Question

Biochemistry question.

In a mutant protein, a buried phenylalanine residue was replaced by a lysine residue and the melting temperature decreased by 30 Celcius.

Explain this observation. Choose 1 of the 4 Answer choices

1) Replacing a buried hydrophobic side chain with a charged polar one is highly unfavorable in terms of hydrophobic interactions.

2) Replacing a buried polar side chain with a charged hydrophobic one is highly unfavorable in terms of hydrophobic interactions.

3) Replacing a buried hydrophobic side chain with a charged polar one is highly favorable in terms of hydrophobic interactions.

4) Replacing a buried polar side chain with a charged hydrophobic one is highly favorable in terms of hydrophobic interactions.

Answer 1

Answer 1.

Most proteins have a hydrophobic amino acid core, whereas charged polar or non-polar amino acids cover the surace which is in contact with the solvent due to their ability to form hydrogen bonds.

Charged polar amino acids often form salt-bridges ,which are essential for the stabilization of the three-dimensional structure of the protein molecule. Also, these salt-bridges contribute towards thermostability of proteins,preventing denaturation at high temperatues.

Now if hydrophobic core is replaced by polar charged residue such as lysine,the stability of protein structure will be affected.Hydrophobic proteins if placed on the exterior region,will be incapable of forming hydrogen bonds with surrounding solvent. Thus leading to unstable structure which will probably have a lower melting point.