Question

Identify the non protein \(\alpha\)-amino acids from the list.
A. Ornithine
B. Citrulline
C. Homoserine
D. Taurine
Choose the correct answer from the options given below:

• A. A, B and C only
• B. A, B and D only
• C. A and B only
• D. C and D only

Hint:

Taurine is unique because it contains a sulfonic acid group instead of a carboxylic acid group, making it a \(\beta\)-amino sulfonic acid rather than an \(\alpha\)-amino carboxylic acid.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Amino acids are organic compounds containing both amino and carboxyl functional groups.
An \(\alpha\)-amino acid has both functional groups attached to the same central carbon atom (the \(\alpha\)-carbon).
"Non-protein" amino acids are those that are not incorporated into proteins during ribosomal translation, though they can still play critical roles in cellular metabolism.

Step 2: Detailed Explanation:
Let us analyze the structures of the listed molecules:
- Ornithine (A): Structurally, it is an \(\alpha\)-amino acid with the formula \(\text{NH}_2\text{-(CH}_2\text{)}_3\text{-CH(NH}_2\text{)-COOH}\). It is a key intermediate in the urea cycle but is not encoded by the genetic code for protein synthesis.
- Citrulline (B): It is also an \(\alpha\)-amino acid with a carbamoyl group attached to the side chain. It participates in the urea cycle and nitric oxide production but is not directly incorporated into proteins during translation.
- Homoserine (C): An \(\alpha\)-amino acid with the formula \(\text{HO-CH}_2\text{-CH}_2\text{-CH(NH}_2\text{)-COOH}\). It acts as an intermediate in the biosynthesis of methionine and threonine.
- Taurine (D): Unlike the others, taurine is a \(\beta\)-amino sulfonic acid (\(\text{NH}_2\text{-CH}_2\text{-CH}_2\text{-SO}_3\text{H}\)). Its amino group is attached to the \(\beta\)-carbon, and it lacks the characteristic carboxylic acid (\(\text{-COOH}\)) group of \(\alpha\)-amino acids.
Therefore, Ornithine, Citrulline, and Homoserine are non-protein \(\alpha\)-amino acids, while Taurine is not an \(\alpha\)-amino acid.

Step 3: Final Answer:
The non-protein \(\alpha\)-amino acids from the list are Ornithine, Citrulline, and Homoserine (A, B and C only).