A plausible two-state model for cytochrome c oxidase.

Abstract

The catalytic properties of pulsed and resting cytochrome c oxidase (ferrocytochrome c: oxygen oxidoreductase, EC 1.9.3.1.), expressed in terms of a minimal kinetic scheme and simulated by numerical computations, were successfully described. A two-state model, in which the relative amounts of the enzyme present in each conformation are regulated by the rates ofelectron flux and 0°binding on one side and the interconversion rates on the other, accounts for the activation of cytochrome c oxidase during turnover.

Two‐electron reduction is required for rapid internal electron transfer in resting, pulsed and oxygenated cytochrome c oxidase

“…Thus, it seems well established that this restriction is a property not only of resting but also of pulsed and oxygenated cytochrome oxidase. It has been proposed [25,26] that pulsed cytochrome oxidase is more active than the resting enzyme, because the rate constant for the ratelimiting internal electron transfer has been increased. Our simulations show that an alternative explanation can be provided on the basis of a shift in the redox equilibrium between cytochrome a and CUA towards CUA reduction.…”

Mechanism of control of cytochrome oxidase activity by the electrochemical-potential gradient.

“…Results and Discussion Figure 1 shows the time course of oxidation of reduced cytochrome c by oxygen, catalyzed by the pulsed state (Antonini et al, 1977;Wilson et al, 1981) reported (Sarti et al, 1983) Figure 1 (curves 1 and 2), which escaped previous observation (Sarti et al, 1983) because it is only seen with COV having a RCR 2 6, corresponds to the oxidation of about two equivalents of cytochrome c per functional unit of the oxidase; moreover the bimolecular electron transfer from ferrocytochrome c to oxidase (the initial step in the overall reduction of oxidase) is too fast under these conditions (Malmstrom, 1979;Brunori and Wilson, 1982), and is therefore lost in the dead time of the rapidmixing apparatus, as shown from total-absorbance recovery (and indicated in Figure 1). Thus, when turnover begins close to four cytochrome c molecules have been oxidized per oxidase functional unit.…”

“…In going from a resting to a more active pulsed state of cytochrome oxidase (Antonini et al, 1977;Wilson et al, 1981), the rate of oxidation of cytochrome c increases by a factor of about three both in the absence and presence of ionophores, in agreement with previous observations (Sarti et al, 1983). This fact may be difficult to explain if the turnover phase in the absence of ionophores is limited by proton or charge diffusion through the membrane, while it is known that in going from resting to pulsed oxidase the increase in catalytic rate involves a conformational change in the enzyme (Wilson et al, 1981), which occurs whether a membrane is present or not.…”

“…This fact may be difficult to explain if the turnover phase in the absence of ionophores is limited by proton or charge diffusion through the membrane, while it is known that in going from resting to pulsed oxidase the increase in catalytic rate involves a conformational change in the enzyme (Wilson et al, 1981), which occurs whether a membrane is present or not. Thus the restingto-pulsed transition appears to be an intrinsic feature of the enzyme, which can be expressed both in the solubilized state or in the presence of a compartnentalizing membrane, whether energized or not.…”

“…These features have been incorporated into a simple model in which the catalytic activity of the enzyme, once reconstituted into vesicles, is dependent on the imposed electrochemical-potential gradient across the membrane; this effect is mediated by a conformational change of the protein which is different from the resting-to-pulsed transition (Wilson et al, 1981), and which controls the rate-limited step in catalysis. The model is as follows: P 0-9 S Circled arrows schematically indicate the turnover; P and S are two states of COV characterized by different catalytic activities represented by kp and ks (i.e., the first-order rate constant for the oxidation of cytochrome c determined from the exponential phase of the plot in Figure 1).…”