Hydrogen bonding capacity — Which amino acids listed can form hydrogen bonds through their side-chain (R-group) functional groups in aqueous proteins?
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AAsparagine
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BAspartic acid
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CGlutamine
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DAll of these
Answer
Correct Answer: All of these
Explanation
Introduction / Context:Hydrogen bonds are critical for protein folding, enzyme–substrate recognition, and macromolecular assemblies. Side-chain functional groups that can donate or accept hydrogen bonds frequently determine specificity and stability.
Given Data / Assumptions:
- Asparagine (amide), aspartic acid (carboxylate), and glutamine (amide) are considered.
- We evaluate their R-groups for hydrogen bond donor/acceptor roles.
- Physiological pH context applies.
Concept / Approach:Asparagine and glutamine have carboxamide side chains capable of donating (via –NH) and accepting (via C=O) hydrogen bonds. Aspartic acid, when protonated (–COOH), can donate; when deprotonated (–COO−), it can accept hydrogen bonds through its carboxylate oxygens and engage in electrostatic interactions.
Step-by-Step Solution:
Examine Asn: side-chain amide supports both donor and acceptor interactions.Examine Asp: side-chain carboxyl participates as an acceptor (COO−) or donor (COOH) depending on microenvironment and pH.Examine Gln: analogous to Asn but with a longer chain; robust H-bonding capacity.Therefore, all listed residues can form hydrogen bonds via their side chains.Verification / Alternative check:Protein crystal structures show these residues frequently located at active sites or solvent-exposed regions making H-bond networks.
Why Other Options Are Wrong:
- Asparagine only or glutamine only: incomplete; ignores aspartate's roles.
- Aspartic acid only: incomplete; amides are strong H-bond participants too.
Common Pitfalls:Assuming deprotonated carboxylates cannot hydrogen bond; while less effective donors, they are strong acceptors and engage in intricate networks.
Final Answer:All of these