Activationless electron transfer through the hydrophobic core of cytochrome c oxidase.
Electron transfer (ET) within proteins occurs by means of chains of redox intermediates that favor directional and efficient electron delivery to an acceptor. Individual ET steps are energetically characterized by the electronic coupling V, driving force DeltaG, and reorganization energy lambda. lambda reflects the nuclear rearrangement of the redox partners and their environment associated with the reactions; lambda approximately 700-1,100 meV (1 eV = 1.602 x 10(-19) J) has been considered as a typical value for intraprotein ET. In nonphotosynthetic systems, functional ET is difficult to assess directly. However, using femtosecond flash photolysis of the CO-poised membrane protein cytochrome c oxidase, the intrinsic rate constant of the low-DeltaG electron injection from heme a into the heme a(3)-Cu(B) active site was recently established at (1.4 ns)(-1). Here, we determine the temperature dependence of both the rate constant and DeltaG of this reaction and establish that this reaction is activationless. Using a quantum mechanical form of nonadiabatic ET theory and common assumptions for the coupled vibrational modes, we deduce that lambda is <200 meV. It is demonstrated that the previously accepted value of 760 meV actually originates from the temperature dependence of Cu(B)-CO bond breaking. We discuss that low-DeltaG, low-lambda reactions are common for efficiently channeling electrons through chains that are buried inside membrane proteins.
Jasaitis
Audrius
A
Laboratory for Optical Biosciences, INSERM U696, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 7645, Ecole Polytechnique-Ecole Nationale Supérieure de Techniques Avancées, 91128 Palaiseau Cedex, France.
Rappaport
Fabrice
F
Pilet
Eric
E
Liebl
Ursula
U
Vos
Marten H
MH
D000818:Animals; D002417:Cattle; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000737:chemistry* / Q000378:metabolism*; D004789:Enzyme Activation; D057927:Hydrophobic and Hydrophilic Interactions; D066298:In Vitro Techniques; D007700:Kinetics; D008929:Mitochondria, Heart / Q000201:enzymology; D013816:Thermodynamics
D003576:Electron Transport Complex IV