Arachidonic acid is a polyunsaturated fatty acid with 20 carbon atoms, playing a vital role in cell signaling and inflammation. When metabolized for energy, arachidonic acid undergoes β-oxidation, a process that breaks down fatty acids two carbons at a time into acetyl CoA molecules.

Understanding how many β-oxidation cycles are required to completely hydrolyze arachidonic acid into acetyl CoA is important for grasping fatty acid metabolism and energy production.

Basics of β-Oxidation

• β-Oxidation cleaves fatty acids sequentially by removing two-carbon acetyl CoA units from the acyl chain.

• Each cycle shortens the fatty acid chain by two carbons and produces one acetyl CoA.

• The process continues until the entire fatty acid is converted into acetyl CoA molecules.

Calculating the Number of Cycles for Arachidonic Acid

• Arachidonic acid has 20 carbon atoms.

• The number of acetyl CoA molecules produced is:

  Acetyl CoA=Number of carbons2=202=10Acetyl CoA=2Number of carbons=220=10

• The number of β-oxidation cycles required is one less than the number of acetyl CoA produced, because the last cleavage produces two acetyl CoA without further cycling:

  Number of cycles=(202)−1=10−1=9Number of cycles=(220)−1=10−1=9

Thus, 9 cycles of β-oxidation are required to completely hydrolyze arachidonic acid into acetyl CoA.

Summary

Additional Notes

• Each β-oxidation cycle also produces one molecule each of NADH and FADH₂, which feed into the electron transport chain for ATP generation.

• Arachidonic acid’s double bonds require additional enzymatic steps (e.g., isomerase and reductase) during β-oxidation, but these do not change the number of cycles needed.

• The acetyl CoA produced enters the citric acid cycle for further oxidation and energy production.

Conclusion

The complete β-oxidation of arachidonic acid (20 carbons) requires 9 cycles to fully convert it into 10 molecules of acetyl CoA.

Correct answer: (1) 9