pH-induced alterations in the structure and activity of cytochrome c oxidase

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Abstract
The effects of pH on the activity and structure of beef heart cytochrome c oxidase were studied in the pH range 5.0-7.6. A group with pK of approximately 5.45 has been readily detected in the pH vs. activity curve. This group must be deprotonated to achieve maximal activity. A group with a similar pK (5.45) has been detected and contributes to the spectral character of the reduced oxidase. Over the pH range 5.0-7.6 no other acid-sensitive group contributes to the spectrum of the reduced oxidase. The oxidized oxidase shows at least 3 acid-sensitive groups contributing to the spectrum. One occurs in the pH 7 range and another in the pH 5.6 range; below pH 5.2 additional pH-sensitive groups are apparent. Accurate estimation of the pK of the groups responsible for the spectral changes in the oxidized oxidase has not been possible. The spectrum of the oxygenated (428 nm) conformer of the oxidized protein is invariant over the pH range 5.5-7. The changes occuring in the spectrum of the purified oxidase also occur in the protein contained in phospholipid vesicles. The data are discussed in terms of the mechanism by which the oxidase, during its in situ catalytic cycle, may rise to the primary events in energy coupling.