Question about substituting a positively charged amino acid for a neutral one. TPR 2016 Biochem P.55

[liyinhui2013]

An acidic Glu residue in a protein (neutral in its protonated form) has a pKa value of 2.3 in the wild-type protein and is found near a neutral Ile residue. What will be the effect on the pKa of the Glu residue if a mutation substitutes a positively charged Lys residue for the Ile residue?

A. The pKa will decrease due to favorable ionic interactions between the deprotonated Glu and Lys residues.
B. The pKa will decrease due to unfavorable ionic interactions between the deprotonated Glu and Lys residues.
C. The pKa will increase due to favorable ionic interactions between the deprotonated Glu and Lys residues.
D. The pKa will increase due to unfavorable ionic interactions between the deprotonated Glu and Lys residues.

Why the answer is A? How could a positively charged Lys residue stabilize Glu?

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[theonlytycrane]

A positively charged Lys and negativity charged Glu (deprotonated) will have some stabilizing ionic interactions. This favors deprotonation of Glu, thus lowering its pKa.

 

[aldol16]

Salt bridge.