Identifying protein bonds from structural diagrams

Steps:

• Examine diagram for arrangement at S: charged side chains close together indicate ionic bond.
• Examine diagram for arrangement at T: polar groups aligned suggest hydrogen bond.
• Note absence of sulfur-containing cysteines at both sites, ruling out disulfide bonds.
• Match to choice describing ionic at S, hydrogen at T, and absent disulfide.

Why A is correct:

• Ionic bonds form between oppositely charged amino acid side chains (e.g., lysine and aspartate), while hydrogen bonds occur between polar groups like serine hydroxyls, per protein secondary/tertiary structure definitions.

Why the others are wrong:

• B misidentifies both as absent, ignoring ionic and hydrogen bonds present.
• C incorrectly assigns S only as absent and T only as a bond type without specifying.
• D reverses the assignments, claiming T only absent and both S and T as bonds.

Final answer: A