Resonance Raman spectroscopy and enhanced photoreducibility for the 420 nm pulsed form of cytochrome oxidase

Evidence of dithionite contribution to the low-frequency resonance Raman spectrum of reduced and mixed-valence cytochrome c oxidase

The resonance Raman spectra of deoxygenated solutions of mixed-valence cyanide-bound and fully reduced cytochrome oxidase derivatives that have been reduced in the presence of aqueous or solid sodium dithionite exhibit two new low-frequency lines centered at 474 and 590 cm-1. These lines were not observed when the reductant system was changed to a solution containing ascorbate and N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD). Under enzyme turnover conditions, the addition of dithionite to the reoxidized protein (the 428-nm or "oxygenated" form) increases the intensity of these lines, while reoxidation and rereduction of the enzyme in the presence of ascorbate/TMPD resulted in the absence of both lines. Our data suggest that both lines must have contributions from species formed from aqueous dithionite, presumably the SO2 species, since these two lines are also observed in the Raman spectrum of a solution of aqueous dithionite, but not in the spectrum of an ascorbate/TMPD solution. Since heme metal-ligand stretch vibrations are expected to appear in the low-frequency region from 215 to 670 cm-1, our results indicate that special care should be exercised during the interpretation of the cytochrome a3 resonance Raman spectrum.

Raman/absorption simultaneous measurements for cytochrome oxidase compound A at room temperature with a novel flow apparatus

A novel flow apparatus for continuously producing reaction intermediates of cytochrome oxidase was constructed and applied successfully to observe the transient absorption and resonance Raman spectra in its reaction with oxygen. Time-resolved difference absorption spectra in 500-650-nm region clearly indicated the formation of compound A upon photolysis of the fully reduced CO-bound form at 5 degrees C, and at this stage electrons were not transferred from cytochrome c to cytochrome oxidase. However, at the stage of formation of compound B, cytochrome c was oxidized. Resonance Raman spectra of these intermediates measured simultaneously with the absorption spectra are also reported.

Spectrophotometric characterization of intermediate redox states of cytochrome oxidase

The spectrophotometric characteristics of hemes a and a3 in cytochrome oxidase have been examined over the range 380 nm to 900 nm. Difference spectra (relative to the oxidized form) are presented for ferrous, high-spin oxidized, low-spin oxidized, early "pulsed," late "pulsed," and two-peroxide-treated states of the enzyme. Comparisons indicate that the decay product of the initial peroxide complex of the enzyme is identical to a low-spin pulsed form of the enzyme. A high-spin pulsed form of the enzyme persists for several hours to days after preparation.

Conversion of CuA to a type II copper in cytochrome c oxidase

When cytochrome c oxidase is incubated at 43 degrees C for approximately 75 min in a solution containing the zwitterionic detergent sulfobetaine 12, the CuA site is converted into a type II copper as judged by changes in the 830-nm absorption band and the EPR spectrum of the enzyme. SDS-PAGE and sucrose gradient ultracentrifugation indicate concomitant loss of subunit III and monomerization of the enzyme during the heat treatment. Comparison of the optical and resonance Raman spectra of the heat-treated and native protein shows that the heme chromophores are not significantly perturbed; the resonance Raman data indicate that the small heme perturbations observed are limited to the cytochrome a3 site. Proton pumping measurements, conducted on the modified enzyme reconstituted into phospholipid vesicles, indicate that these vesicles are unusually permeable toward protons during turnover, as previously reported for the p-(hydroxymercuri)benzoate-modified oxidase and the modified enzyme obtained by heat treatment in lauryl maltoside. The sulfobetaine 12 modified enzyme is no longer capable of undergoing the recently reported conformational transition in which the tryptophan fluorescence changes upon reduction of the low-potential metal centers. Control studies on the monomeric and subunit III dissociated enzymes suggest that the disruption of this conformational change in the heat-treated oxidase is most likely associated with perturbation of the CuA site. These results lend support to the suggestion that the fluorescence-monitored conformational change of the native enzyme is initiated by reduction of the CuA site [Copeland et al. (1987) Biochemistry 26, 7311].

