chapter  20
Unsaturated Fatty Acids
Pages 26

I. Introduction ........................................................................................................................ 514 II. Fatty Acid Biosynthesis...................................................................................................... 514

A. Carbon Source and De Novo Synthesis ..................................................................... 516 B. Desaturation................................................................................................................ 516

1. D9 Desaturase ...................................................................................................... 517 2. D12 Desaturase .................................................................................................... 518 3. D15 Desaturase .................................................................................................... 518 4. D6 and D5 Desaturases........................................................................................ 519 5. Regulation of Desaturase Activity....................................................................... 519

C. Elongation................................................................................................................... 520 D. Peroxisomal PUFA Synthesis..................................................................................... 520

III. PUFA Metabolism in Membranes...................................................................................... 521 A. Remodeling................................................................................................................. 521

1. Acyl-Specific Incorporation................................................................................. 522 IV. Dietary Sources of Unsaturated Fatty Acids...................................................................... 523

A. Microbes ..................................................................................................................... 523 B. Agricultural Products.................................................................................................. 524 C. Effect of Agriculture on the Composition of the Food Supply ................................. 525

V. Nutritional Effects of Unsaturated Fatty Acids.................................................................. 525 A. Role of PUFA in Cell Physiology.............................................................................. 526

1. Unsaturated Fatty Acids and Membrane Structure ............................................. 526 2. Alterations in Membrane-Associated Enzyme Activity ...................................... 528

VI. Synthesis and Abundance of PUFA................................................................................... 528 A. n-7 Fatty Acids ........................................................................................................... 528

1. Palmitoleic Acid (16:1n-7) .................................................................................. 528 2. Vaccenic Acid (18:1n-7) ..................................................................................... 528

B. n-9 Fatty Acids ........................................................................................................... 528 1. Oleic Acid (18:1n-9)............................................................................................ 528 2. Erucic Acid (22:1n-9) .......................................................................................... 529 3. Mead Acid (20:3n-9) ........................................................................................... 529 4. Other n-9 Fatty Acids .......................................................................................... 529

C. n-6 Fatty Acids ........................................................................................................... 529 1. Linoleic Acid (18:2n-6) ....................................................................................... 529 2. g-Linolenic Acid (18:3n-6).................................................................................. 529 3. Dihomo-g-Linolenic Acid (20:3n-6) ................................................................... 529 4. Arachidonic Acid (20:4n-6)................................................................................. 530 5. Docosatetraenoic Acid (22:4n-6)......................................................................... 530 6. Other n-6 Fatty Acids .......................................................................................... 530

530 1. a-Linolenic Acid (18:3n-3) ................................................................................. 530 2. Eicosapentaenoic Acid (20:5n-3) ........................................................................ 530 3. Docosapentaenoic Acid (22:5n-3) ....................................................................... 530 4. Docosahexaenoic Acid (22:6n-3) ........................................................................ 530 5. Other n-3 Fatty Acids .......................................................................................... 531

E. Unusual and Non-Methylene-Interrupted Fatty Acids ............................................... 531 VII. Summary............................................................................................................................. 532 References ..................................................................................................................................... 533

Fatty acids serve awide variety ofmetabolic functions critical to all forms of life. They are a rich source of energy and carbon and well designed as a convenient unit for energy storage. However, the importance of fatty acids in human nutrition and physiology goes well beyond their role as a source of calories. Fatty acids provide the structure and hydrophobicity crucial to the maintenance of a semipermeable membrane barrier. Their structures can be modified by desaturation and elongation to produce a substantial variety of species with individual chemical and physical properties. Ester linkages to glycerides allow fatty acids to be easily exchanged for one another and allow cells to manipulate the physical properties of their membranes. Fatty acids also serve as precursors to active signal molecules such as eicosanoids, which are capable of producing potent biological effects. Evolution has produced a distinction between plants and animals in their capabilities for the metabolism of fatty acids. Higher animals are unable to synthesize all of the fatty acids required for certain tissue functions and are obligated to ingest fatty acids that are synthesized by plants. Animals have evolved a separate and distinct series of metabolic modifications of fatty acids, but are still unable to alter the original modifications inserted by plants. As a result, the membrane, signal, and storage lipids of animals vary widely according to their dietary intakes. In addition, the ability of an animal to produce a specific fatty acid relies either on an inherent mechanism for desaturation of saturated fatty acids or on the ingestion of a convertible precursor. The ingestion or metabolism of particular unsaturated fatty acids is necessary for a great variety of physiological and cellular functions. Inadequate intake or defective metabolism leads to various dysfunctions due to deficiencies of these fatty acids in particular cellular locations. In addition, dietary fatty acids have beenwell correlatedwith metabolic and physiological alterations associated with heart disease and cancer [1-3]. Unsaturated fatty acids in particular play an important role in these non-energy-producing metabolic functions.