Cytochrome c oxidase: two models

Nature of the FeO2 bonding in myoglobin and hemoglobin: A new molecular paradigm

The iron(II)-dioxygen bond in myoglobin and hemoglobin is a subject of wide interest. Studies range from examinations of physical-chemical properties dependent on its electronic structure, to investigations of the stability as a function of oxygen supply. Among these, stability properties are of particular importance in vivo. Like all known dioxygen carriers synthesized so far with transition metals, the oxygenated forms of myoglobin and hemoglobin are known to be oxidized easily to their ferric met-forms, which cannot bind molecular oxygen and are therefore physiologically inactive. The mechanistic details of this autoxidation reaction, which are of clinical, as well as of physical-chemical, interest, have long been investigated by a number of authors, but a full understanding of the heme oxidation has not been reached so far. Recent kinetic and thermodynamic studies of the stability of oxymyoglobin (MbO2) and oxyhemoglobin (HbO2) have revealed new features in the FeO2 bonding. In vivo, the iron center is always subject to a nucleophilic attack of the water molecule or hydroxyl ion, which can enter the heme pocket from the surrounding solvent and thereby irreversibly displace the bound dioxygen from MbO2 or HbO2 in the form of O2- so that the iron is converted to the ferric met-form. Since the autoxidation reaction of MbO2 or HbO2 proceeds through a nucleophilic displacement following one-electron transfer from iron(II) to the bound O2, this reaction may be viewed as a meeting point of the stabilization and the activation of molecular oxygen performed by hemoproteins. Along with these lines of evidence, we finally discuss the stability property of human HbO2 and provide with the most recent state of hemoglobin research. The HbA molecule contains two types of alphabeta contacts and seems to differentiate them quite properly for its functional properties. The alpha1beta2 or alpha2beta1 contact is associated with the cooperative oxygen binding, whereas the alpha1beta1 or alpha2beta2 contact is used for controlling the stability of the bound O2. We can thus form a unified picture for hemoglobin function by closely integrating the cooperative and the stable binding of molecular oxygen with iron(II) in aqueous solvent. These new views on the nature of FeO2 bonding and the possible role of globin moiety in stabilizing MbO2 and HbO2 are of primary importance, not only for a full understanding of various hemoprotein reactions with O2, but also for planning new molecular designs for synthetic oxygen carriers which may be able to function in aqueous solvent and at physiological temperature.

Autoxidation of oxymyoglobin: a meeting point of the stabilization and the activation of molecular oxygen

1. The primary events of haemoprotein reactions with molecular oxygen have been re-examined by placing special emphasis upon the reduction properties of dioxygen. 2. In the stepwise reduction of O2 to water via hydrogen peroxide, the addition of the first electron is an unfavourable, uphill process with the midpoint potential of -0.33 V, all the subsequent steps being downhill. This thermodynamic barrier to the first step is, therefore, a most crucial ridge located between the stabilization and the activation of dioxygen performed by haemoproteins. 3. If the proteins have a redox potential much higher than -0.33 V, molecular oxygen must bind to the proteins stably and reversibly. In Mb or Hb, however, the FeO2 centre is always subject to a nucleophilic attack of the water molecule or hydroxyl ion, which can enter the haem pocket from the surrounding solvent. These can cause irreversible oxidation of the FeO2 bonding to the ferric met-form with generation of the superoxide anion. 4. In cases of the oxygen activation, if haemoproteins have a redox potential lower than or close to -0.33 V, the first reduction of O2 to O2- would be a spontaneous process. Cytochrome P-450 provides such an example and can facilitate the subsequent addition of electrons that leads to the breaking of the O-O bond to yield the hydroxylating species. 5. As to the proteins whose redox potential is not facilitative and appreciably higher than -0.33 V, a bimetallic, concerted, two-equivalent reduction of the bound dioxygen to the peroxide level would be much more favoured without the intermediate formation of O2-. This is probably the case of cytochrome c oxidase for the reduction of O2 to water. 6. The redox potential diagrams thus visualize various aspects of the ways haemoproteins overcome their thermodynamic constraints and carry out their specific functions in the stabilization and the activation of molecular oxygen.

