Electronic structure analysis of the oxygen-activation mechanism by Fe(II)- and ?-ketoglutarate (?KG)-dependent dioxygenases.

ABSTRACT: ?-Ketoglutarate (?KG)-dependent nonheme iron enzymes utilize a high-spin (HS) ferrous center to couple the activation of oxygen to the decarboxylation of the cosubstrate ?KG to yield succinate and CO(2), and to generate a high-valent ferryl species that then acts as an oxidant to functionalize the target C-H bond. Herein a detailed analysis of the electronic-structure changes that occur in the oxygen activation by this enzyme was performed. The rate-limiting step, which is identical on the septet and quintet surfaces, is the nucleophilic attack of the distal O atom of the O(2) adduct on the carbonyl group in ?KG through a bicyclic transition state ((5, 7) TS1). Due to the different electronic structures in (5, 7) TS1, the decay of (7)TS1 leads to a ferric oxyl species, which undergoes a rapid intersystem crossing to form the ferryl intermediate. By contrast, a HS ferrous center ligated by a peroxosuccinate is obtained on the quintet surface following (5)TS1. Thus, additional two single-electron transfer steps are required to afford the same Fe(IV)-oxo species. However, the triplet reaction channel is catalytically irrelevant. The biological role of ?KG played in the oxygen-activation reaction is dual. The ?KG LUMO (C=O ?*) serves as an electron acceptor for the nucleophilic attack of the superoxide monoanion. On the other hand, the ?KG HOMO (C1-C2 ?) provides the second and third electrons for the further reduction of the superoxide. In addition to density functional theory, high-level ab initio calculations have been used to calculate the accurate energies of the critical points on the alternative potential-energy surfaces. Overall, the results delivered by the ab initio calculations are largely parallel to those obtained with the B3LYP density functional, thus lending credence to our conclusions.

SUBMITTER: Ye S 

PROVIDER: S-EPMC3955955 | biostudies-literature | 2012 May

REPOSITORIES: biostudies-literature