What is the effect of stereochemistry on Drug-Receptor interactions?
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A receptor is like a lock and a drug is like a key. The binding pocket of a receptor is three-dimensional and is itself chiral (it is built from chiral amino acids). So the shape of the drug matters a lot.
Both enantiomers fit equally well into a receptor site
Only one isomer may fit in the chiral binding site of the receptor
Geometric isomers always have the same activity
Chirality does not influence receptor binding
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The Correct Option isB
Solution and Explanation
Step 1: Understanding stereochemistry in drugs. Many drug molecules have one or more chiral centres, producing two mirror-image forms called enantiomers. These have identical physical properties but can behave very differently in the body.
Step 2: How receptors recognise molecules. A receptor's binding site is itself chiral (made of L-amino acids). Think of it like a lock: only the key with the right shape fits. One enantiomer will complement the binding pocket perfectly; the other will not.
Step 3: Why option 2 is correct. Only one enantiomer (usually the eutomer) fits snugly into the chiral receptor site and produces the desired pharmacological effect. The other enantiomer (distomer) may be inactive or even cause side effects.
Step 4: Why the other options are wrong. Option 1 is wrong: both enantiomers do NOT fit equally; receptor sites are stereoselective. Option 3 is wrong: geometric isomers often have different activities. Option 4 is wrong: chirality greatly influences binding and activity.
Step 5: Classic examples. S-ibuprofen is the active form; R-ibuprofen is much less active. L-dopa works; D-dopa does not. These confirm the principle.
Answer: Option (2) — Only one isomer may fit in the chiral binding site of the receptor