Shortening carbon chain

In contrast to pheromones that involve single complex compounds, many moth species have been found to utilize a specific blend of relatively simple fatty acid-derived compounds. It appears that the evolution of a unique enzyme, A1 desaturase, used in combination with 2-carbon chain-shortening reactions (Figure 3) has allowed moth species to produce a variety of unsaturated acetates, aldehydes, and alcohols that can be combined in almost unlimited blends to impart species specificity. For example, biosynthetic precursors for the six-component pheromone blend of acetates for the cabbage looper moth (12) (Figure 2) can be determined easily from the cascade of acyl intermediates produced by the A11-desaturase and chain-shortening reactions (Figure 3). 

Evidence for the occurrence of a controlled, 2-carbon chain shortening reaction was obtained using the orange tortrix moth,... 

First, the use of two specific reactions — All desaturation and controlled 2 carbon chain shortening of fatty acid precursors to account for the biosynthesis of a large number of pheromones — has been an extremely fruitful approach. Even in a case where it seemed uncertain if this approach was appropriate (22)r it turned out that it was (23.). Other reactions should now be added to increase the range of products accounted for. Examples already mentioned include the A10 desaturase and the chain elongation of branched-chain starting materials. Other functional groups that appear in sex pheromones should also be accounted for, such as epoxides. 

In some cases, acyl-CoA conjugates formed from xenobiotic acids can also enter the physiological pathways of fatty acids catabolism or anabolism. A few examples are known of xenobiotic alkanoic and arylalkanoic acids undergoing two-carbon chain elongation or two-, four- or even six-carbon chain shortening. In addition, intermediate metabolites of (3-oxidation may be seen, as illustrated in Figure 32.10 with valproic acid, whose acyl-CoA intermediate (22) is a substrate for some first steps of 3-oxidation. ... 

Finally hydroxy-acyl-CoA is dehydrogenated to 3-keto-acyl-CoA. Then, thiolytic cleavage of the 3-keto-acyl-CoA produces a two-carbon chain-shortened aeyl-CoA plus acetyl-CoA. Each cycle yields an acyl-CoA shortened by two earbon atoms, an acetyl-CoA, and one nicotinamide adenine dinucleotide (NADH) and one flavin adenine dinucleotide (FADH2) as electron carriers (or reducing equivalents). 

Alkyl radicals, R, react very rapidly with O2 to form alkylperoxy radicals. H reacts to form the hydroperoxy radical HO2. Alkoxy radicals, RO, react with O2 to form HO2 and R CHO, where R contains one less carbon. This formation of an aldehyde from an alkoxy radical ultimately leads to the process of hydrocarbon chain shortening or clipping upon subsequent reaction of the aldehyde. This aldehyde can undergo photodecomposition forming R, H, and CO or, after OH attack, forming CH(0)00, the peroxyacyi radical. 

The carbon chains of samrated fatty acids form a zigzag pattern when extended, as at low temperamres. At higher temperatures, some bonds rotate, causing chain shortening, which explains why biomembranes become thinner with increases in temperamre. A type of geometric isomerism occurs in unsaturated fatty acids, depending on the orientation of atoms or groups around the axes of double bonds, which do not allow rotation. If the acyl chains are on the same side of the bond, it is cis-, as in oleic acid if on opposite sides, it is tram-, as in elaidic acid, the tram isomer of oleic acid (Fig-... 

Fig. 2 Action of desaturases and limited chain shortening can produce a variety of mono-unsaturated acyl-CoA precursors that can be modified to form unsaturated pheromone compounds. The arrow pointing down indicates limited chain shortening by two carbons. Modification of all 16-, 14-, 12-, and 10-carbon acyl-CoA derivatives on the carbonyl carbon can account for the majority of monounsaturated acetate esters, aldehydes, and alcohols identified as sex pheromones...

Macrolide aggregation pheromones produced by male cucujid beetles are derived from fatty acids. Feeding experiments with labeled oleic, linoleic, and palmitic acids indicate incorporation into the macrolide pheromone component [ 117 ]. The biosynthesis of another group of beetle pheromones, the lactones, involves fatty acid biosynthetic pathways. Japonilure and buibuilactone biosynthesized by the female scarab, Anomalajaponica, involves A9 desaturation of 16 and 18 carbon fatty acids to produce Z9-16 CoA and Z9-18 CoA,hydroxylation at carbon 8 followed by two rounds of limited chain shortening and cyclization to the lactone [118]. The hydroxylation step appears to be stereospecific [118]. 

Bile acids are synthesized from cholesterol in the liver (see p.314). Their structures can therefore be derived from that of cholesterol. Characteristic for the bile acids is a side chain shortened by three C atoms in which the last carbon atom is oxidized to a carboxyl group. The double bond in ring B is reduced and rings... 

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