Reaction chain-shortening

A large number of moth sex pheromones are biosynthesized from common fatty acids by pathways involving three reactions chain shortening,... 

While two reactions, chain shortening and 11-desaturation, are key steps in the biosynthesis of many pheromones, these pathways also require a reductive step where the acid group is converted to an alcohol, acetate or aldehyde. Studies on the spruce budworm moth (12), Choristoneura fumiferana, and various Heliothis species (13.) indicate that there is a series of reactions, such as those in Fig. 3, to account for the various functionalities. Alcohols and acetates may be stored or released as pheromone aldehydes are too reactive (and potentially dangerous to the insect) to store and so are only produced immediately before release. 

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. 

Conversion of the aldehyde into a nitrile is accomplished by treatment of an aldose with hydroxvlamine to give an oxime (Section 19.8), followed by dehydration of the oxJme with acetic anhydride. The Wohl degradation does not give particularly high yields of chain-shortened aldoses, but the reaction is general for all aldopentoses and aldohexoses. For example, D-galactose is converted by Wohl degradation into n-lyxose. 

The general idea of peptide sequencing by Edman degradation is to cleave one amino acid at a time from an end of the peptide chain. That terminal amino acid is then separated and identified, and the cleavage reactions are repeated on the chain-shortened peptide until the entire peptide sequence is known. Automated protein sequencers are available that allow as many as 50 repetitive sequencing cycles to be carried out before a buildup of unwanted by products interferes with the results. So efficient are these instruments that sequence information can be obtained from as little as 1 to 5 picomoles of sample—less than 0.1 /xg. 

The four steps of the /3-oxidation pathway, resulting in the cleavage of an acetyl group from the end of the fatty-acid chain. The key chain-shortening step is a retro-Claisen reaction of a /3-keto thioester. Individual steps are explained in the text. 

Q Nucleophilic addition of coenzyme A to the keto group occurs, followed by a retro-Claisen condensation reaction. The products are acetyl CoA and a chain-shortened fatty acyl CoA. 

The retro-Claisen reaction occurs by initial nucleophilic addition of a cysteine -SH group on the enzyme to the keto group of the /3-ketoacyl CoA to yield an alkoxide ion intermediate. Cleavage of the C2-C3 bond then follows, with expulsion of an acetyl CoA enolate ion. Protonation of the enolate ion gives acetyl CoA, and the enzyme-bound acyl group undergoes nucleophilic acyl substitution by reaction with a molecule of coenzyme A. The chain-shortened acyl CoA that results then enters another round of tire /3-oxidation pathway for further degradation. 

Cultures are subject to time delay. New methodology using real-time polymerase chain reaction (PCR) shortens the reporting time. 

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). 

A peroxisomal dehydrogenase initiates the 3-oxidation reactions that shorten the chain to -18 carbons or less, at which point the fatty acyl CoA is transferred to mitochondria for complete degradation by (3-oxidation. 

Sex pheromone component ratio in the cabbage looper moth altered by a mutation affecting the fatty acid chain-shortening reactions in the pheromone biosynthetic pathway. Insect Biochemistry and Molecular Biology 24 373-381. 

Transketolase removes a two-carbon fragment from ketols such as fructose 6-phosphate (alternatively xylulose 5-phosphate or sedoheptu-lose 7-phosphate) through the participation of thiamine diphosphate. Nucleophilic attack of the thiamine diphosphate anion on to the carbonyl results in an addition product which then fragments by a reverse aldol reaction, generating the chain-shortened aldose erythrose 4-phosphate, and the two-carbon carbanion unit attached to TPP (Figure 8.5) (compare the role of TPP in the decarboxylation of a-keto... 

It is possible to degrade 2-amino-2-deoxyhexoses to 2-amino-2-deoxy-pentoses by means of chain-shortening reactions from the nonreducing end of the molecule. Thus, Wolfrom and Anno38 converted ethyl 2-acetamido-2-deoxy-1 -thio-a-d-glucofuranoside (XV) into 2-amino-2-deoxy-a-D-xylose hydrochloride (XVI) by sequential application of periodate oxidation, borohydride reduction, and hydrolysis, as in the following reaction scheme. 

Jurenka R. A., Haynes K. F., Adlof R. O., Bengtsson M. and Roelofs W. L. (1994) Sex pheromone component ratio in the cabbage loopermoth altered by a mutation affecting the fatty acid chain-shortening reactions in the pheromone biosynthetic pathway. Insect Biochem. Mol. Biol. 24, 373-381. 

Wolf W. A. and Roelofs W. L. (1983) A chain-shortening reaction in orange tortrix moth sex pheromone biosynthesis. Insect Biochem. 13, 375-379. 

The above data suggest that pheromone component biosynthesis is related to fatty acid biosynthesis, but that some unusual reactions need to be added. Consideration of a large number of pheromone components (2) suggests that these reactions include chain-shortening by two carbons and desaturation at the 11-12 position. Fig. 1 shows the variety of potential intermediates that can be produced from palmitic or stearic acids employing these reactions. 

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

A more explicit demonstration of chain-shortening was achieved by performing assays using 3-14C-hexadecanoic acid as substrate (IL) Over 80% of the radioactivity in the isolated chain-shortened tetradecanoic acid was in the 1-position, showing that most of the reaction was direct, though a small amount may have undergone degradation/resynthesis. 

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. 

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