Effect of a pH Gradient on the Protonation States of Cytochrome c Oxidase: A Continuum Electrostatics Study

The steady-state spectroscopic behaviour and the turnover of cytochrome c oxidase incorporated into proteoliposomes have been investigated as functions of membrane potential and pH gradient. The respiration rate is almost linearly dependent on [cytochrome c2+] at high flux, but while the cytochrome a redox state is always dependent on the [cytochrome c2+] steady state, it reaches a maximum reduction level less than 100% in each case. The maximal aerobic steady-state reduction level of cytochrome a is highest in the presence of valinomycin and lowest in the presence of nigericin. The proportion of [cytochrome c2+] required to achieve 50% of maximal reduction of cytochrome a varies with the added ionophores; the apparent redox potential of cytochrome a is most positive in the fully decontrolled system (plus valinomycin and nigericin). At low levels of cytochrome a reduction, the rate of respiration is no longer a linear function of [cytochrome c2+], but is dependent upon the redox state of both cytochromes a and c. That is, proteoliposomal oxidase does not follow Smith–Conrad kinetics at low cytochrome c reduction levels, especially in the controlled states. The control of cytochrome oxidase turnover by ΔpH and by ΔΨ can be explained either by an allosteric model or by a model with reversed electron transfer between the binuclear centre and cytochrome a. Other evidence suggests that the reversed electron transfer model may be the correct one.Key words: proteoliposomes, cytochrome c, cytochrome oxidase, membrane potential, pH gradient, cytochrome a, electron transfer.

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Control of proteoliposomal cytochrome c oxidase: the overall reaction

Biochemistry and Cell Biology ◽

10.1139/o90-168 ◽

1990 ◽

Vol 68 (9) ◽

pp. 1128-1134 ◽

Cited By ~ 7

Author(s):

Peter Nicholls ◽

Chris E. Cooper ◽

John M. Wrigglesworth

Keyword(s):

Steady State ◽

Membrane Potential ◽

Cytochrome C ◽

Cytochrome Oxidase ◽

Cytochrome C Oxidase ◽

Respiration Rate ◽

Respiratory Control ◽

Ph Gradient ◽

Activity Levels ◽

Simulation Programme

The control of cytochrome c oxidase incorporated into proteoliposomes has been investigated as a function of membrane potential (ΔΨ) and pH gradient (ΔpH). The oxidase generates a pH gradient (alkaline inside) and a membrane potential (negative inside) when respiring on external cytochrome c. Low levels of valinomycin collapse ΔΨ and increase ΔpH; the respiration rate decreases. High levels of valinomycin, however, decrease ΔpH as valinomycin can also act as a protonophore. Nigericin (in the absence of valinomycin) increases ΔΨ and collapses ΔpH; the respiration rate increases. On a millivolt equivalent basis ΔpH is a more effective inhibitor of activity than is ΔΨ. In the absence of any ionophores the cytochrome oxidase proteoliposomes enter a steady state, in which there are both ΔpH and ΔΨ components of control. Present and previous data suggest that the respiration rate responds in a linear way ("ohmically") to increasing ΔpH but in a nonlinear way to ΔΨ ("non-ohmically"). High levels of both ΔΨ and ΔpH do not completely inhibit turnover (maximal respiratory control values lie between 6 and 10). The controlled steady state involves the electrophoretic entry and electroneutral exit of K+ from the vesicles. A model is presented in which the enzyme responds to both ΔpH and ΔΨ components of the proton-motive force, but is more sensitive to ΔpH than to ΔΨ at an equivalent ΔμH+. The steady state of the proteoliposome system can be represented for any set of permeabilities and enzyme activity levels using the computer simulation programme Stella™.Key words: cytochrome c, cytochrome oxidase, proteoliposomes, respiratory control, modelling, valinomycin, nigericin.

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Effects of triorganotin-mediated anion–hydroxide exchange upon reconstituted cytochrome c oxidase proteoliposomes

Biochemistry and Cell Biology ◽

10.1139/o86-089 ◽

1986 ◽

Vol 64 (7) ◽

pp. 647-655 ◽

Cited By ~ 9

Author(s):

A. P. Singh ◽

P. Nicholls

Keyword(s):

Steady State ◽

Membrane Potential ◽

Cytochrome C ◽

Cytochrome C Oxidase ◽

Respiration Rate ◽

Ph Gradient ◽

Dye Uptake ◽

Tin Chloride ◽

Carbocyanine Dye ◽

Potential Formation

Proteoliposomes containing cytochrome c oxidase and an internally trapped fluorescent pH probe (pyranine) were used to monitor respiration-dependent internal alkalinization and membrane potential formation. A maximum steady-state pH gradient of about 0.4 pH unit (vesicle interior alkaline) was obtained during active respiration in presence of reducing substrates and cytochrome c. This pH gradient was abolished by the triorganotin compounds tripropyl-, tributyl-, and triphenyl-tin chloride. At the same time, the membrane potential, measured by carbocyanine dye uptake, was slightly increased in value. Valinomycin, which abolishes the membrane potential, restores the value of ΔpH at low trialkyltin concentrations. The organotin compounds acted as electroneutral ionophores which exchanged intravesicular OH− ions with external SCN−, I−, and Cl− ions, but not [Formula: see text] or [Formula: see text] ions. Abolition of ΔpH is accompanied by an increase in respiration rate, but full resiratory stimulation only occurs when both Δψ and ΔpH are abolished by addition of both triorganotin and valinomycin. The triorganotin–valinomycin combination leads to active KC1 accumulation by the respiring proteoliposome, and it is necessary to postulate an electrically neutral KC1 efflux process to explain the continued steady respiration of the proteoliposomes in the presence of this ionophore combination.

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Protonmotive functions of cytochrome c oxidase in reconstituted vesicles. Influence of turnover rate on ‘proton translocation’

Biochemical Journal ◽

10.1042/bj2100199 ◽

1983 ◽

Vol 210 (1) ◽

pp. 199-205 ◽

Cited By ~ 52

Author(s):

G Proteau ◽

J M Wrigglesworth ◽

P Nicholls

Keyword(s):

Cytochrome C ◽

Cytochrome C Oxidase ◽

Turnover Rate ◽

External Medium ◽

Proton Translocation ◽

Buffering Capacity ◽

Ph Gradient ◽

Ferrocytochrome C ◽

Carbonyl Cyanide ◽

Enzyme Turnover

1. Oxidation of ferrocytochrome c by cytochrome c oxidase incorporated into proteoliposomes induces a transient acidification of the external medium. This change is dependent on the presence of valinomycin and can be abolished by carbonyl cyanide p-trifluoromethoxyphenylhydrazone or by nigericin. The H+/e- ratio for the initial acidification varies with the internal buffering capacity of the vesicles, and under suitable conditions approaches + 1, the pulse slowly decaying to give a net alkalinity change with H+/e- value approaching −1. 2. Inhibition of cytochrome c oxidase turnover by ferricytochrome c or by azide addition results in ferrocytochrome c-dependent H+ pulses with decreasing H+/e- ratios. The rate of the initial H+ production remains higher than the rate of equilibration of the pH gradient, indicating an intrinsic dependence of the H+/e- ratio on enzyme turnover. The final net alkalinity changes are relatively unaffected by turnover inhibition.

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