Open AccessElectron transfer between hemes in mammalian cytochrome c oxidase
Author(s) -
Eric Pilet,
Audrius Jasaitis,
Ursula Liebl,
Marten H. Vos
Publication year - 2004
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.0405032101
Subject(s) - flash photolysis , heme a , electron transfer , chemistry , heme , photochemistry , redox , electron transport chain , photodissociation , cytochrome c oxidase , ultrafast laser spectroscopy , kinetics , spectroscopy , reaction rate constant , enzyme , physics , inorganic chemistry , biochemistry , quantum mechanics
Fast intraprotein electron transfer reactions associated with enzymatic catalysis are often difficult to synchronize and therefore to monitor directly in non-light-driven systems. However, in the mitochondrial respiratory enzyme cytochrome oxidase aa(3), the kinetics of the final electron transfer step into the active site can be determined: reverse electron flow between the close-lying and chemically identical hemes a(3) and a can be initiated by flash photolysis of CO from reduced heme a(3) under conditions where heme a is initially oxidized. To follow this reaction, we used transient absorption spectroscopy, with femtosecond time resolution and a time window extending to 4 ns. Comparison of the picosecond heme a(3)-CO photodissociation spectra under different redox states of heme a shows significant spectral interaction between both hemes, a phenomenon complicating the interpretation of spectral studies with low time resolution. Most importantly, we show that the intrinsic electron equilibration, corresponding to a DeltaG(0) of 45-55 meV, occurs in 1.2 +/- 0.1 ns. This is 3 orders of magnitude faster than the previously established equilibration phase of approximately 3 mus, which we suggest to reflect a change in redox equilibrium closely following CO migration out of the active site. Our results allow testing a number of conflicting predictions regarding this reaction between both experimental and theoretical studies. We discuss the potential physiological relevance of fast equilibration associated with this low-driving-force redox reaction.