Suppose that an aspartic acid (aspartate) residue in the active site of an enzyme was mutated to alanine. As expected, the alanine mutant was inactive, suggesting that the aspartic acid residue was critical to the catalytic mechanism.
Which mutation is most likely to restore wild-type level of activity to the alanine mutant?

a) A toY
b) A to E
c) A toL
d) A to M
e) A to K