Mechanism of the Hydroxylation Reactions Catalyzed By 4-Hydroxyphenylpyruvate Dioxygenase and Hydroxymandelate Synthase

Abstract

4-Hydroxyphenylpyruvate dioxygenase (HPPD) and Hydroxymandelate synthase (HMS) carry out highly similar complex dioxygenation reactions using the substrates, 4-hydroxyphenylpyruvate (HPP) and dioxygen. HPPD catalyzes decarboxylation, aromatic hydroxylation and substituent migration (NIH shift) in a single catalytic cycle to form homogentisate (HG), whereas HMS catalyzes decarboxylation and aliphatic hydroxylation to give hydroxymandelate (HMA). Wild-type HPPD, HPPD variants and HMS variants produce both native and non-native products. Based on this observation, we have employed a product analysis method with HPP deuterium substitutions (ring or benzylic) that reveal kinetic isotope effects from intermediate partitioning ratios. In this study we offer evidence for the 1) oxygenation intermediates for HPPD and HMS pathways and 2) mechanism of NIH shift in HPPD. Our data with ring-deutero HPP suggest that the native hydroxylation reaction of HPPD to form HG occurs via a ring epoxide intermediate whereas secondary normal KIEs with 3’,3’-diduetero HPP in HPPD indicate that bond cleavage in the substituent shift step occurs via a homolytic biradical mechanism (biradical). HMS variants show a small normal KIE with 3’,3’-diduetero HPP, indicating displacement of a benzylic deuteron in the hydroxylation step which has a multiplicative component from geometry changes for the non-abstracted deuterium. When R-3’-monodeutero-HPP is used as a substrate for HMS reaction, the secondary KIE observed indicates sp3 geometry at the benzylic carbon in the transition state for the hydroxylation step.