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If we look at text books for the human fatty acid metabolism (FA synthesis or β oxidation), they typically shows even length fatty acids (C8, C10, etc). Now, from a chemical perspective, I am wondering how the involved enzymes recognize that the fatty acid is an even number of carbons.

Can someone point me to (historic) experiments that have been done that show that these human pathways indeed only apply to even chain length fatty acids? Or do they also act on odd length fatty acids?

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The first experiment which showed that fatty acids are oxidized in C2-units has been done by Georg Franz Knoop and been published 1904 as "Der Abbau aromatischer Fettsäuren im Tierkörper.". The paper in reference 1 states:

Georg Franz Knoop discovered fatty acid β-oxidation. In 1904, he published his classical experiments using odd and even chain ω-phenyl fatty acids such as ω-phenylvaleric acid and ω-phenylbutyric acid (Knoop 1904). Knoop fed these compounds to dogs and analysed their urine. In dogs that had been fed the odd chain fatty acids, he found hippuric acid (conjugate of benzoic acid and glycine), whereas, the dogs that had been fed even chain fatty acids excreted phenaceturic acid (conjugate of phenylacetic acid and glycine). From this he concluded that the metabolism of fatty acids proceeds by the successive removal of two carbon fragments. The remaining fatty chain had to contain a carboxylic acid. He postulated that oxidation took place on the β carbon atom, an oxidation unknown to organic chemistry.

If the fatty acid chain has an uneven number of carbon atoms, the resulting propionyl-CoA is modified into succinyl-CoA which is then used by different processes.

During the breakdown of the chain, Acetyl-CoA groups are released (at the end of each step), which can then be further metabolized by the body. Acetyl-CoA is fed into the Krebs cycle and then oxidized.

References:

  1. A general introduction to the biochemistry of mitochondrial fatty acid β-oxidation
  2. Fatty Acid Oxidation
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    $\begingroup$ Good answer, which can be summarized as "they don't". I would add that for synthesis, there is also no specific odd/even check, they are just synthesized two atoms at a time. Though there are fatty acid modifying enzymes that have a particular length preference (that's why we tend to see particular chain lengths) $\endgroup$ – Victor Chubukov Aug 25 '16 at 13:56
  • $\begingroup$ Victor, indeed, it shows they do not. OK, that's half of the answer. What about the FA synthesis pathway? Does that also not have a preference? $\endgroup$ – Egon Willighagen Aug 25 '16 at 14:14
  • $\begingroup$ The basic answer is that the synthesis enzymes start with a 2-carbon molecule and then build the chain 2 carbons at a time. If you introduce unusual enzymes that can start with a 3-carbon molecule, you can get odd-chain fatty acids. However, the terminating enzymes do have some specificity in terms of chain length preference. en.wikipedia.org/wiki/Fatty_acid_synthesis $\endgroup$ – Victor Chubukov Aug 25 '16 at 17:27

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