Most of the fatty acids in animal biology consist of even number of carbons in its parent chain. What property of such fatty acids cause the biological systems to prefer them over the odd counterparts? What's so bad about fatty acids containing odd number of carbon atoms in their parent chain?

Why are fatty acids consisting of even number carbons in their parent chain predominant?

  • $\begingroup$ you might want to justify your underlying claim. $\endgroup$ – John Mar 16 '18 at 20:28
  • $\begingroup$ What does TL:DR mean? Don't tell me, it isn't English. Replace it. $\endgroup$ – David Mar 16 '18 at 22:44
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    $\begingroup$ @David Are you opposed to all abbreviations? $\endgroup$ – canadianer Mar 16 '18 at 23:04
  • $\begingroup$ @canadianer — In general I discouraged my students from using all but the most common (RNA, SDS) or any that came up repeatedly. There is nothing to gain from abbreviating a word that only occurs a couple of times, and there is likely to be someone unfamiliar with it. In this case, the abbreviation is some sort of internet slang which I looked up once but forget. It is not in the Collins English Dictionary on my iPhone and I regard its use as inexcusable. $\endgroup$ – David Mar 16 '18 at 23:12
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    $\begingroup$ It actually was added to Oxford Dictionaries as an abbreviation in 2013! "TL;DR, abbrev.: ‘too long didn’t read’: used as a dismissive response to a lengthy online post, or to introduce a summary of a lengthy post." (blog.oxforddictionaries.com/august-2013-update). Also in Collins English (collinsdictionary.com/us/dictionary/english/tl-dr). But, regardless, I can understand that abbreviations can be hard to keep up with. $\endgroup$ – julianstanley Mar 16 '18 at 23:14

In short, it's because fatty acids are made from two-carbon blocks.

The way that most organisms make fatty acids is by the successive addition of two-carbon units (acetyl-CoA). So we usually get even-numbered fatty acids just because the building blocks are also even.

In plants and in synthetic contexts, we can have some reactions that can produce odd-numbered fatty acids (by building with two-carbon units three times to get six, and then breaking that six in half to get three, for example). Such odd fatty acids are seen in some organisms, but humans are usually thought to get them from other sources (e.g. microbiota, diet).


Point 1: Fatty acids are generally built two-carbons at a time.

The European Bioinformatics Institute has a really good explanation of fatty acid synthesis. The quote below is from that (https://www.ebi.ac.uk/interpro/potm/2007_6/Page2.htm):

"Whether existing as one complex or as independent enzymes, the reactions catalysed by types I and II fatty acid synthases are the same. Both systems are primed with acetyl-CoA, and then add 2-carbon units to the growing chain using malonyl-CoA as substrate."

Point 2: Odd-numbered fatty acids can be produced by various organisms, and can be linked to dysfunction in humans.

doi:10.1038/srep44845 talks about the importance of odd chain fatty acids in disease as a preface to its findings about the sources of such fatty acids. They try to narrow down where odd-chain fatty acids come from, but they definitely seem to be building their case on the pretense that odd chain fatty acids are not great in humans.

"According to the literature, the origin of C15:0 and C17:0 has long been attributed to the diet, specifically from ruminant fat as the main contributor in a typical Western diet. This has been explained by the fact that these two OC-FAs are produced by the rumen microbiome and then incorporated into the fat deposits of the host animal destined for human consumption."

Point 3: Plants produce odd-chain fatty acids

An established/old paper (doi:10.1006/abbi.1994.1110) mentions this, and there's a Wikipedia article (https://en.wikipedia.org/wiki/Odd-chain_fatty_acid) that paraphrases in a nice way:

Some plant-based fatty acids, however, have an odd number of carbon atoms, and Phytanic fatty acid absorbed from plant chlorophyll has multiple methyl branch points. As a result, it breaks down into three odd numbered 3C Propionyl segments as well as three even numbered 2C Acetyl segments and one even numbered 4C Isobutynoyl segment.

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    $\begingroup$ You may be correct, but how are we to know? On SE you have to demonstrate why your answer is correct. In this case you could provide a reference to a Wikipedia article or, better, the section in Berg online. Even better would be a chemical equation. Read the Help on How do I write a good answer. $\endgroup$ – David Mar 16 '18 at 22:42
  • $\begingroup$ Thank you for the correction! I've added a bottom component with a few sources that hopefully help to make my answer a little more rigorous. $\endgroup$ – julianstanley Mar 16 '18 at 22:56
  • $\begingroup$ Fine now. This is a question that is likely to come up again, so making it more comprehensive will make it more useful to many people. $\endgroup$ – David Mar 16 '18 at 23:15

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