Resolution of cytochrome oxidase into two component complexes

Protein and lipid structural transitions in cytochrome c oxidase-dimyristoylphosphatidylcholine reconstitutions

The thermotropic behavior of the mitochondrial enzyme cytochrome c oxidase (EC 1.9.3.1) reconstituted in dimyristoylphosphatidylcholine (DMPC) vesicles has been studied by using high-sensitivity differential scanning calorimetry and fluorescence spectroscopy. The incorporation of cytochrome c oxidase into the phospholipid bilayer perturbs the thermodynamic parameters associated with the lipid phase transition in a manner analogous to other integral membrane proteins: it reduces the enthalpy change, lowers the transition temperature, and reduces the cooperative behavior of the phospholipid molecules. Analysis of the dependence of the enthalpy change on the protein:lipid molar ratio indicates that cytochrome c oxidase prevents 99 +/- 5 lipid molecules from participating in the main gel-liquid-crystalline transition. These phospholipid molecules presumably remain in the same physical state below and above the transition temperature of the bulk lipid, thus providing a more or less constant microenvironment to the protein molecule. The effect of the phospholipid bilayer matrix on the thermodynamic stability of the cytochrome c oxidase complex was examined by high-sensitivity differential scanning calorimetry. Detergent (Tween 80)-solubilized cytochrome c oxidase undergoes a complex, irreversible thermal denaturation process centered at 56 degrees C and characterized by an enthalpy change of 550 +/- 50 kcal/mol of enzyme complex. Reconstitution of the cytochrome c oxidase complex into DMPC vesicles shifts the transition temperature upward to 63 degrees C, indicating that the phospholipid bilayer moiety stabilizes the native conformation of the enzyme. The lipid bilayer environment contributes approximately 10 kcal/mol to the free energy of stabilization of the enzyme complex. The thermal unfolding of cytochrome c oxidase is not a two-state process.(ABSTRACT TRUNCATED AT 250 WORDS)

Resolution of bovine heart cytochrome c oxidase into smaller complexes by controlled subunit denaturation

Bovine heart cytochrome c oxidase has been partially denaturated under mild conditions with 0.1-0.25% lithium dodecyl sulfate and 0.05% Triton X-100. From its reactivity towards CO and CN-, an unmasking of the heme a was inferred in this enzyme. The catalytic activity was lost during the denaturation and small spectral differences became visible. Spectra and ligand binding properties of the denatured enzyme were reversed by dilution in 2% Triton X-100. This suggests that during the denaturation procedure the hemes were not displaced from their original sites. By gel filtration of the partially denatured enzyme the following complexes of subunits were obtained: I-III, I-II-III, II-IV-V-VI-VII and IV-V-VI-VII. The first three complexes retained almost all the heme, and their spectral characteristics were very similar to those of the partially denatured cytochrome c oxidase. The data, in combination with the information that subunit III does not contain heme [Saraste et al. (1980) FEBS Lett. 114, 35-38], suggest that the hemes are attached to subunit I and II. After denaturation of cytochrome c oxidase under more drastic conditions some of the heme was also found to be associated with the smaller subunits, but its spectral characteristics were radically altered, becoming almost identical to those of free heme.

Subunit stoichiometry of cytochrome c oxidase of bovine heart

Cytochrome c oxidase from bovine heart was dissociated into its protein subunits by sodium dodecylsulphate, the subunits were separated on a preparative scale by sodium dodecylsulphate gel permeation chromatography. The subunits elute upon gel chromatography in order of decreasing apparent relative molecular mass (I, 40000; II, 26000; III, 21000; IV, 17000; V and VI, 12000; VII, 10000 and VIII, 6000). The very hydrophobic subunits I and III tend to form small aggregates both in the presence and absence of sodium dodecylsulphate. The molar ratio of the subunits was determined by two methods: firstly by quantitative amino acid analysis of each subunit peak, and secondly from the absorbance of each subunit at 280 nm caused by tryptophan and tyrosine. We conclude that the subunits I to VI are present in 1 : 1 ratio; our fraction VII contains two stoichiometric polypeptides which may or may not be identical. Fraction VIII contains enough protein for four stoichiometric chains which may belong to three different types. The 12 stoichiometric chains add up to a Mr of about 170000 if the sizes of I and III are 40000 and 21000, respectively. After correction for the presence of aggregates, subunits I and III appear to be present in more than 1 : 1 stoichiometric amounts with respect to other subunits, which probably means that subunits I and III are considerably larger than hitherto assumed. This is in line with recently published mtDNA sequence work [Anderson, S. et al. (1981) Nature, 290, 457--465].

