Conformational switching at cytochrome a during steady-state turnover of cytochrome c oxidase

Response of Heme Symmetry to the Redox State of Bovine Cytochrome c Oxidase

Second-derivative absorption spectroscopy was employed to monitor the response of effective symmetry of cytochromes a and a₃ to the redox and ligation states of bovine cytochrome c oxidase (CcO). The Soret band π → π* electronic transitions were used to display the changes in symmetry of these chromophores induced by the reduction of CcO inhibited by the exogenous ligands and during catalytic turnover. The second derivative of the difference absorption spectra revealed only a single Soret band for the oxidized cytochromes a and a₃ and cyanide-ligated oxidized cytochrome a₃. In contrast, two absorption bands were resolved in ferrous cytochrome a and ferrous cytochrome a₃ ligated with cyanide. A transition from one-band spectrum to two-band spectrum indicates the lowering of symmetry of these hemes due to the alteration of their immediate surroundings. It is suggested that the changes in polarity occurring in the vicinity of these cofactors are main reason for the split of the Soret band of both ferrous cytochrome a and cyanide-bound ferrous cytochrome a₃.

Cytochrome c/cytochrome c oxidase interaction. Direct structural evidence for conformational changes during enzyme turnover

We investigated the interaction between cytochrome c oxidase and its substrate cytochrome c by catalyzing the covalent linkage of the two proteins to yield 1 : 1 covalent enzyme-substrate complexes under conditions of low ionic strength. In addition to the 'traditional' oxidized complex formed between oxidized cytochrome c and the oxidized enzyme we prepared complexes under steady-state reducing conditions. Whereas for the 'oxidized' complex cytochrome c became bound exclusively to subunit II of the enzyme, for the 'steady-state' complex cytochrome c became bound to subunit II and two low molecular mass subunits, most likely VIb and IV. For both complexes we investigated: (a) the ability of the covalently bound cytochrome c to relay electrons into the enzyme, and (b) the ability of the covalently bound enzyme to catalyze the oxidation of unbound (exogenous) ferrocytochrome c. Steady-state spectral analysis (400-630 nm) combined with stopped-flow studies, confirmed that the bound cytochrome c mediated the efficient transfer of electrons from the reducing agent ascorbate to the enzyme. In the case of the latter, the half life for the ascorbate reduction of the bound cytochrome c and that for the subsequent transfer of electrons to haem a were both < 5 ms. In contrast the covalent complexes, when reduced, were found to be totally unreactive towards oxidized cytochrome c oxidase confirming that the previously observed reduction of haem a within the complexes occurred via intramolecular rather than intermolecular electron transfer. Additionally, stopped-flow analysis at 550 nm showed that haem a within both covalent complexes catalyzed the oxidation of exogenous ferrocytochrome c: The second order rate constant for the traditional complex was 0.55x10(6) m(-1) x s(-1) while that for the steady-state was 0.27x10(6) m(-1) x s(-1). These values were approximately 25-50% of those observed for 1 : 1 electrostatic complexes of similar concentrations. These results combined with those of the ascorbate and the electrophoresis studies suggest that electrons are able to enter cytochrome c oxidase via two independent pathways. We propose that during enzyme turnover the enzyme cycles between two conformers, one with a substrate binding site at subunit II and the other along the interface of subunits II, IV and VIb. Structural analysis suggests that Glu112, Glu113, Glu114 and Asp125 of subunit IV and Glu40, Glu54, Glu78, Asp35, Asp49, Asp73 and Asp74 of subunit VIb are residues that might possibly be involved.

The second derivative electronic absorption spectrum of cytochrome c oxidase in the Soret region

The electronic absorption spectrum of solubilized beef heart cytochrome c oxidase was analyzed in the 400-500 nm region to identify the origin of doublet features appearing in the second derivative spectrum associated with ferrocytochrome a. This doublet, centered near 22,600 cm(-1), was observed in the direct absorption spectrum of the a(2+)a(3)(3+).HCOO(-) form of the enzyme at cryogenic temperatures. Since evidence for this doublet at room temperature is obtained only on the basis of the second derivative spectrum, a novel mathematical approach was developed to analyze the resolving power of second derivative spectroscopy as a function of parameterization of spectral data. Within the mathematical limits defined for resolving spectral features, it was demonstrated that the integrated intensity of the doublet feature near 450 nm associated with ferrocytochrome a is independent of the ligand and oxidation state of cytochrome a(3). Furthermore, the doublet features, also observed in cytochrome c oxidase from Paracoccus denitrificans, were similarly associated with the heme A component and were correspondingly independent of the ligand and oxidation state of the heme A(3) chromophore. The doublet features are attributed to lifting of the degeneracy of the x and y polarized components of the B state of the heme A chromophore associated with the Soret transition.

