols

Other chemical properties of triacylglycerols are determined by the Structure and properties of their constituent fatty acids, with reactions taking place at the hydrocarbon radicals of the fatty acids. Accordingly, triacylglycerols undergo the same reactions as free fatty acids. Reduction leads to margarine production, while oxidation results in the rancidification of oils.

Triacylglycerols are the primary component of fat depots in PLANT AND ANIMAL Cells. They are absent from membranes. Notably, triacylglycerols are nonpolar, hydrophobic molecules because they lack charged or strongly polar functional groups. In most plant and animal cells, triacylglycerols reside in the Cytosol as finely dispersed, emulsified oil droplets. In specialized animal Connective Tissue cells—namely, adipocytes (fat cells)—vast quantities of triacylglycerols can be stored as lipid droplets that occupy nearly the entire Cell volume. Fat cells are abundant subcutaneously, within the Abdominal cavity, and in the Mammary Glands.

Triacylglycerols perform the following functions:

Energy storage. They serve as a more compact and efficient energy source than CARBOHYDRATES. The oxidation of lipids yields twice as much energy as the oxidation of an equivalent amount of carbohydrates. Furthermore, lipids are more stable because they can be accumulated in very large quantities in a virtually pure, unhydrated state. Consequently, animals that expend substantial energy on locomotion and other activities rely primarily on triacylglycerols for long-term energy reserves. In overweight individuals, fat cells accumulate kilograms of triacylglycerols, the stored energy of which could sustain the body's basal METABOLISM for several months. In contrast, energy stored as Glycogen can generally support the organism for no more than a day.

Source of endogenous water. Lipid oxidation generates significant amounts of metabolic water. Therefore, desert animals such as camels store large quantities of fat, yielding both energy and water upon oxidation; because fats are hydrophobic, they do not disrupt the body's water balance. Gradual oxidation replenishes depleted water levels without interfering with osmotic Homeostasis. The same principle applies to animals undergoing seasonal hibernation (such as bears and hedgehogs).

Mechanical protection. Deposited within the subcutaneous adipose tissue of the soles and palms, fats form a cushioning layer that protects bones from impact during locomotion.

Thermal protection. It forms a heat-insulating layer that reduces the body's heat loss and protects it from the effects of extremely low temperatures (found in seals, walruses, penguins, and other warm-blooded Arctic animals).

Chemical protection. Hydrophobic foreign substances are sequestered in lipid droplets, thereby isolating them from other cells in the organism.

Neutral lipids known as alkyl glycerol ethers have been isolated from human tumor cells and bovine erythrocytes. In these compounds, a long-chain (saturated or unsaturated) alcohol is linked to glycerol via an ether bond. Available data suggest that these lipids do not exist in a free state in nature, but rather as esterified derivatives of long-chain fatty acids called alkyldiacylglycerols.

Glycosyldiacylglycerols have been isolated from plants and Bacteria. One representative of this lipid type, extracted from Chloroplasts and chromatophores of photosynthetic bacteria, contains 6-sulfo-6-deoxyglucose (quinovose) linked glycosidically to diacylglycerol. The Biological Role of this Class of lipids has not yet been fully established.

This group of glycerol-containing lipids comprises derivatives of L-glycerol-3-phosphate. Their Classification is based on the type of bond between the hydrocarbon moiety and glycerol, as well as The Nature of the polar groups (other than phosphate) within the molecule. The biological role of these amphiphilic molecules is highly significant, as they account for up to 50% of all lipids in Introduction/36.html">Biological Membranes.

Phospholipids include phosphatidic acid, phosphatidylglycerols, phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, lysophospholipids, plasmalogens, and sphingomyelins (Figure 48).

Figure 48: Structural formulas of phospholipids and plasmalogens: A - phosphatidic acid; B - cardiolipin; C - phosphatidylethanolamine; D - phosphatidylserine; E - phosphatidylcholine; F - plasmalogen

Phosphatidic Acid and Phosphatidylglycerols

Phosphoglycerides are widely distributed in plants, animals, and microorganisms, and can be viewed as derivatives of L-phosphatidic acid (1,2-diacyl-L-glycerol-3-phosphate). Among this group, the most common are phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine, which contain the nitrogenous bases Choline, ethanolamine, and Serine, respectively. Phosphatidylcholines are also referred to as lecithins. In most natural phosphatides, saturated fatty acids (C₁₆—C₁₈) are typically located at the C-1 position, whereas unsaturated fatty acids (C₁₆—C₂₀) containing one to four double bonds occupy the C-2 position, although exceptions to this rule are known.

