Effect of heat treatment on oxidase activity and proton-pumping capability of proteoliposome-incorporated beef heart cytochrome aa3

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Abstract
By incubating beef heart cytochrome c oxidase at 43-45.degree. C, selective inactivation of the H+-pumping function is possible without affecting cytochrome c oxidase activity; proteoliposomes reconstituted with heated enzyme (43.5.degree. C for 60 min at pH 7.0) showed an apparent H+/e- ratio of only 0.3 and a turnover with cytochrome c plus ferrocyanide as substrate of 20 s-1, while those with the intact enzyme showed an apparent H+/e- ratio somewhat greater than 1.0 and a turnover of 19 s-1. This decrease in the H+/e- ratio could not be attributed to a stimulation of H+ permeability upon heating, since the respiratory control ratio and the magnitude of membrane potential formation remained almost the same in the 2 cases. A pH-dependent Em (midpoint redox potential) change of cytochrome a in the presence of cyanide was still observed after the heat treatment. Heating induced a small spectral shift in the Soret region of the oxidized (resting) enzyme; the peak of the heated enzyme was at 421 nm, while that of the intact enzyme was at 419 nm. The spectral shift obtained by pulsing the enzyme with O2 under turnover conditions is also altered.

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