Fatty acid structures – Arachidonic acid (arachidonate) contains 20 carbon atoms with how many double bonds?

Introduction / Context:
Arachidonic acid is a polyunsaturated omega-6 fatty acid central to eicosanoid biosynthesis (prostaglandins, thromboxanes, leukotrienes). Recognizing its carbon length and degree of unsaturation is fundamental in lipid biochemistry and physiology.

Given Data / Assumptions:

• Molecular shorthand for arachidonate is 20:4 (n−6).
• Double bond count determines fluidity and substrate specificity for oxygenases (COX, LOX).
• Eicosanoid signaling affects inflammation, thrombosis, and vascular tone.

Concept / Approach:
The notation 20:4 indicates 20 carbons and 4 cis double bonds. In mammalian membranes, these are typically at Δ5,8,11,14 positions (counting from the carboxyl end). This structural feature underlies its conversion to potent lipid mediators upon phospholipase A2 activation.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical catalogs and lipidomic analyses consistently annotate arachidonic acid as 20:4 (n−6), confirming the four double bonds.

Why Other Options Are Wrong:

• 2 or 3 double bonds: would correspond to other fatty acids (e.g., linoleic acid is 18:2).
• 8 double bonds or 0: not consistent with arachidonate’s known structure.

Common Pitfalls:
Confusing arachidonic (20:4) with arachidic acid (20:0), the saturated analog.

Final Answer:
4 double bonds