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The soluble cytochrome o from Vitreoscilla contains two identical subunits and two hemes. The reduced form binds 2 mol of CO in a cooperative manner with a Hill coefficient near 2 (Tyree, B., and Webster, D. A. (1978) J. Biol. Chem. 253, 6988-6991). This carbonyl compound was photolysed with a dye laser and recombination followed at 437 or 420 nm where maximal absorbance changes were registered. Recombination kinetics were biphasic, and the fast phase was approximately 10 times the rate of the slow phase. Apparent rate constants of both phases showed a nonlinear dependence on CO concentration, respectively, in conformity with a reaction scheme which assumes the transient formation of an intermediate species in both slow and fast reactions. A study of temperature dependence of the reactions gave EA = 2.7 kcal/mol for the slow reaction and EA = 3.2 kcal/mol for the fast reaction below 23 degrees C; above this temperature the slope of the Arrhenius plot for the fast reaction became positive. Maximal rates for both phases were around pH 6.5 and fell to approximately 40% of maximal at pH 12. The binding reaction was affected by even a low concentration of sodium dodecyl sulfate (0.0025%), which changed both the kinetic constant of each phase and the relative contribution of each phase to the reaction. A model which assumes the existence of fast and slow reaction conformers in equilibrium is proposed.

Oxygenated cytochrome o(s) from Vitreoscilla was photodissociated by a laser flash but the quantum yield was low. The rebinding of oxygen to the ferrous cytochrome proceeded monophasically, and the second order rate constant was 7.8 X 10(7) M-1 s-1, the off rate constant 5.6 X 10(3) s-1, and the calculated dissociation constant for the oxygenated compound 7.2 X 10(-5) M at pH 7.3 and 25 degrees C. Rapid scanning spectroscopy revealed the formation of chytochrome o-O2 directly from ferrous chytochrome o and oxygen without any evidence for an intermediary formation of Compound D, another type of oxygenated chytochrome o. Photodissociation in solution containing CO/O2 mixtures resulted in rapid binding of oxygen followed by slow replacement by CO. This property as well as the photodissociability of chytochrome o-O2 suggests that the heme iron of the compound is in the ferrous state. In addition, the primary oxygen compound was fairly stable and did not decay further in the absence of CO, in marked contrast with that of mammalian cytochrome oxidase primary oxygen compound which rapidly decayed. This result suggests a possible role of this cytochrome as an oxygen carrier or storage.

Cytochrome o, a protoheme IX pigment, has been proposed as the terminal oxidase of the filamentous bacterium, Vitreoscilla. Aerobic and anaerobic photolysis of CO-liganded whole cells demonstrated the presence of a second CO-reactive pigment, cytochrome o'. At temperatures lower than -100 degrees C, anaerobic photolysis dissociated only about 25% of the total CO-liganded components to reveal the unliganded cytochrome o'. At these temperatures, the photolysis of cytochrome o could not be demonstrated. At warmer temperatures, recombination of CO with the reduced cytochrome o' occurred with an apparent energy of activation of 5.8 kcal/mol. Aerobic photolysis of whole cells demonstrated two oxygen-bound intermediates. At temperatures lower than -95 degrees C, a spectrally distinct compound with absorption maxima at 428, 534, and 564 nm appeared (form I'); the apparent second order rate constant (k+1) for the formation of this intermediate was found to be 9.1 M-1 s-1, the reverse rate (k-1) was 9.9 X 10(-5) s-1, and the equilibrium constant (Kd) was 1.1 X 10(-5) M. This oxygen intermediate of cytochrome o' is spectrally and kinetically similar to the oxygen intermediate of cytochrome o seen in Escherichia coli. At temperatures warmer than -90 degrees C, photolysis of aerobic samples resulted in the immediate formation of a second oxygen-bound intermediate (form I) with absorption maxima at 422, 534, and 564 nm. This second intermediate results from the binding of oxygen to the cytochrome o (oxygenated cytochrome o). These data support the proposal that whole cells of Vitreoscilla contain two alternative pathways of electron transport, one terminating with cytochrome o and the other with cytochrome o'.

The formation of hydrogen peroxide during the oxidation of NADH by purified preparations of cytochrome o has been demonstrated by employing three independent methods: polarographic, colorimetric, and fluorometric. The first two methods were used to assay for the accumulation of hydrogen peroxide and showed that hydrogen peroxide did accumulate as a product, but only about 30% of the oxygen consumed or 15 to 20% of the NADH oxidized was recoverable as hydrogen peroxide. This lack of 1:1 stoichiometry was not due to residual catalase activity in these preparations which could be eliminated by freeze-thawing. Thus, hydrogen peroxide may not be the sole or primary product of the NADH-cytochrome o oxidase reaction. The fluorometric assay could be coupled directly to the NADH-cytochrome o oxidase reaction in one medium, and this method showed that hydrogen peroxide was generated continuously from the beginning of the reaction in a 1:1 stoichiometry, hydrogen peroxide generated to NADH oxidized. This result suggests that hydrogen peroxide is an intermediate that can be trapped efficiently under the conditions of the fluorometric assay, whereas under the conditions of the first two assays most of the hydrogen peroxide generated undergoes further reaction. Exogenously added FAD or FMN increased the percentage of hydrogen peroxide that accumulated in the NADHcytochrome o oxidase reaction. Flavin is believed to act on the reductase side of cytochrome o so the increased percentage of hydrogen peroxide is not likely to result from the direct reaction of reduced flavin with oxygen.

The oxygenerated form of cytochrome o can be readily observed in whole cell suspensions of Vitreoscilla because it is the predominant steady state form of the cytochrome in respiring cells. When oxygen is introduced to an anaerobic suspension of cells, the steady state is established immediately and absorption maxima are observed at 578, 545, and 425 nm in difference spectra using ammonium persulfate-oxidized cells as reference. The oxygenerated form of cytochrome o is identified as the predominant steady state species during turnover of the cytochrome in vivo from the close coincidence of these spectra with those obtained using purified cytochrome. When the dissolved oxygen is consumed, the spectrum of the oxygenated form is replaced by the spectrum of the reduced form. Glutamate increased the respiration of these cells and decreased the duration of the steady state. Spectra of the oxygenated form of cytochrome o could also be obtained by using starved cells as reference, i.e. (untreated cells) - (starved cells) difference spectra. These results implicate the oxygenated form of cytochrome o as an active intermediate in terminal electron transport of these cells. In the presence of 1 mM cyanide, which inhibits the respiration of Vitreoscilla, the duration of the steady state was increased and the steady state spectrum was altered, showing that turnover of the cytochrome was inhibitied and formation of the oxygenated form wsa decreased. All these results suggest that the cyclic changes of cytochrome o in terminal respiration involve the oxidized, reduced, and oxygenated forms.