Determination and novel features of the absolute absorption spectra of the heme a moieties in cytochrome c oxidase

The absolute absorption spectra of the two heme a moieties in cytochrome c oxidase were determined in the Soret region where spectral contributions from copper ions are negligible. This determination employs a set of absorption spectra of the enzyme recorded during anaerobic reduction with sodium dithionite, and does not require any other spectral data. The unique feature of the component spectra revealed in the present study suggests the existence of a specific interaction of heme a with the immediate environment as its origin. The usefulness of the absolute spectra in quantitative analysis of the spectral data is presented.

Molecular oxygen modulates cytochrome c oxidase function

This study sought to determine whether molecular oxygen interacts with cytochrome c oxidase to modify its catalytic activity. Such an interaction could explain the observation that mitochondria incubated under low O2 concentrations exhibit a reversible suppression of State 3 respiration. Oxidized bovine heart cytochrome c oxidase was incubated in oxygen concentrations of <50 microM for 4 h. The enzyme exhibited a reversible decrease in Vmax after incubation, compared with control enzyme incubated at higher oxygen concentrations. This change was accompanied by a small increase in the apparent Km of the enzyme for both cytochrome c and oxygen, although the optical absorption spectra of oxidized, cycling, or reduced enzyme were not affected. Spectroscopy studies after 4 h of incubation revealed that heme a3 was 33% reduced during cycling at [O2] = 25 microM whereas enzyme at [O2] = 135 microM was only 18% reduced, suggesting that the site of inhibition occurred at the electron transfer step between heme a3 and O2. These results provide a mechanistic explanation for the observation that intact cells or mitochondria exhibit a reversible inhibition of respiration during prolonged exposure to [O2] <25 mM, by demonstrating that the catalytic activity of cytochrome c oxidase function is similarly inhibited, possibly through an allosteric effect of molecular O2 on the enzyme.

Probing protein-cofactor interactions in the terminal oxidases by second derivative spectroscopy: study of bacterial enzymes with cofactor substitutions and heme A model compounds

Second derivative absorption spectra are reported for the aa3-cytochrome c oxidase from bovine cardiac mitochondria, the aa3-600 ubiquinol oxidase from Bacillus subtilis, the ba3-cytochrome c oxidase from Thermus thermophilis, and the aco-cytochrome c oxidase from Bacillus YN-2000. Together these enzymes provide a range of cofactor combinations that allow us to unequivocally identify the origin of the 450-nm absorption band of the terminal oxidases as the 6-coordinate low-spin heme, cytochrome a. The spectrum of the aco-cytochrome c oxidase further establishes that the split Soret band of cytochrome a, with features at 443 and 450 nm, is common to all forms of the enzyme containing ferrocytochrome a and does not depend on ligand occupancy at the other heme cofactor as previously suggested. To test the universality of this Soret band splitting for 6-coordinate low-spin heme A systems, we have reconstituted purified heme A with the apo forms of the heme binding proteins, hemopexin, histidine-proline-rich glycoprotein and the H64V/V68H double mutant of human myoglobin. All 3 proteins bound the heme A as a (bis)histidine complex, as judged by optical and resonance Raman spectroscopy. In the ferroheme A forms, none of these proteins displayed evidence of Soret band splitting. Heme A-(bis)imidazole in aqueous detergent solution likewise failed to display Soret band splitting. When the cyanide-inhibited mixed-valence form of the bovine enzyme was partially denatured by chemical or thermal means, the split Soret transition of cytochrome a collapsed into a single band at 443 nm.(ABSTRACT TRUNCATED AT 250 WORDS)