At physiological pH values, phosphatides exist as zwitterions predominantly in the ionic forms shown above. Thus, the ionized groups form the highly polar hydrophilic region of the molecule, while the two acyl residues constitute its hydrophobic domain. The Functional groups of ethanolamine and serine have the following pK values: serine 9.15 (amino group) and 2.25 (carboxyl group); ethanolamine 9.5 (amino group). Choline (β-hydroxyethyltrimethylammonium hydroxide) is a strong base comparable to KOH and dissociates completely in water to yield choline and hydroxyl ions.

The melting point is determined by the fatty acids composing the phospholipids, similarly to triacylglycerols.

Phosphoglycerides undergo the same reactions as triacylglycerols, namely hydrolysis and saponification, as well as Reactions Involving the hydrocarbon chains of their fatty acids.

Lysophospholipids. This group of compounds comprises phosphoacylglycerols containing only a single acyl radical. An example is lysolecithin, which plays a crucial role in Phospholipid Metabolism. Phosphatidic acid serves as an important intermediate in the Biosynthesis of triacylglycerols and phospholipids, yet it is present in tissues only in trace amounts.

With the exception of lysophospholipids and phosphatidic acid, all other lipids are Structural components of membranes. Within membranes, however, they may perform additional functions that depend on the specific type of phospholipid, as discussed below.

Phosphatidylcholine (Lecithin). Lecithins, much like simple fats, contain glycerol and fatty acids, but they also incorporate phosphoric acid and choline. Widely distributed across various tissue cells, lecithins perform both metabolic and structural functions within membranes. Dipalmitoyl lecithin is a highly effective surfactant that reduces surface tension, thereby preventing the collapse of the inner surfaces of the respiratory Airways in the Lungs. Its absence in the lungs of premature newborns leads to The Development of respiratory distress syndrome. The majority of phospholipids contain a saturated acyl radical at the C-1 position and an unsaturated radical at the C-2 position.

Phosphatidylethanolamine (Cephalin). Cephalins differ from lecithins solely in that their choline moiety is replaced by ethanolamine.

Phosphatidylinositol. Another group of nitrogen-free derivatives of α-phosphatidic acid contains Inositol. The hydrolysis of 1 mole of phosphoinositide typically yields 1 mole of glycerol, 1 mole of the hexahydric alcohol myo-inositol, 2 moles of fatty acid, and 1–3 moles of phosphate. 4-diphospho- and 4,5-triphosphoinositides are found in Brain tissues, accounting for over half of all phosphoinositides.

Triphosphoinositide. 1-phosphatidyl-myo-inositol-4,5-bisphosphate. In this compound, inositol is represented by one of its stereoisomers, myo-inositol. Myo-inositol is one of the nine stereoisomeric forms of the carbocyclic hexahydric alcohol and belongs to the class of substances known as cyclitols, which are cycloalkanes bearing a hydroxyl group on each of three or more carbon atoms of the ring. Myo-inositol is a vital cyclitol widely distributed in various microorganisms, higher plants, and animals. In plants, it is found as its hexaphosphate (phytic acid) or as its magnesium-calcium salt (phytin). The biochemical interest in myo-inositol stems from its nutritional role as well as its involvement in lipid and Carbohydrate Metabolism. Phosphatidylinositol-4,5-bisphosphate is a key component of cellular membrane phospholipids; upon stimulation by an appropriate hormone, it is cleaved into diacylglycerol and inositol trisphosphate, both of which act as intracellular messengers, or second messengers.

