You have discovered a novel protein that has a pI=5.5. To study the functional p

Question

You have discovered a novel protein that has a pI=5.5. To study the functional properties of this new protein, your research group has made a mutant that contains two amino acid changes-namely, a surface Phe residue in the normal protein has been replaced by His (side chain pKa=6.1), and a surface Gln has been replaced by Glu (side chain pKa=4.2).

Support your answer with the calculation of the pI of the mutant at pH=5.5. Assume that the only factors impacting the change in the pI are the side chains of the mutant protein.

Explanation / Answer

For the mutant protein,

when Phe was replaced with His and Gln was replaced with Glu

at pH 5.5, the charge on His is +1 and on Glu is -1

So,

pI = 1/2(6.1 + 4.2)

    = 5.1

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