Question

Naturally occurring proteins consist exclusively of L-amino acids. Below is the structure of a peptide Ala-Thr-Asp. Which of these amino acids have been drawn as L-amino acids?

following are options
Ala and Thr

Ala, Thr and Asp

Ala and Asp

Thr and Asp

Answer 1

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Ala and Asp

Answer 2

All three amino acids in the peptide Ala-Thr-Asp have been drawn as L-amino acids. This is because naturally occurring proteins consist exclusively of L-amino acids, which have a specific configuration of the amino and carboxyl groups, as well as a hydrogen atom and a side chain bonded to the alpha carbon.

• The L- and D- configuration refers to the position of the amino group relative to the carboxyl group. Since naturally occurring proteins consist of L-amino acids, all amino acids in the peptide, including Ala, Thr, and Asp, must be drawn as L-amino acids.

• Naturally occurring proteins consist exclusively of L-amino acids, which are mirror images of D-amino acids. This is because life on Earth uses enzymes that only produce and recognize L-amino acids.

• Alanine has a non-polar side chain consisting of a methyl group (-CH3) attached to the alpha carbon.

• Threonine has a polar side chain consisting of a hydroxyl group (-OH) attached to the beta carbon.

• Aspartic acid has a negatively charged side chain consisting of a carboxyl group (-COOH) attached to the beta carbon.

Therefore, the amino acids that have been drawn as L-amino acids in the peptide Ala-Thr-Asp are Ala and Thr. Asp is also an L-amino acid, but it was not explicitly stated in the question stem.

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