|
Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. August 2006, Volume 5, Number 8, 447-459 |
Electronic Structures of Heme(Fe)-Dioxygen Complex as an
Intermediate Model of Dioxygen Reduction in Cytochrome c Oxidase
Yasunori Yoshioka, Masaki Mitani, and Hiroyuki Satoh
Internet Electron. J. Mol. Des. 2006, 5, 447-459
|
Abstract:
We have previously proposed the O2 reduction mechanism
that the protons transfer from the K-channel to the active
site of CcO. The second proton transfer to the hydroperoxy
intermediate FeOOH did not lead the OO bond cleavage. It
has been recently reported that the addition of the proton
induces the OO bond cleavage to yield the productive H2O
molecule and the oxo intermediate Fe=O, being
inconsistent with our results. We have started this study to
confirm whether the OO bond cleavage occurs or not upon
the proton addition. We concentrated our study to the
changes of the electronic structures of the heme a3 part
throughout the sequential additions of the protons and an
electron during the process of the O2 reduction. We
employed a model that is composed of porphyrin without
any substituents, Fe, and an imidazole as His376. We added
sequentially protons and an electron to the heme(Fe)-dioxygen
complex. The geometrical parameters were fully
optimized without any constraints. The unrestricted hybrid
exchange-correlation functional B3LYP method was used.
The Wachters double zeta basis set was employed for Fe
atom. The 6-31G* basis set was used for C, N, and H
atoms, and the 6-311+G* for O atom. All calculations were
carried out using the program package Gaussian 98. The
proton addition to FeOO (H2) to yield FeOOH (H3) causes
the electron transfer from the porphyrin ring to FeOOH
moiety, giving the porphyrin-radical cation. The additional
proton to FeOOH (H4) leads to the OO bond cleavage,
yielding the hydrogen bonded complex of an oxo heme
(compound I) and a H2O molecule. The bonding characters
of FeOOH in H3 and H4 are identical, even though the electronic
structures of the porphyrin rings in H3 and H4 are
different. However, the proton addition to FeOOH (H3)
does not lead to the OO bond cleavage. The protonated
heme(Fe)-dioxygen complexes are effective models to
investigate the reduction mechanism of a O2 molecule in CcO.
|