Question

Describe briefly how \(\beta\) -oxidation of an odd-chain fatty acid is different from that for an even-chain fatty acid.

Short answer

Odd-chain fatty acids produce propionyl-CoA, which converts to succinyl-CoA, unlike even-chain fatty acids that produce acetyl-CoA.

Step-by-step solution

- Understand Even-Chain Fatty Acid Oxidation

Even-chain fatty acids undergo β-oxidation where two-carbon units in the form of acetyl-CoA are removed sequentially until the entire fatty acid chain is converted into acetyl-CoA molecules.

- Understand Odd-Chain Fatty Acid Oxidation

Odd-chain fatty acids also undergo β-oxidation similarly, removing two-carbon units, until three carbons remain, constituting a molecule of propionyl-CoA.

- Conversion of Propionyl-CoA

Propionyl-CoA undergoes a series of enzymatic reactions. First, it is carboxylated to methylmalonyl-CoA. Then, methylmalonyl-CoA is rearranged to form succinyl-CoA, which enters the Krebs cycle.

- Main Difference

The key difference is that the final product for even-chain fatty acids is acetyl-CoA, while for odd-chain fatty acids, it is propionyl-CoA, which is further converted to succinyl-CoA.

Key concepts

Even-Chain Fatty Acids

Even-chain fatty acids are the most common type of fatty acids found in nature. They contain an even number of carbon atoms. During β-oxidation, these fatty acids undergo a sequence of enzymatic reactions where two-carbon units are removed incrementally from the carboxyl end. These two-carbon units take the form of acetyl-CoA.

This process continues until the entire fatty acid chain is converted into acetyl-CoA molecules. The acetyl-CoA molecules produced then enter the Krebs cycle, where they are further oxidized for energy production.

To summarize, for even-chain fatty acids, the main takeaway is:

• Present in even numbers.
• Undergo β-oxidation to produce acetyl-CoA sequentially.
• All carbon atoms are fully converted into acetyl-CoA.

Odd-Chain Fatty Acids

Odd-chain fatty acids have an interesting twist compared to their even-chain counterparts. They contain an odd number of carbon atoms. Like even-chain fatty acids, odd-chain fatty acids also undergo β-oxidation. However, the process continues until a three-carbon unit remains, forming a molecule known as propionyl-CoA.

Here's what happens during this last step: the removal of two-carbon units proceeds until the three-carbon propionyl-CoA is left. This molecule cannot be directly used in the Krebs cycle and requires further conversion.

This difference is crucial:

• Odd number of carbon atoms.
• Ends β-oxidation with propionyl-CoA.
• Requires additional conversion to enter the Krebs cycle.

Acetyl-CoA

Acetyl-CoA is a central molecule in metabolism. This two-carbon molecule results from several catabolic pathways, including β-oxidation of even-chain fatty acids.

Acetyl-CoA enters the Krebs cycle (also known as the citric acid cycle), where it is further oxidized to produce ATP, the energy currency of the cell. The process also generates NADH and FADH2, which play roles in the electron transport chain.

Key points about Acetyl-CoA:

• Derived from the β-oxidation of even-chain fatty acids.
• Entered into the Krebs cycle for ATP production.
• Integral to metabolic processes.

Propionyl-CoA

Propionyl-CoA is a vital intermediate derived from the β-oxidation of odd-chain fatty acids. This three-carbon molecule cannot directly enter the Krebs cycle.

Propionyl-CoA is first carboxylated to form methylmalonyl-CoA. This reaction requires a cofactor like biotin. Methylmalonyl-CoA is then rearranged by an enzyme called methylmalonyl-CoA mutase to form succinyl-CoA.

Key highlights on Propionyl-CoA:

• Produced from the β-oxidation of odd-chain fatty acids.
• Converted to methylmalonyl-CoA.
• Further converted to succinyl-CoA to enter the Krebs cycle.

Succinyl-CoA

Succinyl-CoA is an important molecule in the Krebs cycle. Formed from the conversion of odd-chain fatty acid derived propionyl-CoA, succinyl-CoA plays a pivotal role in energy production.

Once converted, succinyl-CoA enters the Krebs cycle, where it participates in the cyclical series of reactions leading to ATP production. This integration is crucial for late-stage energy extraction from odd-chain fatty acids.

Remember these points about Succinyl-CoA:

• Formed from propionyl-CoA through methylmalonyl-CoA.
• Essential for entering the Krebs cycle from odd-chain fatty acids.
• Facilitates energy production.

Krebs cycle

The Krebs cycle, also known as the citric acid cycle or TCA cycle, is a fundamental metabolic pathway occurring in the mitochondria. Both acetyl-CoA from even-chain fatty acids and succinyl-CoA from odd-chain fatty acids enter this cycle.

This cycle involves a series of enzymatic reactions that produce ATP, NADH, and FADH2. These high-energy molecules are later used in the electron transport chain for the production of more ATP.

Key takeaways about the Krebs cycle:

• Central to cellular energy production.
• Processes both acetyl-CoA and succinyl-CoA.
• Creates ATP, NADH, and FADH2 for energy.