Understanding the AroA Mechanism: Evidence for Enolpyruvyl Activation and Kinetic Isotope Effect Measurements

Abstract

<p> AroA catalyzes a carboxyvinyl transfer reaction, forming enolpyruvyl shikimate 3-phosphate (EPSP) from shikimate 3-phosphate (S3P) and phosphoenolpyruvate (PEP). Upon extended incubation, it forms EPSP ketal by intramolecular nucleophilic attack of O4H on C2' of the enolpyruvyl group. EPSP ketal was previously proposed to form by non-enzymatic breakdown of the tetrahedral intermediate (THI) which had dissociated from AroA. In this study, EPSP ketal formed in the presence of excess AroA, which demonstrated that it was formed in the active site. This eliminated non-enzymatic THI breakdown as its source, and demonstrated that AroA forms either a discrete EPSP cationic intermediate, or cl transition state with high cationic character. The pH dependence of non-enzymatic EPSP hydrolysis was examined in order to understand the intrinsic reactivity of the enolpyruvyl group. Acid catalysis accelerated EPSP hydrolysis> 10^8-fold. These results provide evidence for enolpyruvyl activation through protonation at C3', forming an unstable cationic intermediate, or a highly cation-like transition state. The incorporation of 2H into EPSP from solvent 2H20 during AreA-catalyzed hydrolysis was much slower than the hydrolysis rate, in the absence of inorganic phosphate in the reaction. This demonstrated that KIEs on AroA-catalyzed EPSP hydrolysis, when they are measured in the future, will reflect protonation of EPSP. A method was developed for KIE measurements on acid-catalyzed EPSP hydrolysis, which showed good reproducibility and no buffer dependence. Further experiments need to be completed on the acid-catalyzed KIEs and enzyme-catalyzed KIEs, followed by transition state analysis. This will precisely define the transition state structure of the enzyme-catalyzed EPSP hydrolysis reaction, and provide a good starting point for designing AroA inhibitors.</p>