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J. Biol. Chem., Vol. 264, Issue 14, 7976-7980, May, 1989
K Kobayashi, H Une and K Hayashi
The reduction of bovine heart cytochrome oxidase by the 1-
methylnicotinamide (MNA) radical was investigated by the use of pulse
radiolysis. With the decay of the MNA radical, the absorption at 445 and
605 nm, a characteristic to ferrous heme a of the oxidase, increased. The
kinetic difference spectrum obtained was similar to that of the fully
reduced minus the fully oxidized form of the oxidase, and was not different
from that obtained in the reaction of the MNA radical with the mixed
valence CO complex of the oxidase, where heme a3 is the CO-bound reduced
form with heme a oxidized. This suggests that the absorption changes at 445
and 605 nm arise from the reduction of heme a, not heme a3. In order to
elucidate the contribution of "visible" copper in this reaction, the
absorption of the oxidase in the near- infrared region was measured. A
decrease of the 830 nm band due to the reduction of visible copper was
detected with a half-life of 5 microseconds. This absorption change obeyed
pseudo-first order kinetics and its rate constant increased with the
concentration of the oxidase. This suggests that the absorption change at
830 nm is followed by a bimolecular reaction of the MNA radical with
visible copper of the oxidase. After the first phase of the reduction, the
return of the 830 nm band corresponding to oxidation of the copper was
observed with a half-life of 100 microseconds. Concomitantly, the
absorption at 605 and 445 nm due to the reduction of heme a increased. The
rates of oxidation of the copper were identical to those of the reduction
of heme a and independent of the oxidase concentration. This suggests that
the MNA radical reacts with visible copper of the oxidase with a second
order rate constant of 1.5 X 10(9) m-1 s-1 and subsequently the electron
flows to heme a by intramolecular electron migration with a first order
rate constant of 1.8 X 10(4) s-1. An activation energy of the
intramolecular electron transfer was calculated to be 2.8 kcal/mol in the
range 4-33 degrees C.
Electron transfer process in cytochrome oxidase after pulse radiolysis
Institute of Scientific and Industrial Research, Osaka University, Japan.
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