Covalent binding of arylazido derivatives of cytochrome c to cytochrome oxidase

Redox equilibration after one-electron reduction of cytochrome c oxidase: radical formation and a possible hydrogen relay mechanism

Kinetic studies using UV/visible and EPR spectroscopy were carried out to follow the distribution of electrons within beef heart cytochrome c oxidase (CcO), both active and cyanide-inhibited, following addition of reduced cytochrome c as electron donor. In the initial one-electron reduced state the electron is shared between three redox centers, heme a, CuA and a third site, probably CuB. Using a rapid freeze system and the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) a protein radical was also detected. The EPR spectrum of the DMPO adduct of this radical was consistent with tyrosyl radical capture. This may be a feature of a charge relay mechanism involved in some part of the CcO electron transfer system from bound cytochrome c via CuA and heme a to the a3CuB binuclear center.

Substrate binding-dissociation and intermolecular electron transfer in cytochrome c oxidase are driven by energy-dependent conformational changes in the enzyme and substrate

Reduction of O₂ by cytochrome c oxidase (COX) is critical to the cellular production of adenosine-5'-triphosphate; COX obtains the four electrons required for this process from ferrocytochrome c. The COX-cytochrome c enzyme-substrate complex is stabilized by electrostatic interactions via carboxylates on COX and lysines on cytochrome c. Conformational changes are believed to play a role in ferrocytochrome c oxidation and release and in rapid intramolecular transfer of electrons within COX, but the details are unclear. To gather specific information about the extent and relevance of conformational changes, we performed bioinformatics studies using the published structures of both proteins. For both proteins, we studied the surface accessibility and energy, as a function of the proteins' oxidation state. The residues of reduced cytochrome c showed greater surface accessibility and were at a higher energy than those of the oxidized cytochrome c. