8. An a-helix would be stabilized most by: A the presence of an Arg + residue ne

Question

8. An a-helix would be stabilized most by:

       A     the presence of an Arg+ residue near the carboxyl terminus of the a-helix.

       B     the transmission of the protons of several peptide bonds throughout the a-helix.

       C     the presence of two Lys+ residues near the amino terminus of the a-helix.

       D     interactions between neighboring Asp- and Glu- residues

       E      interactions between two adjacent amino acid residues with large and bulky side chains.

Can you please explain why the answer is A

Thanks

5. During glycolysis, glucose-1-phosphate is converted to fructose-6- phosphate in two successive reactions: Glucose-1-phosphate glucose-6-phosphate kJ/mol Glucose-6-phosphate fructose-6-phosphate kJ/mo Go -71 Go'-+1.7 AGo' for the overall reaction is: A -8.8 kJ/mo B 7.1 kJ/mo C 5.4 kJ/mol. D +5.4 kJ/mol.

Explanation / Answer

Ans. Correct option. A. Arginine has a basic side chain, which is positively charged at physiological pH. The carboxyl terminal has a negative charge at physiological pH.

So, the favorable ionic interaction between positively charged side chain of Arg and negatively charged carboxyl terminal stabilizes helix through mutual attraction.

Note that a-helices are primarily stabilized by H-bonds. Ionic interactions are less abundant than H-bonds.

# Option B. Incorrect. Protons do not transmit from one to another region of the peptide.

# Option C. Incorrect. Lys has a basic side chain and bear positive charge at physiological pH. Being positively charged, two lys residues repel each other.

Therefore, repulsion between the negatively charged residues destabilizes the helix.

# Option D. Incorrect. Both the Asp and Glu are acidic residues and bear negative charge at physiological pH. Because both the residues bear same negative charge, they repel each other – because same charge repel each other.

Therefore, repulsion between the negatively charged residues destabilizes the helix.

# Option E. Incorrect. Adjacently placed large and bulky side chains destabilizes the helix by causing stearic repulsion.

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