Proton involvement in the transition from the "peroxy" to the ferryl intermediate of cytochrome c oxidase.
In the absence of any external electron donor, the "peroxy" intermediate of cytochrome c oxidase (CcO-607) is converted to the ferryl form (CcO-580) and subsequently to oxidized enzyme. The rate of conversion of CcO-607 to the CcO-580 form is pH dependent between pH 3.0 and pH 7.6. A plot of the logarithm of the rate constant for this conversion is a linear function of pH with a slope of -0.92, implying the involvement of a single proton in the transition. Upon rapidly lowering the pH from 8.1 to 5.8, the uptake of one proton was observed by direct pH measurement, and the kinetics of proton uptake coincide with the spectral conversion of CcO-607 to CcO-580. We interpret the slow endogenous decay of CcO-607 to CcO-580 to be the result of proton transfer to a deprotonated group generated in the binuclear cavity during CcO-607 formation. This group is not freely accessible to protons from the medium, and its pK(a) is probably higher than 9.0.
Fabian
M
M
Department of Biochemistry and Cell Biology, MS 140, Rice University, 6100 Main, Houston, TX 77005-1892, USA. fabian@bioc.rice.edu
Palmer
G
G
D002021:Buffers; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000378:metabolism*; D005292:Ferricyanides / Q000378:metabolism; D005296:Ferrous Compounds / Q000378:metabolism*; D006531:HEPES; D006861:Hydrogen Peroxide / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010084:Oxidation-Reduction; D011522:Protons; D013053:Spectrophotometry; D013431:Sulfates / Q000378:metabolism
D002021:Buffers; D005292:Ferricyanides; D005296:Ferrous Compounds; D011522:Protons; D013431:Sulfates; C007931:hexacyanoferrate III; C031512:potassium sulfate; D006861:Hydrogen Peroxide; D003576:Electron Transport Complex IV; D006531:HEPES