Question

The amino acid considered as a branched-chain amino acid (BCAA) is:

• A. Serine
• B. Leucine
• C. Asparagine
• D. Tyrosine

Hint:

Only three amino acids, Leucine, Isoleucine and Valine, count as branched-chain amino acids.

Answer 1

The Correct Option is B

The question asks us to spot the branched-chain amino acid (BCAA) from a list of four amino acids. BCAAs are a specific, small group, Leucine, Isoleucine and Valine, and their side chains all include a branch point instead of a single straight carbon chain.

1. Serine: has a hydroxymethyl side chain and belongs to the polar, uncharged amino acid group, not the BCAA group.
2. Leucine: its side chain is an isobutyl group with a clear branch, making it one of the three true BCAAs.
3. Asparagine: has an amide side chain and is classed as a polar amino acid, unrelated to branching.
4. Tyrosine: carries an aromatic phenol ring side chain, placing it with the aromatic amino acids.

Only Leucine fits the strict definition of a branched-chain amino acid, since its side chain actually branches, unlike the other three options.

Let's summarize:

• The only true BCAAs among all twenty amino acids are Leucine, Isoleucine and Valine.
• Serine, Asparagine and Tyrosine belong to other structural classes, polar, amide and aromatic.

So the correct answer is Leucine.