Effect of administration of 2-methyl-4-dimethylaminoazobenzene on the half-lives of rat liver mitochondria and cytochrome oxidase

The turnover of total mitochondrial proteins and cytochrome oxidase in the livers of rats administered with 2-methyl-4-dimethylaminoazobenzene (2-Me-DAB) has been determined. The incorporation of [14C]bicarbonate revealed a half-life of 3.1 days in control and 6 to 9 days in azodye administered animals for whole mitochondrial proteins. The incorporation of [35S]methionine yielded t1/2 values of 8.5 days and 15.4 days, respectively. The t1/2 of cytochrome oxidase, 10.8 days for control and 19.3 days for 2-Me-DAB-treated animals, indicated that the delay in the decay of the enzyme was of the same order as that of whole mitochondria. Short term incorporation revealed that the administration of the azodye stimulated the synthesis of the enzyme. Mitochondria isolated from azodye-administered animals appeared less susceptible to lysosomal proteolysis. Also, azodye administration seemed to impair the ability of lysosomes to degrade mitochondria.

An assessment of the chemiosmotic hypothesis of mitochondrial energy transduction

It is argued that a proton concentration difference and/or a membrane potential is not the form into which the free energy of the oxidation-reduction reactions of the mitochondrial respiratory chain is first transduced. It is suggested that the search for a chemical intermediate should be continued in spite of the conclusion by some investigators that the chemical hypothesis is untenable. It is asked whether pH changes when measured in solutions containing mitochondria can be interpreted as evidence for H+ movements, also, whether there is a continuous, renewable and stable electrochemical proton concentration difference (delta mu H+) across the mitochondrial membrane, and whether in fact the delta mu H+ is a necessary intermediate in the synthesis of ATP. The four postulates of Mitchell's chemiosmotic hypothesis of energy transduction are discussed point by point. It is agreed that "The systems are plugged through a topologically closed insulating membrane," which probably is not "a nonaqueous osmotic barrier," and which probably does not have an unusually "low permeability to solutes and to H+ and OH- in particular" when compared with other membranes. There is disagreement with the statement that "Respiratory and photoredox systems are chemiosmotic membrane-located protonmotive chains" in that it is suggested by others that chemiosmosis is chemically nonexistent and that thermodynamically it would lack control. The subsequent statement, "having a characteristic----H+/2 epsilon- stoichiometry," is rendered uncertain by the experimental findings of values greater than 2H+/2 epsilon-/site and probably as large as 4H+/2 epsilon-/site. The proposal that "The synthetase is a chemiosmotic membrane-located reversible motive ATPase" requires the assumption that the ATP synthetase is the same enzyme as the ATPase, but functioning in the reverse direction. It is considered possible that there are two enzymes in the multi-subunit ATPase complex: one the hydrolase, and the other the synthetase. The further proposal, "having characteristic----H+/P stoichiometry" requires that the ratio be 2 according to Mitchell. However, values of 3, as well as larger values, have been reported by others, which introduces a large element of uncertainty. There is no disagreement with the statement that "There are proton-linked (or hydroxyl ion-linked) solute porter systems for osmotic stabilization and metabolite transport." In fact, this may be the principal reason for having proton efflux or "proton-pumping.''(ABSTRACT TRUNCATED AT 400 WORDS)

Interactions in cytochrome oxidase: functions and structure

Mitochondrial cytochrome c oxidase is an exceedingly complex multistructural and multifunctional membranous enzyme. In this review, we will provide an overview of the many interactions of cytochrome oxidase, stressing developments not covered by the excellent monograph of Wikström, Krab, and Saraste (1981), and continuing into early 1983. First we describe its functions (both in the nominal sense, as a transporter of electrons between cytochrome c and oxygen, and in its role in energy transduction). Then we describe its structure, emphasizing the protein (its structure as a whole, the number and stoichiometry of its subunits, their biosynthetic origin, and their interactions with each other, with other components of the enzyme complex, and with the membrane as a whole). Finally, we present a model in which the protein conformation serves as the focus for the dynamic interaction of its two major functions.