Selective Stabilization of the Chorismate Mutase Transition State by a Positively Charged Hydrogen Bond Donor

Citrulline was incorporated via chemical semisynthesis at position 90 in the active site of the AroH chorismate mutase from Bacillus subtilis. The wild-type arginine at this position makes hydrogen-bonding interactions with the ether oxygen of chorismate. Replacement of the positively charged guanidinium group with the isosteric but neutral urea has a dramatic effect on the ability of the enzyme to convert chorismate into prephenate. The Arg90Cit variant exhibits a >10⁴-fold decrease in the catalytic rate constant k_cat with a 2.7-fold increase in the Michaelis constant K_m. In contrast, its affinity for a conformationally constrained inhibitor molecule that effectively mimics the geometry but not the dissociative character of the transition state is only reduced by a factor of ≈6. These results show that an active site merely complementary to the reactive conformation of chorismate is insufficient for catalysis of the mutase reaction. Instead, electrostatic stabilization of the polarized transition state by provision of a cationic hydrogen bond donor proximal to the oxygen in the breaking C−O bond is essential for high catalytic efficiency.