Fatty acids even-carbon

The synthesis of fatty acids two carbon atoms at a time from acetyl-CoA has been considered in Chapter 17 and is outlined in Fig. 17-12. In this pathway, which resembles the P oxidation sequence in reverse, the products are saturated fatty acids with an even number of carbon atoms as shown in Fig. 21-2. In this section, we will consider some of the factors that lead to variations in the chain lengths and types of fatty acids. 

Fatty Acids Fatty acids are long-chained carboxylic acids. Although their name includes the word fatty, fatty acids are not the same as fats. Fats are triglycerides, and fatty acids are one component of triglycerides. Naturally occurring fatty acids always have an even number of carbon atoms. Saturated fatty acids have carbon chains that contain only carbon-carbon single bonds. Unsaturated fatty acids have carbon chains that contain at least one carbon-carbon double bond. Monounsaturated fatty acids have one carbon-carbon double bond in the... 

Since the treatment of fatty acids with carbonated iJka-lies, instead of employing them in the caustio state, as in ordinary soap-making, involves the escape of carbonic acid, and a consequent swelling up of the materials when brought in contact, even without boiling, ample room most be left in the pan to allow for the great increase in bulk which occurs after repeated additions of alkali. 

Despite the strain inherent in cyclopropanes, Nature finds ways to make them. Bacteria, in particular, contain surprising amounts of cyclopropanated fatty acids. Even more remarkable is the conversion of such molecules into cyclopropenes. The source of the single extra carbon is 5-adenosylmethionine, the same agent involved in the methyl transfers discussed in Chapters 3 and 7 (pp. 142, 288 Fig. 10.85). The mechanism of this intriguing change hasn t been worked out yet. 

A concern (i.e., whether all the FA chains present in TAG species are detected) always exists when the described MDMS-SL approach is employed for identification of TAG species. Indeed, scanning 10 common fatty acids (even not including any fatty acids containing odd numbers carbon atoms) is enough to cover the major content of TAG species (i.e., >90 mol%) present in the majority of biological samples. It is always advised to scan all possibly occurring fatty acids if sample and resource permit. 

Branched-chain fatty acids occur widely in nature, but tend to be present as minor components except in bacteria, where they appear to replace unsaturated fatty acids functionaiiy. Usually, the branch consists of a single methyl group, either on the penultimate (/so) or antepenultimate (anteiso) carbon atoms (Figure 2.2). In the biosynthesis of these fatty acids, the primer molecules for chain-elongation by the fatty acid synthetase are 2-methylpropanoic and 2-methylbutanoic acids, respectively. Methyl branches can be found in other positions of the chain (on even-numbered carbon atoms), if methylmalonyl-coenzyme A rather than malonyl-coenzyme A is used in for chain extension this can occur in bacteria and in animal tissues, especially those of ruminant animals, where polymethyl-branched fatty acids even can be synthesised [275]. 

Fig. 5A. R vs. Ri for fatty acids of various chain lengths. The figure applies only to the special case of acetoacetate formation where 2-carbon fragments are converted quantitatively to acetoacetate (see text). R = C 0 C 00H ratio for carboxyl-labeled fatty acids Ri = C 0 C 00H ratio for penultimately labeled fatty acids c = number of carbons in a fatty acid (even series) / fractional degree of the conversion, (CH3CO—) —> (—CH2CO—) / = 1.0 is represented by the point (R = 1.0, Ri = 1.0). Note that when R = 1.0, Ri > 1.0. Small rectangular area is enlarged in Fig. 5B.

The fatty acids occur in nature chiefly as glycerides see fats), which constitute the most important part of the fats and oils, and as esters of other alcohols, the waxes. The naturally occurring fatty acids are mostly the normal straight-chain acids with an even number of carbon atoms. 

Acetyl coenzyme A is the biosynthetic precursor to the fatty acids, which most often occur naturally as esters Fats and oils are glycerol esters of long chain carboxylic acids Typically these chains are unbranched and contain even numbers of carbon atoms... 

Fats and oils (Section 26 2) Tnesters of glycerol Fats are solids at room temperature oils are liquids Fatty acid (Section 26 2) Carboxylic acids obtained by hydro lysis of fats and oils Fatty acids typically have unbranched chains and contain an even number of carbon atoms in the range of 12-20 carbons They may include one or more double bonds... 

