Question

In wild-type (normal) enzyme, there is an asparagine residue near the reactive site cysteine. Predict the effects of mutating this asparagine residue to an aspartic acid residue on the pKa of the reactive site cysteine and the activity of the enzyme at a given pH. Explain.

Answer 1

pKa of asparginine is 10.76

pKa of aspartic acid is 2.98

So PKA values of these two amino acid are totally different. This will going to change the overall pI of the enzyme and will affect charge on active site of amino acid .

For example protein which has asparginine at active site , its pKa value will be most probably more than 9 or 10. So this enzyme will be active at basic pH. But if protein has aspartic acid at active site than it will lead to change the overall pI of protein which will shift to more acidic side. Now this protein is not active the previous described pH ( basic pH) due to aspartic acid residue.