Separation of enzymically active bovine cytochrome c oxidase monomers and dimers by high performance liquid chromatography

Cytochrome c oxidase: chemistry of a molecular machine

The plethora of proposed chemical models attempting to explain the proton pumping reactions catalyzed by the CcO complex, especially the number of recent models, makes it clear that the problem is far from solved. Although we have not discussed all of the models proposed to date, we have described some of the more detailed models in order to illustrate the theoretical concepts introduced at the beginning of this section on proton pumping as well as to illustrate the rich possibilities available for effecting proton pumping. It is clear that proton pumping is effected by conformational changes induced by oxidation/reduction of the various redox centers in the CcO complex. It is for this reason that the CcO complex is called a redox-linked proton pump. The conformational changes of the proton pump cycle are usually envisioned to be some sort of ligand-exchange reaction arising from unstable geometries upon oxidation/reduction of the various redox centers. However, simple geometrical rearrangements, as in the Babcock and Mitchell models are also possible. In any model, however, hydrogen bonds must be broken and reformed due to conformational changes that result from oxidation/reduction of the linkage site during enzyme turnover. Perhaps the most important point emphasized in this discussion, however, is the fact that proton pumping is a directed process and it is electron and proton gating mechanisms that drive the proton pump cycle in the forward direction. Since many of the models discussed above lack effective electron and/or proton gating, it is clear that the major difficulty in developing a viable chemical model is not formulating a cyclic set of protein conformational changes effecting proton pumping (redox linkage) but rather constructing the model with a set of physical constraints so that the proposed cycle proceeds efficiently as postulated. In our discussion of these models, we have not been too concerned about which electron of the catalytic cycle was entering the site of linkage, but merely whether an ET to the binuclear center played a role. However, redox linkage only occurs if ET to the activated binuclear center is coupled to the proton pump. Since all of the models of proton pumping presented here, with the exception of the Rousseau expanded model and the Wikström model, have a maximum stoichiometry of 1 H+/e-, they inadequately explain the 2 H+/e- ratio for the third and fourth electrons of the dioxygen reduction cycle (see Section V.B). One way of interpreting this shortfall of protons is that the remaining protons are pumped by an as yet undefined indirectly coupled mechanism. In this scenario, the site of linkage could be coupled to the pumping of one proton in a direct fashion and one proton in an indirect fashion for a given electron. For a long time, it was assumed that at least some elements of such an indirect mechanism reside in subunit III. While recent evidence argues against the involvement of subunit III in the proton pump, subunit III may still participate in a regulatory and/or structural capacity (Section II.E). Attention has now focused on subunits I and II in the search for residues intimately involved in the proton pump mechanism and/or as part of a proton channel. In particular, the role of some of the highly conserved residues of helix VIII of subunit I are currently being studied by site directed mutagenesis. In our opinion, any model that invokes heme alpha 3 or CuB as the site of linkage must propose a very effective means by which the presumedly fast uncoupling ET to the dioxygen intermediates is prevented. It is difficult to imagine that ET over the short distance from heme alpha 3 or CuB to the dioxygen intermediate requires more than 1 ns. In addition, we expect the conformational changes of the proton pump to require much more than 1 ns (see Section V.B).

Separation, stability and kinetics of monomeric and dimeric bovine heart cytochrome c oxidase

The stability of monomeric and dimeric bovine heart cytochrome c oxidase in laurylmaltoside-containing buffers of high ionic strength allowed separation of the two forms by gel-filtration high-performance liquid chromatography (HPLC). A solution of the dimeric oxidase could be diluted without monomerisation. Both monomeric and dimeric cytochrome c oxidase showed biphasic steady-state kinetics when assayed spectrophotometrically at low ionic strength. Thus, the biphasic kinetics did not result from negative cooperativity between the two adjacent cytochrome c binding sites of the monomers constituting the dimeric oxidase. On polyacrylamide gels in the presence of sodium dodecyl sulphate (SDS) a fraction of subunit III of the dimeric enzyme migrated as a dimer, a phenomenon not seen with the monomeric enzyme. This might suggest that in the dimeric oxidase subunit III lies on the contact surface between the protomers. If so, the presumably hydrophobic interaction between the two subunits III resisted dissociation by SDS to some extent. Addition of sufficient ascorbate and cytochrome c to the monomeric oxidase to allow a few turnovers induced slow dimerisation (on a time-scale of hours). This probably indicates that one of the transient forms arising upon reoxidation of the reduced enzyme is more easily converted to the dimeric state than the resting enzyme. Gel-filtration HPLC proved to be a useful step in small-scale purification of cytochrome c oxidase. In the presence of laurylmaltoside the monomeric oxidase eluted after the usual trace contaminants, the dimeric Complex III and the much larger Complex I. The procedure is fast and non-denaturing, although limited by the capacity of available columns.

