Question

Which of these amino acids has a chiral carbon in its side chain? Serine Threonine Isoleucine (A) I only (B) II only (C) II and III only (D) I, II, and III

Short answer

C

Step-by-step solution

Identify Chiral Carbon

A chiral carbon is a carbon atom that is attached to four different groups. Look for such a carbon in the side chains of each amino acid.

Examine Serine

Serine has a side chain with the structure -CH2OH. None of the carbon atoms in the side chain are attached to four different groups, so serine does not have a chiral carbon in its side chain.

Examine Threonine

Threonine has a side chain with the structure -CH(OH)-CH3. The middle carbon (attached to -OH, -CH3, a hydrogen, and the rest of the molecule) is attached to four different groups, so threonine has a chiral carbon in its side chain.

Examine Isoleucine

Isoleucine has a side chain with the structure -CH(CH3)-CH2-CH3. The first carbon in the side chain (attached to -CH3, -CH2, a hydrogen, and the rest of the molecule) is attached to four different groups, so isoleucine has a chiral carbon in its side chain.

Determine the Correct Answer

Based on the analysis, threonine and isoleucine both have chiral carbons in their side chains, but serine does not. Therefore, the correct answer is (C) II and III only.

Key concepts

Amino Acid Structure

Amino acids are the building blocks of proteins. Each amino acid has a similar core structure but differs by the unique side chain (R-group) that is attached to the central (alpha) carbon. The common structure includes an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable R-group. The specific R-group determines the properties and function of the amino acid. For example, serine has a side chain of -CH2OH, threonine has -CH(OH)-CH3, and isoleucine has -CH(CH3)-CH2-CH3.

Chirality

Chirality is a property where a molecule cannot be superimposed on its mirror image. This is similar to how your left and right hands are mirror images but cannot be perfectly aligned. A carbon atom is chiral if it is bonded to four distinct groups. This is important in amino acids, as chirality can affect the molecule's biological function. In the exercise above, serine, threonine, and isoleucine were examined for chiral carbons. A chiral carbon plays a crucial role in the three-dimensional structure and functionality of the molecule.

Side Chain Identification

Identifying the side chains in amino acids helps determine their distinct properties. In the exercise:

• Serine: Side chain is -CH2OH. None of the carbons are chiral.


• Threonine: Side chain is -CH(OH)-CH3. It has one chiral carbon bonded to four distinct groups: -OH, -CH3, -H, and the rest of the molecule.


• Isoleucine: Side chain is -CH(CH3)-CH2-CH3. It has one chiral carbon bonded to four distinct groups: -CH3, -CH2, -H, and the rest of the molecule.

Threonine and isoleucine both have chiral carbons in their side chains, while serine does not.

MCAT Biochemistry

Understanding the concepts of chirality and amino acid structures is essential for the MCAT Biochemistry section. Recognizing chiral centers in biomolecules can help predict interactions and functions. Questions often test your ability to apply knowledge of molecular structures to biological systems. To prepare:

• Practice identifying chiral centers in different molecules.


• Make sure you know the side chains of the 20 standard amino acids.


• Understand how these structures affect protein behavior and interaction.

Mastering these areas will assist you in answering related MCAT questions confidently.