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Titration and steady-state behaviour of the 830 nm chromophore in cytochrome c oxidase.

ABSTRACT: Titration of cyanide-incubated cytochrome c oxidase (ox heart cytochrome aa3) with ferrocytochrome c or with NNN'N'-tetramethyl-p-phenylenediamine initially introduces two reducing equivalents per mol of cytochrome aa3. The first equivalent reduces the cytochrome a haem iron; the second reducing equivalent is not associated with reduction of the 830 nm chromophores (e.p.r.-detectable copper) but is probably required for reduction of the e.p.r.-undetectable copper. Excess reductant introduces a third reducing equivalent into the cyanide complex of cytochrome aa3. During steady-state respiration in the presence of cytochrome c and ascorbate, the 830 nm chromophore is almost completely oxidized. It is reduced more slowly than cytochrome a on anaerobiosis. In the presence of formate or azide, some reduction at 830 nm can be seen in the steady state; in an oxygen-pulsed system, a decrease in steady-state reduction of cytochromes c and a is associated with ab increased reduction of the 830 nm species. In the formate-inhibited system the reduction of a3 on anaerobiosis shows a lag phase, the duration of which corresponds to the time taken for the 830 nm species to be reduced. It is concluded that the e.p.r.-undetectable copper (CuD) is reduced early in the reaction sequence, whereas the detectable copper (CUD) is reduced late. The latter species is probably that responsible for reduction of the cytochrome a3 haem. The magnetic association between undetectable copper and the a3 haem may not imply capability for electron transfer, which occurs more readily between cytochrome a3 and the 830 nm species.

SUBMITTER: Nicholls P

PROVIDER: S-EPMC1158267 | biostudies-other | 1982 Jun

REPOSITORIES: biostudies-other

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Project description:The kinetics of oxidation of azurin and cytochrome c-551 catalysed by Pseudomonas aeruginosa cytochrome oxidase were re-investigated, and the steady-state parameters were evaluated by parametric and non-parametric methods. At low concentrations of substrates (e.g. less than or equal to 50 microM) the values obtained for Km and catalytic-centre activity are respectively 15 +/- 3 microM and 77 +/- 6 min-1 for azurin and 2.15 +/- 0.23 microM and 66 +/- 2 min-1 for cytochrome c-551, in general accord with previous reports assigning to cytochrome c-551 the higher affinity for the enzyme and to azurin a slightly higher catalytic rate. However, when the cytochrome c-551 concentration was extended well beyond the value of Km, the initial velocity increased, and eventually almost doubled at a substrate concentration greater than or equal to 100 microM. This result suggests a 'half-hearted' behaviour, since at relatively low cytochrome c-551 concentrations only one of the two identical binding sites of the dimeric enzyme seems to be catalytically active, possibly because of unfavourable interactions influencing the stability of the Michaelis-Menten complex at the second site. When reduced azurin and cytochrome c-551 are simultaneously exposed to Ps. aeruginosa cytochrome oxidase, the observed steady-state oxidation kinetics are complex, as expected in view of the rapid electron transfer between cytochrome c-551 and azurin in the free state. In spite of this complexity, it seems likely that a mechanism involving a simple competition between the two substrates for the same active site on the enzyme is operative. Addition of a chemically modified and redox inactive form of azurin (Hg-azurin) had no effect on the initial rate of oxidation of either azurin and cytochrome c-551, but clearly altered the time course of the overall process by removing, at least partially, the product inhibition. The results lead to the following conclusions: (i) reduced azurin and cytochrome c-551 bind at the same site on the enzyme, and thus compete; (ii) Hg-azurin binds at a regulatory site, competing with the product rather than the substrate; (iii) the two binding sites on the dimeric enzyme, though intrinsically equivalent, display unfavourable interactions. Since water is the product of the reduction of oxygen, point (iii) has important implications for the reaction mechanism.

Project description:1. The flux pathways for H+ and K+ movements into and out of proteoliposomes incorporating cytochrome c oxidase have been investigated as a function of the electrical and geometrical properties of the vesicles. 2. The respiration-induced pH gradient (delta pH) and membrane potential (delta psi) are mutually dependent and individually sensitive to the permeability properties of the membrane. A lowering or abolition of delta psi by the addition of valinomycin increased the steady-state level of delta pH. Conversely, removal of delta pH by the addition of nigericin resulted in a higher steady-state delta psi. 3. Vesicles prepared by sonication followed by centrifugation maintained similar pH gradients at steady state to those in vesicles prepared by dialysis, although the time taken to reach steady state was longer. Higher pH gradients can be induced in non-centrifuged sonicated preparations. 4. No significant differences were found in H+ and K+ permeability between proteoliposomes prepared by dialysis or by sonication. The permeability coefficient of the vesicle bilayers for H+ was 6.1 x 10(-4) cm.s-1 and that for K+ was 7.5 x 10(-10) cm.s-1. An initial fast change in internal pH was seen on the addition of external acid or alkali, followed by a slower, ionophore-sensitive, change. The initial fast phase can be increased by the lipid-soluble base dibucaine and the weak acid oleate. In the absence of ionophores, increasing concentrations of oleate increased the rate of H+ translocation to a level similar to that seen in the presence of nigericin. Internal alkalinization could also be induced by oleate upon the addition of potassium sulphate. 5. The initial, pre-steady-state and steady-state delta pH and delta psi changes can be simulated using a model in which the enzyme responds to both delta pH and delta psi components of the protonmotive force. At steady state, the electrogenic entry of K+ is countered by electroneutral exit via a K+/H+ exchange. 6. The permeability coefficient, PH, calculated from H+ flux under steady-state turnover conditions, was approx. 100 times higher than the corresponding 'passive' measurements of PH. Under conditions of oxidase turnover, the vesicles appear to be intrinsically more permeable to protons.

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Titration and steady-state behaviour of the 830 nm chromophore in cytochrome c oxidase.

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