Effect of calcium ions on electron transfer between hemes a and a 3 in cytochrome c oxidase
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Kinetics of the reduction of the hemes in cytochrome c oxidase in the presence of high concentration of ruthenium(III)hexaammine chloride was examined using a stopped-flow spectrophotometer. Upon mixing of the oxidized enzyme with dithionite and Ru(NH3) 6 3+ , three well-resolved phases were observed: heme a reduction reaching completion within a few milliseconds is followed by two slow phases of heme a 3 reduction. The difference spectrum of heme a 3 reduction in the visible region is characterized by a maximum at ∼612 nm, rather than at 603 nm as was believed earlier. It is shown that in the case of bovine heart cytochrome c oxidase containing a special cation-binding site in which reversible binding of calcium ion occurs, heme a 3 reduction is slowed down by low concentrations of Ca2+. The effect is absent in the case of the bacterial cytochrome oxidase in which the cation-binding site contains a tightly bound Ca2+ ion. The data corroborate the inhibition of the cytochrome oxidase enzymatic activity by Ca2+ ions discovered earlier and indicate that the cation affects intramolecular electron transfer.
Key wordscytochrome c oxidase Ca2+ ions fast kinetics heme a3
cytochrome c oxidase
hexaammineruthenium (Ru(NH3) 6 3+ )
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