Studies on the resolution of cytochrome oxidase

Cytochrome oxidase has been resolved in acetic acid and high salt/detergent media. In 0.5% acetic acid, the smaller subunits of the enzyme are selectively extracted with retention of an insoluble protein fraction containing subunits I-IV, VII. This fraction retains all the heme and copper of the original enzyme in a spectrally unaltered state, and possesses enzymic activity comparable to the unresolved enzyme. The further removal of subunit IV from this fraction results in migration of heme and copper and modification of their spectral characteristics. Resolution of the enzyme in a high salt/detergent medium extracts smaller subunits (V-VII) together with subunit IV and some heme and copper. The heme associated with this enzymically active extract has spectral characteristics that are partially suggestive of heme a3. It is suggested that the fraction of subunits I-IV,VII, resolved in dilute acetic acid, may represent the limit of resolution of the cytochrome oxidase complex that remains actively and spectrally indistinguishable from the original enzyme.

Ion-transport chain of cytochrome oxidase: the two chain-direct coupling principle of energy coupling

Cytochrome oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) couples the aerobic oxidation of ferrocytochrome c to the cyclical transport of monovalent cations or to the active transport of monovalent and divalent cations. This transport capability is mediated by an intracomplex ion-transport chain of two protein-bound molecules of cardiolipin per molecule of cytochrome oxidase. Cardiolipin in a two-phase system shows the identical ionophoric pattern as does the cytochrome oxidase coupled system. A molecular model of the cardiolipin chain suggests the possibility of a cage-like structure through which cations can be transferred from phosphate group to phosphate group. The ion-transport chain and the electron-transport chain are anchored to the same set of subunits (I+IV); the close proximity of the two chains argues for the direct coupling of electron and cation flow. The ion-transport chain of cytochrome oxidase provides an introduction to the molecular mechanisms by which ions are moved across membranes in energy-coupling systems.

On reagents that convert cytochrome oxidase from an inactive to an active coupling state

Cytochrome oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) of beef heart mitochondria, prepared by a standard method and brought to the highest purity level, is essentially inactive when tested in the aerobic assay involving oxidation of reduced cytochrome c by molecular oxygen. Three reagents (lysolecithin, Tween 20, and exogenous phospholipids) can convert cytochrome oxidase from an inactive to an active coupling state. These conversions are reversible: i.e., removal of the inducing agent leads to loss of activity. The evidence for the intrinsic coupling capability is that cytochrome oxidase in the active state invariably generates a proton gradient during respiration, and such gradient formation is demonstrable even when cytochrome oxidase is not inserted into a liposome.

Phospholipids as the molecular instruments of ion and solute transport in biological membranes

Partition studies have established that phospholipids generally have the capabilities to mediate the transmembrane transport of the full range of ions and solutes that physiologically cross biological membranes. The list of transportable species includes cations, anions, amino acids, citric acid cycle intermediates, nucleotides, and sugars. Phospholipid-mediated transport can be readily modulated by altering the phospholipid mixture or by addition of detergents, nucleotides, divalent metals, proteins, peptides, or ring compounds. Containment of phospholipid within channels in protein appears to be the precondition for the formation of the micellar structure requisite for solute transport. Phospholipid-mediated transport is postulated to be a central feature of energy coupling, membrane-spanning systems, and membrane-bound, phospholipid-requiring enzymes.

Restoration of active transport of solutes and oxidative phosphorylation by naphthoquinones in irradiated membrane vesicles from Mycobacterium phlei

Irradiation of the inverted membrane vesicles of Mycobacterium phlei with light at 360 nm inactivated the natural menaquinone [MK(9)(II-H)] and resulted in a loss of substrate oxidation, pH gradient, membrane potential, active transport of proline or calcium ions, and oxidative phosphorylation. Restoration of the protonmotive force and active transport occurred on addition of naphthoquinones such as vitamin K(1), menadione, or lapachol to the irradiated membrane vesicles. However, coupled phosphorylation was restored only by vitamin K(1). Menadione and lapachol did not act as uncoupling agents. The magnitude of the pH gradient and membrane potential in the quinone-restored system was a reflection of the rate of oxidation and was correlated with the rate of uptake of proline or Ca(2+). These results are consistent with the chemosmotic hypothesis proposed for the energy transducing mechanism for active transport and further demonstrate that the complete respiratory chain is not required to drive active transport. In contrast, the data suggest that in addition to the driving force (protonmotive force) necessary to establish oxidative phosphorylation, a specific spatial orientation of the respiratory components, such as the naphthaquinones, is essential for the utilization of the proton gradient or membrane potential or both. Bypass of electrons from the respiratory chain with menadione may explain the inability of this quinone to restore oxidative phosphorylation; however, lapachol restores oxidation by the same electron transport pathway as the natural menaquinone but fails to restore phosphorylation. Because all three quinones restore the protonmotive force, other factors that are discussed must be considered in understanding the mechanism of oxidative phosphorylation.

Ion-channel component of cytochrome oxidase

Cytochrome oxidase is resolvable into an electron transfer complex and an ion transfer complex. The ion transfer complex has been shown to have the capability for inducing nonspecific ion transport into liposomes. Subunit I of cytochrome oxidase has been identified as an ion-channel-forming protein.