Question

Why linoleic acid or linolenic acid are essential fatty acids in mammals?

• A. Because of absence of elongase enzyme
• B. Because of absence of cyclooxygenase enzyme
• C. Because of the absence of required desaturases
• D. Linoleic acid or linolenic acid are not essential fatty acids in mammals

Hint:

Plants possess both $\Delta^{12}$ and $\Delta^{15}$ desaturase enzymes in their plastids, which is why plant-derived foods and oils are rich sources of essential fatty acids.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Essential fatty acids are fatty acids that are required for biological processes but cannot be synthesized by the body.
Consequently, they must be obtained through the diet.

Step 2: Detailed Explanation:
Mammals possess fatty acid desaturase enzymes ($\Delta^9$, $\Delta^6$, $\Delta^5$, and $\Delta^4$ desaturases) that introduce double bonds into fatty acid carbon chains.
The $\Delta^9$ desaturase can introduce a double bond at the 9th carbon position from the carboxyl end (converting stearic acid into oleic acid).
However, mammals lack the specific desaturase enzymes ($\Delta^{12}$ and $\Delta^{15}$ desaturases) required to insert double bonds beyond the 9th carbon position.
Linoleic acid (an $\omega$-6 fatty acid) has the structure $18:2\ \Delta^{9,12}$, with a double bond at the 12th position.
$\alpha$-Linolenic acid (an $\omega$-3 fatty acid) has the structure $18:3\ \Delta^{9,12,15}$, with double bonds at the 12th and 15th positions.
Because mammals do not possess $\Delta^{12}$ and $\Delta^{15}$ desaturases, they are unable to synthesize these compounds from scratch or from oleic acid.
As a result, both linoleic acid and $\alpha$-linolenic acid must be supplied by the diet.
Once consumed, mammals can elongate and desaturate these essential fatty acids to produce other critical lipids, such as arachidonic acid ($20:4\ \omega$-6) and docosahexaenoic acid (DHA, $22:6\ \omega$-3).

Step 3: Final Answer:
These fatty acids are essential in mammals because of the absence of the required desaturase enzymes.