The steady state behaviour of cytochrome c oxidase in proteoliposomes

Electron transfer to oxygen catalysed by cytochrome c oxidase is accompanied by spectral changes at the binuclear a3CuB centre, both in the soluble enzyme and in membranous systems, indicating spin or ligand state transitions of an iron that remains ferric. The other haem group, cytochrome a, does not change its spectral characteristics significantly during the steady state, but remains partially reduced until anaerobiosis. Cytochrome a3, is fully oxidized in each of its major steady state forms, and reduced upon anaerobiosis to a single ferrous species. Although cytochrome a is normally the immediate electron donor to the binuclear centre, its redox state does not alter under conditions in which the flux through the enzyme is changing significantly. A second electron transfer pathway to the binuclear centre may therefore exist, possibly one in which direct reduction of the binuclear a3CuB centre by CuA occurs. Both cytochrome a and CuA behave as simple electron transfer centres. The energy-conserving chemistry takes place at the binuclear centre in concert with the four-electron reduction of molecular oxygen.

Long-distance cofactor interactions in terminal oxidases studied by second-derivative absorption spectroscopy

The electronic transitions of the two heme groups of cytochrome c oxidase have been resolved by application of second-derivative and cryogenic absorption spectroscopy. Both methods reveal a splitting of the ferrocytochrome a Soret transition into two features at 443 and 450 nm. The relative intensity of the 450 nm feature appears to depend on the ligation state of cytochrome a3, the solution pH, and complex formation with cytochrome c. The structural origin and mechanistic significance of this second Soret transition of cytochrome a are discussed in terms of the electron transfer and proton translocation activities of the enzyme.

Electronic and vibrational spectroscopy of the cytochrome c:cytochrome c oxidase complexes from bovine and Paracoccus denitrificans

The 1:1 complex between horse heart cytochrome c and bovine cytochrome c oxidase, and between yeast cytochrome c and Paracoccus denitrificans cytochrome c oxidase have been studied by a combination of second derivative absorption, circular dichroism (CD), and resonance Raman spectroscopy. The second derivative absorption and CD spectra reveal changes in the electronic transitions of cytochrome a upon complex formation. These results could reflect changes in ground state heme structure or changes in the protein environment surrounding the chromophore that affect either the ground or excited electronic states. The resonance Raman spectrum, on the other hand, reflects the heme structure in the ground electronic state only and shows no significant difference between cytochrome a vibrations in the complex or free enzyme. The only major difference between the Raman spectra of the free enzyme and complex is a broadening of the cytochrome a3 formyl band of the complex that is relieved upon complex dissociation at high ionic strength. These data suggest that the differences observed in the second derivative and CD spectra are the result of changes in the protein environment around cytochrome a that affect the electronic excited state. By analogy to other protein-chromophore systems, we suggest that the energy of the Soret pi* state of cytochrome a may be affected by (1) changes in the local dielectric, possibly brought about by movement of a charged amino acid side chain in proximity to the heme group, or (2) pi-pi interactions between the heme and aromatic amino acid residues.

Definition of the catalytic site of cytochrome c oxidase: specific ligands of heme a and the heme a3-CuB center

The three-subunit aa3-type cytochrome c oxidase (EC 1.9.3.1) of Rhodobacter sphaeroides is structurally and functionally homologous to the more complex mitochondrial oxidase. The largest subunit, subunit I, is highly conserved and predicted to contain 12 transmembrane segments that provide all the ligands for three of the four metal centers: heme a, heme a3, and CuB. A variety of spectroscopic techniques identify these ligands as histidines. We have used site-directed mutagenesis to change all the conserved histidines within subunit I of cytochrome c oxidase from Rb. sphaeroides. Analysis of the membrane-bound and purified mutant proteins by optical absorption and resonance Raman spectroscopy indicates that His-102 and His-421 are the ligands of heme a, while His-284, His-333, His-334, and His-419 ligate the heme a3-CuB center. To satisfy this ligation assignment, helices II, VI, VII, and X, which contain these histidine residues, must be in close proximity. These data provide empirical evidence regarding the three-dimensional protein structure at the catalytic core of cytochrome c oxidase.