A. Classification •
The lipids are a large and heterogeneous group of substances of biological origin that are easily dissolved in organic solvents such as methanol, acetone, chloroform, and benzene. By contrast, they are either insoluble or only poorly soluble in water. Their low water solubility is due to a lack of polarizing atoms such as O, N, S, and P (see p. 6).
Lipids can be classified into substances that are either hydrolyzable— i. e., able to undergo hydrolytic cleavage—or nonhydrolyzable. Only a few examples of the many lipids known can be mentioned here. The individual classes of lipids are discussed in more detail in the following pages.
Hydrolyzable lipids (components shown in brackets). The simple esters include the fats (triacylglycerol; one glycerol + three acyl residues); the waxes (one fatty alcohol + one acyl residue); and the sterol esters (one sterol + one acyl residue). The phospholipids are esters with more complex structures. Their characteristic component is a phosphate residue. The phospholipids include the phosphatidic acids (one glycerol + two acyl residues + one phosphate) and the phosphatides (one glyc-erol + two acyl residues + one phosphate + one amino alcohol). In the sphingolipids, glycerol and one acyl residue are replaced by sphingosine. Particularly important in this group are the sugar-containing glycolipids (one sphingosine + one fatty acid + sugar). The cerebrosides (one sphingosine + one fatty acid + one sugar) and gangliosides (one sphin-gosine + one fatty acid + several different sugars, including neuraminic acid) are representatives of this group.
The components of the hydrolyzable lipids are linked to one another by ester bonds. They are easily broken down either enzymatically or chemically.
Non-hydrolyzable lipids. The hydrocarbons include the alkanes and carotenoids. The lipid alcohols are also not hydrolyzable. They include long-chained alkanols and cyclic sterols such as cholesterol, and steroids such as es-tradiol and testosterone. The most important acids among the lipids are fatty acids. The eicosanoids also belong to this group; these are derivatives of the polyunsaturated fatty acid arachidonic acid (see p. 390).
1. Fuel. Lipids are an important source of energy in the diet. In quantitative terms, they represent the principal energy reserve in animals. Neutral fats in particular are stored in specialized cells, known as adipocytes. Fatty acids are released from these again as needed, and these are then oxidized in the mitochondria to form water and carbon dioxide, with oxygen being consumed. This process also gives rise to reduced coenzymes, which are used for ATP production in the respiratory chain (see p. 140).
2. Nutrients. Amphipathic lipids are used by cells to build membranes (see p. 214). Typical membrane lipids include phospholipids, glycolipids, and cholesterol. Fats are only weakly amphiphilic and are therefore not suitable as membrane components.
3. Insulation. Lipids are excellent insulators. In the higher animals, neutral fats are found in the subcutaneous tissue and around various organs, where they serve as mechanical and thermal insulators. As the principal constituent of cell membranes, lipids also insulate cells from their environment mechanically and electrically. The impermeability of lipid membranes to ions allows the formation of the membrane potential (see p. 126).
4. Special tasks. Some lipids have adopted special roles in the body. Steroids, eicosa-noids, and some metabolites of phospholipids have signaling functions. They serve as hormones, mediators, and second messengers (see p. 370). Other lipids form anchors to attach proteins to membranes (see p. 214). The lipids also produce cofactors for enzymatic re-actions—e.g., vitamin K (see p. 52) and ubiquinone (see p. 104). The carotenoid retinal, a light-sensitive lipid, is of central importance in the process of vision (see p. 358).
Several lipids are not formed independently in the human body. These substances, as essential fatty acids and fat-soluble vitamins, are indispensable components of nutrition (see pp.364ff.)
Fatty acids and fats
The naturally occurring fatty acids are carbox-ylic acids with unbranched hydrocarbon chains of 4-24 carbon atoms. They are present in all organisms as components of fats and membrane lipids. In these compounds, they are esterified with alcohols (glycerol, sphingosine, or cholesterol). However, fatty acids are also found in small amounts in unesterified form. In this case, they are known as free fatty acids (FFAs). As free fatty acids have strongly amphipathic properties (see p. 28), they are usually present in protein-bound forms.
The table lists the full series of aliphatic carboxylic acids that are found in plants and animals. In higher plants and animals, un-branched, longchain fatty acids with either 16 or 18 carbon atoms are the most common— e.g., palmitic and stearic acid. The number of carbon atoms in the longer, natural fatty acids is always even. This is because they are bio-synthesized from C2 building blocks (see p. 168).
Some fatty acids contain one or more isolated double bonds, and are therefore "unsaturated." Common unsaturated fatty acids include oleic acid and linoleic acid. Of the two possible cis-trans isomers (see p. 8), usually only the cis forms are found in natural lipids. Branched fatty acids only occur in bacteria. A shorthand notation with several numbers is used for precise characterization of the structure of fatty acids—e g., 18:2;9,12 for linoleic acid. The first figure stands for the number of C atoms, while the second gives the number of double bonds. The positions of the double bonds follow after the semicolon. As usual, numbering starts at the carbon with the highest oxidation state (i. e., the carboxyl group corresponds to C-1). Greek letters are also commonly used (a = C-2; p = C-3; ro = the last carbon, ro-3 = the third last carbon).
Essential fatty acids are fatty acids that have to be supplied in the diet. Without exception, these are all polyunsaturated fatty acids: the C20 fatty acid arachidonic acid (20:4;5,8,11,14) and the two Ci8 acids linoleic acid (18:2;9,12) and linolenic acid (18:3;9,12,15). The animal organism requires arachidonic acid to synthesize eicosanoids
(see p. 390). As the organism is capable of elongating fatty acids by adding C2 units, but is not able to introduce double bonds into the end sections of fatty acids (after C-9), arachi-donic acid has to be supplied with the diet. Linoleic and linolenic acid can be converted into arachidonic acid by elongation, and they can therefore replace arachidonic acid in the diet.
Fats are esters of the trivalent alcohol glycerol with three fatty acids. When a single fatty acid is esterified with glycerol, the product is referred to as a monoacylglycerol (fatty acid residue = acyl residue).
Formally, esterification with additional fatty acids leads to diacylglycerol and ultimately to triacylglycerol, the actual fat (formerly termed "triglyceride"). As triacylglycer-ols are uncharged, they are also referred to as neutral fats. The carbon atoms of glycerol are not usually equivalent in fats. They are distinguished by their "sn" number, where sn stands for "stereospecific numbering."
The three acyl residues of a fat molecule may differ in terms of their chain length and the number of double bonds they contain. This results in a large number of possible combinations of individual fat molecules. When extracted from biological materials, fats always represent mixtures of very similar compounds, which differ in their fatty acid residues. A chiral center can arise at the middle C atom (sn -C-2) of a triacylglycerol if the two external fatty acids are different. The monoacylglycerols and diacylglycerols shown here are also chiral compounds. Nutritional fats contain palmitic, stearic, oleic acid, and linoleic acid particularly often. Unsaturated fatty acids are usually found at the central C atom of glycerol.
The length of the fatty acid residues and the number of their double bonds affect the melting point of the fats. The shorter the fatty acid residues and the more double bonds they contain, the lower their melting points.
Number of carbons
Formic acid Acetic acid Propionic acid Butyric acid Valerianic acid Caproic acid Caprylic acid Capric acid Lauric acid Myristic acid Palmitic acid Stearic acid Oleic acid Linoleic acid Linolenic acid Arachidic acid
10 : 0 12 : 0 14 :0 16 : 0 18 : 0 18 : 1 18 : 2 18 :3 20 :0
Not contained in lipids
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