Computational Studies of Electron Tunneling in Respiratory Complex III

Abstract

In the framework of the tunneling current theory, electron tunneling rates and atomistic tunneling pathways were investigated for electron transfer reactions between different redox pairs in complex III comprising different stages in the proton-motive Q-cycle. In the light of our calculations, complex III appears to be a smart machine which under certain conditions undergoes conformational changes gating electron transfer, or channeling electrons to specific pathways. One-electron tunneling approximation was adopted in the tunneling calculations, which were done with hybrid Broken-Symmetry unrestricted DFT/ZINDO levels of theory. The tunneling orbitals were determined using the exact bi-orthogonalization scheme, which uniquely selects a pair of tunneling orbitals with small overlap, and remaining Frank-Condon orbitals with significant overlap. Due to extensive conjugation in porphyrins, the core electrons of redox hemes exhibit substantial induced polarization, resulting in the decreased electron transfer rates by 20 - 80%. Electron transfer rates in different redox pairs show exponential distance dependence, in agreement with other reported data.