In three separate site-directed mutagenesis experiments, scientists altered a catalytic Lys residue in corn PEP carboxylase to an Asn, Glu, or Arg. Which substitution would you expect to have the least effect on enzymatic activity

Arginine (Arg).

Explanation:

Lysine (Lys) is a hydrophobic amino acid that contains an α-amino group and an α-carboxyl acid group, and a side chain lysyl. It is a residue positively charged and has polar and hydrophilic properties, being thus similar to the Arg residue. Therefore, it is expected that the replacement of Lys by Arg should not have significant effects on the structure of the PEP carboxylase enzyme.  

This type of substitution is named conservative because the replacement is made between amino acids with similar properties, thereby they have less impact on protein function and aren't so easily removed by natural selection compared to non-conservative substitutions.