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Our teacher asked us about fatty acid C21:3.

Does this fatty acid even exist?

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    $\begingroup$ Biosynthesis adds two carbons at a time, so this less likely than something that's even. $\endgroup$ – Zhe Dec 12 '16 at 13:03
  • $\begingroup$ You speak of saturated fatty acids where even number of carbons is more abundant then odd number. But in the case of unsaturated fatty acids, does one exist in nature with an odd number of carbons in its chain? $\endgroup$ – EmmaF Dec 12 '16 at 13:05
  • $\begingroup$ Interesting question, but the biosynthesis happens via the unsaturated acid anyway. If nature wants the saturated version, it simply performs the appropriate reduction of (what happens to be, at the time) an $\alpha,\beta$-unsaturated carbonyl. $\endgroup$ – Zhe Dec 12 '16 at 14:03
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  1. C21:3 is a stable fatty acid that could indicate a variety of double-bond isomers. It is "unusual" in biology because of the odd number of carbons, but that doesn't preclude its stability and accessibility to organic synthesis. If someone really wanted to make this acid, they could do so easily.

  2. Such a compound, 5Z,14Z,17Z-heneicosatrienoic acid, has been reported in the natural environment. It was found in a deep-sea clam, Calyptogena phaseoliformis, that lives in or near hydrothermal vents and lives off of the organic matter produced by chemosynthetic bacteria that live in close proximity to it. So in addition to being synthetically accessible, it is also made in living organisms.

Thus, for all reasonable interpretations of "exist" in your question, does this fatty acid even exist?, the answer is yes.

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