Nanosecond electron tunneling between the hemes in cytochrome bo3

Biological electron transfer (eT) between redox-active cofactors is thought to occur by quantum-mechanical tunneling. However, in many cases the observed rate is limited by other reactions coupled to eT, such as proton transfer, conformational changes, or catalytic chemistry at an active site. A prominent example of this phenomenon is the eT between the heme groups of mitochondrial cytochrome c oxidase, which has been reported to take place in several different time domains. The question of whether pure eT tunneling in the nanosecond regime between the heme groups can be observed has been the subject of some experimental controversy. Here, we report direct observations of eT between the heme groups of the quinol oxidase cytochrome bo₃ from Escherichia coli, where the reaction is initiated by photolysis of carbon monoxide from heme o₃. eT from CO-dissociated ferrous heme o₃ to the low-spin ferric heme b takes place at a rate of (1.2 ns)⁻¹ at 20°C as determined by optical spectroscopy. These results establish heme–heme electron tunneling in the bo₃ enzyme, a bacterial relative to the mitochondrial cytochrome c oxidase. The properties of eT between the closely lying heme groups in the heme–copper oxidases are discussed in terms of the reorganization energy for the process, and two methods for assessing the rate of electron tunneling are presented.