Question

In Franz Knoop's classic experiment, which indicated that fatty acids are metabolically oxidized at the $\beta$-carbon atom, $\omega$-phenyl labeled fatty acids containing odd number of carbon atoms are broken down to which of the following metabolite?

• A. Phenylacetic acid
• B. Benzoic acid
• C. Acetoacetic acid
• D. Propionic acid

Hint:

In Knoop's experiment:
Even-chain phenyl fatty acids $\rightarrow$ Degraded to Phenylacetic acid.
Odd-chain phenyl fatty acids $\rightarrow$ Degraded to Benzoic acid.
This study established the two-carbon cleavage mechanism of $\beta$-oxidation.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
In 1904, Franz Knoop performed a series of tracing experiments to determine how fatty acids are degraded in living organisms.
He used a benzene ring as a non-metabolizable tag attached to the terminal ($\omega$) carbon atom of fatty acids fed to dogs.

Step 2: Detailed Explanation:
Knoop analyzed the metabolic end products excreted in the urine of animals fed these phenyl-labeled fatty acids:
1. When the animals were fed phenyl-labeled fatty acids containing an even number of carbon atoms (such as phenylbutyrate, 4 carbons), the metabolic end product recovered was phenylacetic acid (excreted as phenaceturic acid).
2. When the animals were fed phenyl-labeled fatty acids containing an odd number of carbon atoms (such as phenylpropionate, 3 carbons, or phenylvalerate, 5 carbons), the end product recovered was benzoic acid (excreted as hippuric acid).
These results showed that fatty acids are degraded through the systematic, step-wise removal of two-carbon units (acetyl-CoA) starting from the carboxyl end.
This pathway is known as $\beta$-oxidation because the cleavage occurs between the $\alpha$- and $\beta$-carbon atoms of the fatty acyl-CoA chain.
For odd-carbon phenyl-fatty acids, the final cleavage step leaves a single-carbon carboxyl group attached to the phenyl ring, yielding benzoic acid.

Step 3: Final Answer:
Phenyl-labeled fatty acids with an odd number of carbon atoms are degraded to Benzoic acid, which corresponds to Option (B).