Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Molecules of Which We Are Made
Lipids
Lipid Fatty Acids

With the exception of the first few members of the series, which are Water-soluble, Fatty acids exhibit strongly pronounced hydrophobic properties. Nevertheless, they are all acids, with a pK_а of approximately 4.8. Naturally occurring free Fatty acids are typically located at the lipid-water interface, featuring dissociated carboxyl groups projecting into the aqueous phase. However, natural fatty acids are usually esterified or linked via amide bonds to Other components of Complex Lipids.

Despite the great diversity of fatty acids found in any given Organism, only a few usually predominate. Higher plants primarily contain palmitic acid (C₁₆ acid) and two C₁₈ unsaturated acids, oleic and linoleic. The C₁₈ saturated stearic acid is almost entirely absent in plants, and acids ranging from C₂₀ to C₂₄ are rare (with the exception of leaf epidermis). At the same time, members of certain taxonomic groups contain unusual fatty acids, which serves to classify these organisms within that specific group; for instance, members of the family Compositae (such as daisies) contain acetylenic fatty acids, whereas castor bean seeds contain a unique hydroxy fatty acid (Fig. 2-32).

Class="center">Table 2-8 Structure of Selected Phosphatides and Sphingolipids

Animal Tissues also contain palmitic and oleic acids, but in addition, they feature high concentrations of stearic acid. C₂₀, C₂₂, and C₂₄ acids are also present, and overall, the array of fatty acids in animals is significantly more diverse than in plants. Curiously, the Unsaturated fatty acids of most organisms contain double bonds exclusively in the cis-configuration. Table 2-9 presents the fatty acid composition of a typical triglyceride mixture.

Table 2-9 Fatty Acid Composition of Selected Fats and Oils^a

^a Gunstone F. D., An Introduction to the Chemistry and Biochemistry of Fatty Acids and Their Glycerides, 2nd ed., Chapter 6. Chapman and Hall, London, 1967.

Bacteria generally lack polyunsaturated fatty acids, but frequently contain Branched-Chain Fatty Acids, as well as cyclopropane-containing acids, hydroxy acids, and free unesterified fatty acids. The fatty acid composition varies across different Organs of the same organism. For example, the lipid component of Biological Membranes may consist of up to 90% Phospholipids. Phospholipids, in turn, are characterized by a higher content of unsaturated fatty acids compared to triglycerides.