Question

multiple choice 10 points
the figure above shows a model of a ligand precursor being cleaved to produce an active ligand that binds to a
specific receptor. which of the following is most likely to reduce the binding of the active ligand to its receptor?
a mutation in the receptor gene that causes a substitution of a charged amino acid for a nonpolar amino acid in the ligand binding site of the receptor
an increase in the ratio of the number of unsaturated to the number of saturated fatty acid tails of the membrane lipids
a change in the cytoskeletal attachment of transmembrane proteins
the presence of a large amount of the precursor form of the ligand

Explanation:

• Option A: A mutation in the receptor gene that causes a substitution of a charged amino acid for a non - polar amino acid in the ligand binding site of the receptor. This change in the receptor's ligand - binding site can disrupt the complementary shape and chemical interactions (such as hydrogen bonding, van der Waals forces) between the ligand and the receptor. Since the binding of a ligand to its receptor is highly specific based on molecular shape and chemical properties (like polarity), this mutation is likely to reduce ligand - receptor binding.
• Option B: An increase in the ratio of unsaturated to saturated fatty acid tails of membrane lipids affects membrane fluidity. However, membrane fluidity mainly impacts the movement of proteins within the membrane (like lateral diffusion) rather than the direct binding of a ligand to its receptor. The binding site of the receptor for the ligand is more about the receptor's structure (amino acid composition in the binding site) than membrane fluidity.
• Option C: A change in the cytoskeletal attachment of transmembrane proteins affects the localization and stability of the receptor within the membrane. But it does not directly alter the ligand - binding site of the receptor. The cytoskeleton - receptor interaction is more about keeping the receptor in a particular position in the cell (e.g., at the synapse for a neurotransmitter receptor) rather than affecting the ligand - receptor binding affinity.
• Option D: The presence of a large amount of the precursor form of the ligand does not affect the binding of the active ligand to its receptor. The precursor is inactive and does not bind to the receptor (as per the model where only the cleaved, active ligand binds to the receptor).

Answer:

A. A mutation in the receptor gene that causes a substitution of a charged amino acid for a nonpolar amino acid in the ligand binding site of the receptor