In three separate site-directed mutagenesis experiments, scientists altered a Lys residue in corn PEP carboxylase to an Asn, Glu, or Arg. The substitution that you would expect to have the least effect on enzymatic activity would be because it, like Lys, is charged at physiological pH. The substitution that you would expect to have the most effect on enzymatic activity would be because it is charged at physiological pH.