Insight into the mechanism of aromatic hydroxylation by toluene 4-monooxygenase by use of specifically deuterated toluene and p-xylene

The present studies address the mechanism of aromatic hydroxylation used by the natural and G103L isoforms of the diiron enzyme toluene 4-monooxygenase. These isoforms have comparable catalytic parameters but distinct regiospecificities for toluene hydroxylation. Hydroxylation of ring-deuterated p-xylene by the natural isoform revealed a substantial inverse isotope effect of 0.735, indicating a change in hybridization from sp(2) to sp(3) for hydroxylation at a carbon atom bearing the deuteron. During the hydroxylation of 4-H-2(1)- and 3,5-H-2(2)-toluene, similar magnitudes of intramolecular isotope effects and patterns of deuterium retention were observed from both isoforms studied, indicating that the active-site mutation affected substrate orientation but did not influence the mechanism of hydroxylation. The results with deuterated toluenes show inverse intramolecular isotope effects for hydroxylation at the position of deuteration, normal secondary isotope effects for hydroxylation adjacent to the position of deuteration, near-quantitative deuterium retention in m-cresol obtained from 4-H-2(1)-toluene, and partial loss of deuterium from all phenolic products obtained from 3,5-H-2(2)-toluene. This combination of results suggests that an active site-directed opening of position-specific transient epoxide intermediates may contribute to the chemical mechanism and the high degree of regiospecificity observed for aromatic hydroxylation in this evolutionarily specialized diiron enzyme.