​A cytoplasmic monomeric protein containing a single non-surface exposed cysteine residue precipitates upon mutation of the Cys to 'Ile'. However, the mutation of Cys to 'Ala' leads to a soluble and functional protein equivalent to the native form. Which one of the following statements explains the above observations?​

Correct option is D

The protein precipitates (i.e., becomes insoluble and aggregates) when Cys is mutated to Ile, but remains soluble and functional when mutated to Ala. Precipitation typically occurs due to misfolding, aggregation, or destabilization of the protein structure, often linked to changes in the protein’s core.

Let’s evaluate the options:

1.Cys mutated to Ile alters the net charge of the protein, while mutation to Ala does not.
Cys is neutral at physiological pH (though the thiol can be deprotonated in specific contexts). Ile and Ala are also neutral. None of these mutations should significantly alter the net charge of the protein. This option is unlikely to explain the precipitation.
2. Cys mutated to Ala alters the net charge of the protein, while mutation to Ile does not.
As above, none of these amino acids significantly change the net charge. This option is also incorrect.
3. Cys mutated to Ala causes steric hindrance in the core of the protein, while mutation to Ile does not.
Ala is smaller than Cys (side chain: methyl group vs. thiol group), so it’s unlikely to cause steric hindrance. Ile, however, is larger and bulkier than Cys, so if anything, Ile would cause steric hindrance, not Ala. This option is incorrect and contradicts the observation (since Ile causes precipitation, not Ala).
4. Cys mutated to Ile causes steric hindrance in the core of the protein, while mutation to Ala does not.
Ile’s bulky, branched side chain could indeed cause steric hindrance in the protein’s core, disrupting packing and leading to misfolding or destabilization, which could cause precipitation. Ala, being small, is less likely to disrupt the core structure, allowing the protein to remain stable and soluble. This option aligns with the observation.

The most likely explanation is that the bulky side chain of Ile introduces steric hindrance in the protein’s core, leading to misfolding or destabilization that causes precipitation, while the smaller Ala does not have this effect.
Answer: 4. Cys mutated to Ile causes steric hindrance in the core of the protein, while mutation to Ala does not.