Heme–copper oxidases use tunneling pathways

Abstract

Electron transfer (ET) reactions in bioenergetics move electrons 10 A and more between redox cofactors through insulating protein (1). Nowhere else does the wave nature of matter control a biological event in a more striking way: These shifting electrons tunnel quantum mechanically from donor to acceptor. Nature uses the physics of electron tunneling for good reason: Delocalized charge would dissipate energy and induce collateral redox damage. As described in a recent issue of PNAS (2), new studies of electron tunneling between hemes in the quinol-oxidizing cytochrome bo 3, a heme–copper oxidase in the same family as cytochrome c oxidase, find that a specific tunneling conduit, or tunneling pathway, mediates the electron flow. This experiment, which shows that a dominant coupling pathway controls a physiological ET reaction, reveals the limitations of coarse-grained (average medium) models for protein electron tunneling and indicates why atomic-resolution descriptions can be essential. The perspective emerging from this study will help to determine how evolution acts on the molecular structure of electron tunneling pathways in the biological ET chains.