Nanosecond electron tunneling between the hemes in cytochrome bo ₃

Abstract

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 bo 3 from Escherichia coli, where the reaction is initiated by photolysis of carbon monoxide from heme o 3. 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 hemeheme electron tunneling in the bo 3 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.biological electron transfer ͉ heme-copper oxidases ͉ Marcus theory ͉ Moser-Dutton ruler ͉ ultrafast spectroscopy

Heme–copper oxidases use tunneling pathways

“…The heme iron separations differ in the two proteins by less than 0.5 Å, and the nearby methyl groups distances differ by 0.8 Å, based on the x-ray structures [differences well within the range of thermal fluctuations (19)]; the coupling pathway structure is nearly the same in the two proteins, involving the bonded His-Phe-His path linking irons and the direct methyl-tomethyl through-space contact. Taking a minimum donor-acceptor distance of 7.8 Å, a (generic) packing density value (2,14), and a typical ET reorganization energy (0.7 eV) places rates computed with packing analysis in agreement with experimental rates (20). However, recent studies have found a reorganization energy for cytochrome bo 3 of Ͻ0.2 eV, so the kinetics cannot be accommodated with the packing density reported by Dutton and coworkers (20).…”

“…Moreover, small changes in the model parameters or medium sampling rules will shift atoms in or out of the packing density analysis. Jasaitis et al (2) point to the ''large local variation of the packing density'' in the space between the hemes, meaning that the computed density is model dependent. Dominant single pathways are particularly likely to arise at short ET distances, so it is not surprising that difficulties with ''coarsegrained'' models occur for the cytochrome bo 3 and cytochrome c oxidase heme-to-heme reactions.…”

“…However, the packing density model makes many untested assumptions about how best to quantify a protein's packing density. Indeed, Jasaitis et al (2) show that alternative plausible strategies can shift the computed packing density by 50% for cytochrome bo 3 heme o 3 -heme b ET. Pathway and packing density methods will make qualitatively different ET rate predictions if the structure of the strongest pathway is not typical of the medium structure in which it is embedded (11).…”

“…Cytochrome bo 3 Jasaitis et al (2) report ET kinetics between CO-dissociated ferrous heme o 3 and low-spin ferric heme b on the nanosecond time scale. Indeed, the structure and heme-to-heme ET kinetics is very similar in cytochrome bo 3 and in cytochrome c oxidase.…”

“…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.…”