A biochemistry student asks why linoleic acid must be obtained from the diet even though humans can synthesize most fatty acids. The correct explanation is that humans lack:

• A Delta-5 desaturase
• B Delta-4 desaturase
• C Elongases for polyunsaturated fatty acids
• D Enzymes capable of introducing double bonds beyond carbon 9 from the methyl end ✓

Explanation

Mammalian desaturases can introduce double bonds only between carbon 9 and the carboxyl end. Linoleic and alpha-linolenic acids require double bonds between carbons 9 and the methyl terminus (omega-6 and omega-3 positions), and no mammalian delta-12 or delta-15 desaturase exists, making both essential dietary fatty acids. Delta-5 and delta-6 desaturases are present and allow elongation and further desaturation of dietary linoleate toward arachidonic acid, so options A and C are false limitations.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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