Exploring Transition State Stabilization by Mandelate Racemase Using Chloro-Substituted Inhibitors and Substrates

Abstract

Mandelate racemase (MR) from Pseudomonas putida catalyzes the Mg2+-dependent interconversion of (R)- and (S)-mandelate. MR has been employed as a model system to understand how enzymes overcome energy barriers involved in the abstraction of an α-proton. This work explored how chloro-substituents on the phenyl ring of MR inhibitors (phenylboronic acids and benzohydroxamates) and substrates ((R)-mandelates) impact binding and catalysis. Chloro-substituents at the meta and para positions increased inhibitor binding up to ~130-fold, but had minimal effect on the binding and turnover of (R)-mandelate. Investigations of chloro-substituents’ influence on transition state (TS) stabilization revealed that MR recognized the boronic acid and hydroxamate groups as analogues to the TS. However, the chloro-substituents did not confer ground state or TS recognition, suggesting that the chloro-substituents on the inhibitors may be exploiting fortuitous interactions with an altered conformation of the enzyme.