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My textbook gives the structures of the unsaturated fatty acids in food including oleic acid, linoleic acid, linolenic acid and docosatetraenoic acid. I figure that when two or more $\ce{C=C}$ double bonds appear in a fatty acid, they tend to arrange in this way:

$$\ce{-CH=CH-CH2-CH=CH\bond{-}}$$

with a methylene group blocking between them, rather than a conjugated form. Why is that?

And in the synthesis and degradation of fatty acids, a conjugated form seems to be more straightforward and more simple for both synthesis and degradation, needless to move the double bond after constructing the fatty acid chain or before β-oxidation to break down the chain. Furthermore, conjugated systems seem to be fairly popular in polyketones which share a similar synthesis mechanism with fatty acids.

So is it that the conjugated form is too easily oxidized to be stored for a long time, or is it that the rigid structure of conjugated double bonds would lead to difficulties in transportation and storage? Or, is there other explanation for that?

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    $\begingroup$ It goes to a question why life has evolved/has been created this way. A difficult answer, like asking an art paint producer why the painting has been painted the way it was. $\endgroup$
    – Poutnik
    Jan 14, 2022 at 6:29
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    $\begingroup$ Note that naturally-occurring conjguated fatty acids do exist, e.g., the conjugated linoleic acids, typically found in milk and beef (nature.com/articles/s41598-021-88870-9), and the conjugated linolenic acids, typically found in seed oils ( link.springer.com/referenceworkentry/… ). [See also en.wikipedia.org/wiki/Polyunsaturated_fatty_acid ] Nevertheless, your question (which is an interesting one) remains: What is the evolutionary/functional explanation for why the predominant PUFAs are methylene-interrupted rather than conjugated? $\endgroup$
    – theorist
    Jan 14, 2022 at 6:29
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    $\begingroup$ @Karl. Not so simple, I'd say. Sure, you can find epidemiological articles talking about their potential health effects, but those haven't been fully established and, besides, what we're talking about here is instead their actual role in metabolic pathways as regulators/precursors. You can't get that kind of info. from summary articles (e.g., lipidmaps.org/resources/lipidweb/lipidweb_html/lipids/fa-eic/… or en.wikipedia.org/wiki/Conjugated_linoleic_acid)...and you can see the reason why by looking at the review articles in the primary literature, which... $\endgroup$
    – theorist
    Jan 15, 2022 at 1:59
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    $\begingroup$ indicate there continues to be significant uncertainty about just what role CLA plays (see, Fig. 4 in sciencedirect.com/science/article/pii/S175646461500167X , from 2015, and note all the ques. marks). It seems it's just with the most recent articles (e.g., this, from 2021: frontiersin.org/articles/10.3389/fphar.2020.587140/full ) that they seem to be demonstrating where they play specific precursor roles. Bottom line: It appears this is not settled science--it's an active area of research. Thus what we really need is a biochemist (which I'm not) to respond to this. $\endgroup$
    – theorist
    Jan 15, 2022 at 2:02
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    $\begingroup$ One factor is that the double bonds are generally all cis, and two consecutive cis double bonds make a rigid curl in the chain. More generally, avoiding conjugation preserves greater flexibility $\endgroup$
    – Andrew
    Jan 15, 2022 at 20:26

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