Secondary active-site residues in enzymes, including hydrophobic amino acids, may contribute to catalysis through critical interactions that position the reacting molecule, organize hydrogen-bonding residues, and define the electrostatic environment of the active site. To ascertain the tolerance of an important model enzyme to mutation of active-site residues that do not directly hydrogen bond with the reacting molecule, all 19 possible amino acid substitutions were investigated in six positions of the engineered chorismate mutase domain of the Escherichia coli chorismate mutase-prephenate dehydratase. The six secondary active-site residues were selected to clarify results of a previous test of computational enzyme design procedures. Five of the positions encode hydrophobic side chains in the wild-type enzyme, and one forms a helix N-capping interaction as well as a salt bridge with a catalytically essential residue. Each mutant was evaluated for its ability to complement an auxotrophic chorismate mutase deletion strain. Kinetic parameters and thermal stabilities were measured for variants with in vivo activity. Altogether, we find that the enzyme tolerated 34% of the 114 possible substitutions, with a few mutations leading to increases in the catalytic efficiency of the enzyme. The results show the importance of secondary amino acid residues in determining enzymatic activity, and they point to strengths and weaknesses in current computational enzyme design procedures.