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group has made a mutant that contains two amino acid changes-namely, a surface Phe residue in the normal protein has been replaced by His (side chain pKa=6.1), and a surface Gln has been replaced by Glu (side chain pKa=4.2).

Part A

Is the pI of the mutant protein predicted to be greater than, less than, or the same as the pI of the normal protein?

Part B

Support you answer with the calculation of overall charge at pH 5.5

Explanation / Answer

ANSWER: basic amino acids have higher pI and acidic ones have lower pI.

pI = 1/2 (pKa1 + pKa2).

so the pI of this mutant is = 1/2 (6.1 + 4.2) = 5.15

thus this pI is less than the novel protein studied.

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