Final answer:
A histidine residue on a membrane protein binds an anionic ligand stronger under acidic pH due to electrochemical exclusion, where the positively charged histidine at low pH interacts more effectively with negatively charged anions.
Step-by-step explanation:
A membrane protein with a histidine residue on the extracellular surface could bind an anionic ligand stronger under acidic conditions. Histidine is a basic amino acid with a side chain that can ionize to a positively charged -NH₃⁺ group at a pH below its pKa of 6.04, making it more likely to interact with negatively charged anions. In biology, this principle is known as electrochemical exclusion, which affirms that interactions between ion channels and ions are charge-specific.
Therefore, under acidic pH, the histidine residue is more likely to be protonated (gain a hydrogen ion) and thus carry a positive charge, which would enhance its ability to form ionic bonds with an anionic (negatively charged) ligand. Conversely, at a neutral or basic pH, the histidine is less likely to be protonated, making its interaction with an anionic ligand weaker.