Fatty acids derived from animal and vegetable sources generally contain an even number of carbon atoms siace they are biochemically derived by condensation of two carbon units through acetyl or malonyl coenzyme A. However, odd-numbered and branched fatty acid chains are observed ia small concentrations ia natural triglycerides, particularly mminant animal fats through propionyl and methylmalonyl coenzyme respectively. The glycerol backbone is derived by biospeciftc reduction of dihydroxyacetone. 

In recent years, especially in the USSR and Europe, synthetic fatty acids, prepared via hydrocarbon oxidation, have been used to prepare fatty amines (2,9). In 1978 Eastern Europeans produced an estimated 0.55 biUion kg of synthetic fatty acids with odd and even numbers of carbon atoms, whereas in the United States, production of natural fatty acids with even carbon atom chain-length acids was 435 million kg. To date, there has been no significant production of synthetic fatty acids in the United States. 

FIGURE 8.1 The structures of some typical fatty acids. Note that most natural fatty acids coutaiu au even number of carbon atoms and that the double bonds are nearly always ck and rarely conjugated. 

He observed that fatty acid analogs with even numbers of carbon atoms yielded phenyl acetate, whereas compounds with odd nnmbers of carbon atoms produced only benzoate. 

In essence, this series of four reactions has yielded a fatty acid (as a CoA ester) that has been shortened by two carbons, and one molecule of acetyl-CoA. The shortened fatty acyl-CoA can now go through another /3-oxidation cycle, as shown in Figure 24.10. Repetition of this cycle with a fatty acid with an even number of carbons eventually yields two molecules of acetyl-CoA in the final step. As noted in the first reaction in Table 24.2, complete /3-oxidation of palmitic acid yields eight molecules of acetyl-CoA as well as seven molecules of FADHg and seven molecules of NADFI. The acetyl-CoA can be further metabolized in the TCA cycle (as we have already seen). Alternatively, acetyl-CoA can also be used as a substrate in amino acid biosynthesis (Chapter 26). As noted in Chapter 23, however, acetyl-CoA cannot be used as a substrate for gluco-neogenesis. 

Even though acetate units, such as those obtained from fatty acid oxidation, cannot be used for net synthesis of carbohydrate in animals, labeled carbon from " C-labeled acetate can be found in newly synthesized glucose (for example, in liver glycogen) in animal tracer studies. Explain how this can be. Which carbons of glucose would you expect to be the first to be labeled by "Relabeled acetate ... 

Most fatty acids have an even number of carbon atoms, so none are left over after /3-oxidation. Those fatty acids with an odd number of carbon atoms yield the three-carbon propionyl CoA in the final j3-oxidation. Propionyl CoA is then converted to succinate by a multistep radical pathway, and succinate enters the citric acid cycle (Section 29.7). Note that the three-carbon propionyl group should properly be called propnnoyl, but biochemists generally use the non-systematic name. 

One of the most striking features of the common fatty adds is that they have an even number of carbon atoms (Table 27.1, p. 1062). This even number results because all fatty acids are derived biosynthelically from acetyl CoA by sequential addition of two-carbon units to a growing chain. The acetyl CoA, in turn, arises primarily from the metabolic breakdown of carbohydrates in the glycolysis pathway that weTl see in Section 29.5. Thus, dietary carbohydrates consumed in excess of immediate energy needs are turned into fats for storage. 

Write the equation for the final step in the /3-oxidation pathway of any fatty acid with an even number of carbon atoms. 

The fatty adds commonly encountered in biological systems are straight chained alkanoic or alkenoic adds, containing an even number of carbon atoms (usually Ch-Ch). natural n Senera / these fatty adds can be produced readily by extraction of the lipids from sources natural sources and saponifying the neutral triglycerides. This is satisfactory providing a mixture of fatty acids is acceptable. Purification of spedfic fatty adds from the saponification mixture increases the costs considerably. 

Stearic acid is a saturated fatty acid. This means it has only single bonds between its carbon atoms. This means it can coil up and form into random shapes. Double bonds between carbon atoms restrict the bending of the molecule at the point of the bond, like a hinge that lets a door swing back and forth but not up and down. Triple bonds are even more restrictive, locking the joint in place three-dimensionally, like the legs of a tripod. 

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