Enhanced superoxide dismutase activity of pulsed cytochrome oxidase

The superoxide dismutase (SOD) activity of beef heart cytochrome oxidase, both in the resting (as isolated) and pulsed (reduced and reoxidized) states, has been investigated using their ability to inhibit the autoxidation rate of pyrogallol and epinephrine. Resting oxidase showed variable SOD activity, while in the pulsed state the SOD activity of cytochrome oxidase (CcO) increased by an order of magnitude. These results are discussed in terms of a physiological role for the pulsed oxidase.

Resonance Raman evidence for an exchangeable protein hydrogen associated with the heme a group of cytochrome oxidase

When cytochrome-c oxidase is soaked in D2O, downshifts of the cytochrome a formyl C = O stretching mode are seen in the resonance Raman (RR) spectra (413.1 nm excitation) of both the resting and reduced forms. Other changes observed in the reduced protein RR spectra are consistent with involvement of the cytochrome a formyl group in the deuterium effect. The D2O-induced RR changes are fully developed during 3-5 days incubation, but are incomplete after 1 h. Extraction of the heme a chromophore in deuterated solvents eliminates these changes, implying that the exchangeable proton is on a protein group in the cytochrome a pocket which H-bonds to the heme formyl. The rate of the D2O exchange process is unaffected by enzyme turnover, thus reducing the likelihood that the cytochrome a formyl H-bond is directly involved in the redox-linked mechanism of proton pumping.

Structure and function of cytochrome-c oxidase

Recent works on the structure and the function of cytochrome-c oxidase are reviewed. The subunit composition of the mitochondrial enzyme depends on the species and is comprised of between 5 and 13 subunits. It is reduced to 1 to 3 subunits in prokaryotes. The complete amino acid composition has been derived from protein sequencing. Gene sequences are partially known in several eukaryote species. Metal centers are only located in subunits I and II. The mitochondrial cytochrome-c oxidase is Y-shaped; the arms of the Y cross the inner membrane, the stalk protrudes into the intermembrane space. The bacterial enzyme has a simpler, elongated shape. A number of data have been accumulated on the subunit topology and on their location within the protein. All available spectrometric techniques have been used to investigate the environment of the metal centers as well as their interactions. From the literature, attention must be paid to what may be considered or not as an active form. The steady improvement of the instrumentation has yielded evidence for different kinds of heterogeneities which could reflect the in vivo situation. The 'pulsed' and 'resting' conformers have been well characterized. The 'oxygenated' form has been identified as a peroxide derivative of the fully oxidized cytochrome-c oxidase. The mammalian enzyme has been isolated in fully active monomeric form which does not preclude the initially suggested dimeric behavior in situ. The role of the lipids is still largely investigated, mainly through reconstitution experiments. Kinetic studies of electron transfer between cytochrome c and cytochrome-c oxidase lead to a single catalytic site model to account for the multiphasic kinetics. Results related to the low temperature investigation of the intermediate steps in the reaction between oxygen and cytochrome-c oxidase received a sound confirmation by the resolution of compound A at room temperature. It is also pointed out that the so-called mixed valence state might not be a transient state in the catalytic reduction of oxygen. The functioning of cytochrome-c oxidase as a proton pump has been supported by a number of experimental results. Subunit III would be involved in this process. The redox link to the proton pump has been suggested to be at the Fea-CuA site. The molecular mechanism responsible for the proton pumping is still unknown.

Resonance Raman spectra for catalytic intermediates of cytochrome c oxidase detected with a mixed flow transient apparatus

A novel technique was employed to collect resonance Raman spectra of an oxygenated intermediate of cytochrome c oxidase. Instead of laser pulses of high peak power, which may cause photodissociation, a continuous wave laser and a mixed flow apparatus were used. An intermediate formed within 450 microseconds after the reaction of cytochrome c oxidase with molecular oxygen could be detected. From the spectra it could be deduced that the most likely candidate for the intermediate would be a transient oxygenated species having the Fe2+ - O2 or Fe4+ = O heme in cytochrome a3 and the Fe2+ heme in cytochrome a.