The effect of detergents on bovine cytochrome c oxidase: a kinetic approach

(1) Investigation of the relationship between the detergent concentration and steady-state and pre-steady-state kinetics of cytochrome c oxidase proved to be a valid approach in the study of protein-detergent interaction. (2) Laurylmaltoside, sodium cholate and Triton X-100 influenced the kinetics of cytochrome c oxidase cooperatively at detergent concentrations near their critical micelle concentration. This mode of interaction reflects disaggregation of the oxidase as a result of cooperative binding of the detergent. (3) Addition of increasing concentrations of Tween-80 to the aggregated enzyme caused a more gradual decrease in aggregation of the oxidase, which did not result in a change in activity of the enzyme. This suggests that aggregation of cytochrome c oxidase occurs in a highly regular manner in which no catalytic sites are shielded off. (4) Oxidase aggregates present at detergent concentrations below the critical micelle concentration of laurylmaltoside and Triton X-100 showed considerable activity. Their kinetics were equal to those of the oxidase in Tween-80, suggesting that the protein molecules are aligned in a similar way in all oligomers. Aggregates present in low concentrations of sodium cholate showed turnover rates that were twice as low as those observed with other aggregates. (5) Solubilisation of the oxidase by sodium cholate or Triton X-100 resulted in almost complete inhibition of enzymic activity, whereas the association rate of ferrocytochrome c was almost equal to that found for monomeric oxidase in laurylmaltoside. These results are in agreement with a mixed-type inhibition.

Bovine cytochrome c oxidases, purified from heart, skeletal muscle, liver and kidney, differ in the small subunits but show the same reaction kinetics with cytochrome c

(1) Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate of purified cytochrome c oxidase preparations revealed that bovine kidney, skeletal muscle and heart contain different cytochrome c oxidase isoenzymes, which show differences in mobility of the subunits encoded by the nuclear genome. No differences in subunit pattern were observed between the oxidase preparations isolated from kidney and liver. (2) The kinetics of the steady-state reactions between bovine ferrocytochrome c and the four types of bovine cytochrome c oxidase preparation were compared under conditions of both high- and low-ionic strength. Also the pre-steady-state kinetics were studied. Only minor differences were observed in the electron-transfer activity of the isoenzymes. Thus, our experiments do not support the notion that the subunits encoded by the nuclear genome act as modulators conferring different activities to the isoenzymes of cytochrome c oxidase. (3) The cytochrome c oxidase preparation from bovine skeletal muscle was found to consist mainly of dimers, whereas the enzymes isolated from bovine kidney, liver and heart were monomeric.

Human cytochrome c oxidase isoenzymes from heart and skeletal muscle; purification and properties

Human cytochrome c oxidase was isolated in an active form from heart and from skeletal muscle by a fast, small-scale isolation method. The procedure involves differential solubilisation of the oxidase from mitochondrial fragments by laurylmaltoside and KCl, followed by size-exclusion high-performance liquid chromatography. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate showed differences between the subunit VI region of cytochrome c oxidases from human heart and skeletal muscle, suggesting different isoenzyme forms in the two organs. This finding might be of importance in explaining mitochondrial myopathy which shows a deficiency of cytochrome c oxidase in skeletal muscle only. In SDS polyacrylamide gel electrophoresis most human cytochrome c oxidase subunits migrated differently from their bovine counterparts. However, the position of subunits III and IV was the same in the human and in the bovine enzymes. The much higher mobility of human cytochrome c oxidase subunit II is explained by a greater hydrophobicity of this polypeptide than of that of the subunit II of the bovine enzyme.

Studies on the role of the oligomeric state and subunit III of cytochrome oxidase in proton translocation

Anion-exchange fast protein liquid chromatography in the presence of lauryldimethylamine N-oxide (LDAO) was introduced to separate cytochrome oxidase into different complexes that either did or did not contain subunit III. Both kinds of enzyme complex exhibited H+ translocation after reconstitution into phospholipid vesicles, but with a significantly (approx. 50-60%) reduced H+/e- ratio as compared with unchromatographed enzyme. The anion-exchange FPLC fractions of the enzyme (with or without subunit III) sedimented more slowly than the control enzyme upon sucrose gradient centrifugation in the presence of cholate and a high potassium phosphate concentration. When the control enzyme was subjected to the sucrose gradient centrifugation in the presence of LDAO or Triton X-100, instead of cholate, one band containing all subunits was observed, which sedimented slowly like the FPLC fractions. Transfer of this band to cholate medium, and reapplication on the sucrose gradient (with cholate), yielded both a slow- and a fast-migrating band after centrifugation. Enzyme complexes that sedimented slowly or rapidly in the sucrose gradients revealed longer and shorter elution times, respectively, in gel filtration FPLC. This suggests that these complexes corresponds to monomers and dimers of cytochrome oxidase. Solubilization of proteoliposomes and subsequent sucrose gradient centrifugation in cholate yielded one fast-migrating band for the untreated enzyme, but both a fast- and a slow-migrating band for the anion-exchange FPLC-treated enzyme, which was exclusively slow-migrating before reconstitution into liposomes. It is suggested that dimerisation of monomeric cytochrome oxidase may be favoured when the enzyme encounters a membranous milieu, and that the dimeric structure might be necessary for proton translocation.