Fatty acids interconvertibility

In eukaryotes, anabolic and catabolic pathways that interconvert common products may take place in specific subcellular compartments. For example, many of the enzymes that degrade proteins and polysaccharides reside inside organelles called lysosomes. Similarly, fatty acid biosynthesis occurs in the cytosol, whereas fatty... 

Using the omega nomenclature, the major classes of unsaturated fatty acids found in mammalian tissue are co-3, co-6 and co-9. They are not interconvertible (Figure 11.9). 

About 12 chemicals are widely recognised as vitamins (Table 15.3), although some are actually families of interconvertible chemical forms. Choline, inositol and bioflavonoids are considered to be possible vitamins. The essential fatty acids and essential amino acids are excluded... 

Generally, vitamin A serves three classes of functions (1) support of the differentiation of epithelial cells, (2) support of the viability of the reproductive system (fetal growth and vitality of the testes), and (3) utilization in the visual cycle. Dietary retinoic acid can support only the first function. Animals raised on diets containing retinoic acid as the only source of vitamin lose their ability to see in dim light and become sterile. In males, sperm production ceases. In females, fetuses are resorbed. Retinoic acid cannot be stored in the liver, as it lacks the hydroxyl group needed for attachment of the fatty acid. Retinyl esters, retinol, and retinal are interconvertible. Retinal can be oxidized to form retinoic acid. All three functions of vitamin A can be supported by dietary retinyl esters, retinol, or retinal. Although these forms can be converted to retinoic acid, retinoic acid apparently cannot be reduced to form retinal. These relationships are summarized in Figure 9.44. 

If carbohydrates or proteins are ingested in excess of the amounts necessary to maintain optimal supplies of glycogen in tissues or protein, the excess is converted to fat. Conversion of excess carbohydrate and/or protein into fat is biochemically an irreversible process. As a result, the body conserves the compounds that it can interconvert, uses them to supply energy when needed, or converts them to fatty acids when it is more efficient to store the carbon in the form of fat. The body tends to conserve its protein reserves and to draw on fat reserves preferentially in time of energy demand. 

Because glyceraldehyde-3-phosphate and dihydroxyacetone phosphate are readily interconverted, these two molecules (referred to the triose phosphates) are both considered to be Calvin cycle products. The synthesis of triose phosphate is sometimes referred to as the C3 pathway. Plants that produce triose phosphates during photosynthesis are called C3 plants. Triose phosphate molecules are used by plant cells in such biosynthetic processes as the formation of polysaccharides, fatty acids, and amino acids. Initially, most triose phosphate is used in the synthesis of starch and sucrose (Figure 13A). The metabolism of each of these molecules is briefly discussed below. 

Methylmalonyl-CoA is present biologically in both the D and L isomers, which can be interconverted by the enzyme methylmalonyl-CoA epimerase (Figure 18.19). This reaction is important in the metabolism of propionyl-CoA, which arises from oxidation of odd-numbered chains of fatty acids. L-methylmalonyl-CoA is further acted upon by methylmalonyl-CoA mutase. 

Biochemistry and molecular biology of structural components. The schistosome tegument contains several phospholipids, the most abundant being phosphatidylcholine (57%), phosphatidylethanolamine (17%), sphingomyelin, lysophosphatidylcholine, phosphatidylinositol and cerebroside glycolipids (7,34). Palmitic acid and oleic acid are the most abundant fatty acids (35). These molecules are derived from the host, as schistosomes are unable to synthesize cholesterol or long-chain fatty acids de novo (36). However, schistosomes can interconvert fatty acids and cleave the polar head from some phospholipids (37). 

The liver is clearly well equipped to utilize free fatty acids and to interconvert acetoacetate and hydroxybutyrate, but the virtual absence of 3-Oxoacid-CoA transferase and lipoprotein lipase means that any significant uptake of ketone bodies and triglycerides is restricted to extra-hepatic tissues. Heart and kidney contain the necessary enzymes to deal with all four fuels and this may reflect their high metabolic activity. Page and Williamson (1971) have shown that normal human brain has the capacity to utilize ketone bodies. 

The polyunsaturated fatty acids can be interconverted to a limited extent in the body, but there is a requirement for a dietary intake of linoleic acid (C18 2 C06) and linolenic acid (C18 3 0)3), as these two, which can each be considered to be the precursor of a family of related fatty acids and eicosanoids, cannot be synthesized in the body. 

New and important information on the interconvertibility of body fatty acids was obtained in 1940 by Stetten and Schoenheimer. These... 

The inability of animals to desaturate oleic acid towards the methyl end of the chain gives rise to distinct families of polyunsaturated fatty acids that are not interconvertible. In general, polyunsaturation in animals is... 

With hindsight, it is clear that the reasons why some polyunsaturated fatty acids have EFA activity and some do not can only be understood by studying the biochemical pathways by which they are interconverted and metabolized to more complex end-products. 

This sequence of desaturations and elongations enables tissues to produce a variety of polyunsaturated fatty acids tailored to their needs. Because, during the course of evolution, animals have lost the ability (retained by plants) to insert double bonds in positions 12 and 15, the members of these four families (n-3, n-6, n-7, n-9) cannot be interconverted in animal tissues. Linoleic acid and its relatives are termed essential because without them animals will die. Therefore, the first member of the series has to be supplied in the diet from plant sources. Arachidonic add, the main product of the elongation and desaturation of linoleic acid, has essential fatty acid activity in that it can cure the signs of EFA deficiency described earlier but it is not essential in the human diet as long as linoleic acid is supplied, i.e. it is an essential metabolite but not an essential nutrient for man. 

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