Question

How many cycles of \(\beta\) -oxidation are required to process a fatty acid with 17 carbons?

Short answer

8 cycles are needed for a 17-carbon fatty acid.

Step-by-step solution

- Identify the Structure of the Fatty Acid

Understand that a fatty acid with 17 carbons is an odd-chain fatty acid. Beta-oxidation typically processes two carbons per cycle to create acetyl-CoA units.

- Determine the Number of Full Cycles

Each cycle of beta-oxidation removes a 2-carbon unit (acetyl-CoA). For an odd-numbered fatty acid, keep in mind that the last cycle will leave a 3-carbon unit, called propionyl-CoA. First, divide 17 (total carbons) by 2 to get 8 with 1 carbon remainder.

- Calculate the Number of Complete Cycles

Since 8 full cycles will process 16 carbons, and you'll be left with one 3-carbon unit (propionyl-CoA), the number of cycles required is 8.

- Consider the Final Product

Recognize that the final product left after 8 cycles for a 17-carbon fatty acid is propionyl-CoA, which will be converted into succinyl-CoA, entering the TCA cycle.

- Summarize the Answer

Therefore, to process a 17-carbon fatty acid, 8 cycles of beta-oxidation are required, leaving a final propionyl-CoA.

Key concepts

Understanding Odd-Chain Fatty Acids

Odd-chain fatty acids are a bit unique compared to the more common even-chain fatty acids. They have an odd number of carbon atoms. For example, the fatty acid in the exercise has 17 carbons. This distinct structure has implications for how they are broken down and utilized by the body. Despite these differences, the process starts similarly to even-chain fatty acid metabolism with beta-oxidation cycles removing two-carbon fragments in the form of acetyl-CoA.

The Role of Acetyl-CoA in Fatty Acid Metabolism

Acetyl-CoA is a crucial molecule in many metabolic pathways. During beta-oxidation, each cycle produces one acetyl-CoA from every two carbons removed from the fatty acid chain. These acetyl-CoA molecules then enter the citric acid cycle (TCA cycle) to be further oxidized for energy production. In the context of the exercise, each of the 8 beta-oxidation cycles produces one acetyl-CoA, contributing to a total of 8 acetyl-CoA molecules from a 17-carbon fatty acid.

What Happens to Propionyl-CoA?

When breaking down odd-chain fatty acids, the final product after all possible beta-oxidation cycles is a three-carbon molecule called propionyl-CoA. This is different from the two-carbon acetyl-CoA produced in other cycles. Propionyl-CoA doesn't go directly into the TCA cycle. Instead, it is converted into succinyl-CoA via a sequence of reactions involving carboxylation and rearrangement. Succinyl-CoA can then enter the TCA cycle, linking the metabolism of odd-chain fatty acids with other cellular metabolic processes.

Beta-Oxidation in Fatty Acid Metabolism

Beta-oxidation is the central process through which fatty acids are broken down to generate energy. The metabolisms of odd-chain and even-chain fatty acids primarily differ in the final products of their beta-oxidation. Each cycle of beta-oxidation removes a two-carbon unit as acetyl-CoA. However, for odd-chain fatty acids, the last cycle produces propionyl-CoA. Understanding how enzymes handle these different products is crucial. The end products, such as acetyl-CoA and succinyl-CoA, are vital intermediates in the broader metabolic network, ensuring that energy production remains seamless even when different types of fatty acids are used.