Difficulty: Easy
Correct Answer: A reactive hydroxyl (–OH) group in the side chain
Explanation:
Introduction / Context:
Serine (Ser) and threonine (Thr) are central to enzyme catalysis, hydrogen bonding, and post-translational modification. Their classification as polar stems from a shared functional group that engages in aqueous interactions and chemistry.
Given Data / Assumptions:
Concept / Approach:
The hydroxyl (–OH) group is polar, donates/accepts hydrogen bonds, and serves as a nucleophilic site for kinase-mediated phosphorylation (Ser/Thr kinases) and glycosyltransferase reactions (O-glycans).
Step-by-Step Solution:
Identify functional group: side-chain alcohol for Ser and Thr.Link to polarity: –OH increases hydrophilicity via hydrogen bonding with solvent and residues.Connect to biology: –OH enables phosphorylation, a core regulatory switch in signaling pathways.Choose the option specifying the hydroxyl group.
Verification / Alternative check:
Proteomic datasets show abundant Ser/Thr phosphorylation; structural models depict –OH groups forming intra-protein hydrogen bonds that guide folding and active-site geometry.
Why Other Options Are Wrong:
Common Pitfalls:
Confusing threonine with tyrosine (which is aromatic phenolic OH) or with cysteine (thiol); assuming all polar residues are charged (Ser/Thr are polar uncharged).
Final Answer:
A reactive hydroxyl (–OH) group in the side chain
Discussion & Comments