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Proc Natl Acad Sci U S A. 2004 Nov 16;101(46):16198-203. Epub 2004 Nov 8.

Electron transfer between hemes in mammalian cytochrome c oxidase.

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Institut National de la Santé et de la Recherche Médicale U451, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 7645, Ecole Polytechnique, Ecole Nationale Supérieure de Techniques Avancées, 91128 Palaiseau Cedex, France.


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.

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