Phosphatidylserine. Tissues also contain a cephalin-related phospholipid that incorporates a serine residue instead of ethanolamine. Although phosphatidylserine is a constituent of membranes in virtually all prokaryotic and Eukaryotic cells, it typically remains a minor membrane component. Its highest concentration is found in mammalian brain tissue (constituting about 15% of total phospholipids), whereas in other Organs such as The Heart, liver, Kidneys, Spleen, and lungs, its content is less than 10%. Phosphatidylserine plays a vital role in cellular activity by acting as a regulator for numerous membrane-bound enzymes. In many cells, phosphatidylserine can serve as a precursor in The biosynthesis of phosphatidylethanolamine, being converted into the latter through the action of the membrane enzyme phosphatidylserine decarboxylase. Additionally, phospholipids containing a Threonine residue have been isolated.

Diphosphatidylglycerols. As the name implies, diphosphatidylglycerols from animal and plant tissues contain 1 mole of glycerol and 2 moles of L-phosphatidic acid (per mole of diphosphatidylglycerol). A prominent representative of this group found in animal tissues is cardiolipin, first isolated from Cytology/cytology/34.html">Cardiac Muscle tissue. Cardiolipin is a phospholipid localized in The inner mitochondrial membrane. It remains the only phosphatide with well-characterized immunological properties. Structurally similar compounds have also been detected in plant tissues; however, plant phosphatides contain only 1 mole of phosphatidic acid and are therefore classified as monophosphatidylglycerols. All these compounds differ in the composition of their constituent fatty acids. It is synthesized from phosphatidylglycerol.

These phosphoglycerides contain an $\alpha,\beta$-unsaturated alcohol attached via an ether bond at the C-1 position of L-glycero-3-phosphate (in contrast to the ester bond formed by a fatty acid residue). Phosphatidalcholines, phosphatidalethanolamines, and phosphatidalserines are the three main classes of plasmalogens. The ether linkages of the $\alpha,\beta$-unsaturated alcohols in these compounds are stable in dilute alkalis, but they undergo hydrolysis in dilute acids to yield the corresponding $\alpha,\beta$-unsaturated aldehyde. The $\alpha,\beta$-unsaturated alcohols possess a cis-configuration and chain lengths ranging from C12 to C18. In pure phosphatidalcholine from bovine heart, the fatty acid residue at the C-2 position is typically unsaturated, whereas the alcohol moiety at the C-1 position is generally saturated (with the exception of the $\alpha,\beta$-double bond). Plasmalogens lacking nitrogenous bases have been discovered in animal tissues, alongside neutral plasmalogens structurally similar to triacylglycerols, which contain two ester bonds and one ether bond formed by an $\alpha,\beta$-unsaturated alcohol at the C-1 position (1-alkenyl-2,3-diacylglycerol). Derivatives of 1-alkyl-2-acylphosphatidylcholine are widely distributed in animal tissues and share a common general formula. Although these substances account for only a few percent of the total phosphoglycerides in living organisms, they are found in substantial amounts in erythrocytes, and in the tissues of certain invertebrates, their concentration can reach up to 25% of the total phosphoglycerides. The PHYSICOCHEMICAL PROPERTIES OF plasmalogens closely resemble those of phospholipids, and their primary function is serving as structural components of biological membranes.

Although the majority of lipids are glycerol derivatives, it is now known that all organisms — animals, plants, and microorganisms — contain at least trace amounts of lipids derived from diols. Such diols include Ethylene glycol, 1,2- and 1,3-propanediols, 1,3-, 1,4-, and 2,3-butanediols, as well as 1,5-pentanediol. Diol lipids encompass mono- and diacyl derivatives representing esters of various fatty acids, diethes, mixed alkyl (or 1-alkenyl) and acyl derivatives, diol analogs of phosphatidylcholine and phosphatidylethanolamine, diol choline plasmalogen, acylated diol galactosides, and diol lipoamino acids. Typically, diol lipids account for only 0.5–1.5% of the concentration of glycerol-containing lipids. However, certain marine Mollusks and Echinoderms contain comparable amounts of ethylene glycol and glycerol derivatives, particularly in late summer or autumn. Since the content of diol lipids in the tissues of these organisms declines during winter and spring, it has been suggested that they may serve as an energy reserve. Normal rat liver contains relatively low amounts of diols; however, during Early stages of liver regeneration, they are present in significantly higher quantities. Data on the biological function of diol lipids remain scarce, apart from the aforementioned potential role of these compounds as an energy reserve in certain marine invertebrates. It is also hypothesized that certain diols may substitute for a fraction of structurally similar glycerol-containing lipids.