Weak O2 binding and strong H2O2 binding at the non-heme diiron center of trypanosome alternative oxidase

Alternative oxidase (AOX) catalyzes the four-electron reduction of dioxygen to water as an additional terminal oxidase, and the catalytic reaction is critical for the parasite to survive in its bloodstream form. Recently, the X-ray crystal structure of trypanosome alternative oxidase (TAO) complexed with ferulenol was reported and the molecular structure of the non-heme diiron center was determined. The binding of O₂ was a unique side-on type compared to other iron proteins. In order to characterize the O₂ binding state of TAO, the O₂ binding states were searched at a quantum mechanics/molecular mechanics (QM/MM) theoretical level in the present study. We found that the most stable O₂ binding state is the end-on type, and the binding states of the side-on type are higher in energy. Based on the binding energies and electronic structure analyses, O₂ binds very weakly to the TAO iron center (ΔE =6.7 kcal mol^-1) in the electronic state of Fe(II)…OO, not in the suggested charge transferred state such as the superoxide state (Fe(III)OO· ^-) as seen in hemerythrin. Coordination of other ligands such as water, Cl^-, CN^-, CO, N₃^- and H₂O₂ was also examined, and H₂O₂ was found to bind most strongly to the Fe(II) site by ΔE = 14.0 kcal mol^-1. This was confirmed experimentally through the measurement of ubiquinol oxidase activity of TAO and Cryptosporidium parvum AOX which was found to be inhibited by H₂O₂ in a dose-dependent and reversible manner.

A Manganese(IV)-Hydroperoxo Intermediate Generated by Protonation of the Corresponding Manganese(III)-Superoxo Complex

Earlier work revealed that metal-superoxo species primarily function as radicals and/or electrophiles. Herein, we present ambiphilicity of a Mn^III-superoxo complex revealed by its proton- and metal-coupled electron-transfer processes. Specifically, a Mn^IV-hydroperoxo intermediate, [Mn(BDP^BrP)(OOH)]⁺ (1, H₂BDP^BrP = 2,6-bis((2-(S)-di(4-bromo)phenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) was generated by treatment of a Mn^III-superoxo complex, Mn(BDP^BrP)(O₂^•) (2) with trifluoroacetic acid at -120 °C. Detailed insights into the electronic structure of 1 are obtained using resonance Raman and multi-frequency electron paramagnetic resonance spectroscopies coupled with density functional theory calculations. Similarly, the reaction of 2 with scandium(III) triflate was shown to give a Mn(IV)/Sc(III) bridging peroxo species, [Mn(BDP^BrP)(OO)Sc(OTf)_n]^(3-n)+ (4). Furthermore, it is found that deprotonation of 1 quantitatively regenerates 2, and that one-electron oxidation of the corresponding Mn^III-hydroperoxo species, Mn(BDP^BrP)(OOH) (3), also yields 1.

Electronic structures of heme a of cytochrome c oxidase in the redox states--charge density migration to the propionate groups of heme a

The electronic structures of heme a of cytochrome c oxidase in the redox states were studied, using hybrid density functional theory with a polarizable continuum model and a point charge model. We found that the most stable electronic configurations of the d electrons of the Fe ion are determined by the orbital interactions of the d orbitals of the Fe ion with the pi orbitals of the porphyrin ring and the His residues. The redox reaction of the Fe ion influences the charge density on the formyl group through the pi conjugation of the porphyrin ring. In addition, we found the charge transfer from the Fe ion to the propionate group of heme a in the redox change despite the lack of the pi-conjugation. We elucidated that the charge propagation originates from the heme a structure itself and that the origin of the charge delocalization to the heme propionate is the orbital interactions between the d orbital of the Fe ion and the p orbitals of the carboxylate part of the heme propionate via the pi conjugation of the porphyrin ring and the sigma* orbital of the C-C bond of the propionate group. The electrostatic effect by surrounding proteins enhances the charge transfer from the Fe ion to the propionate group. These results indicate that heme propionate groups serve electron mediators in electron transfer as well as electrostatic anchors, and that proteins surrounding the active site reinforce the congenital abilities of the cofactors.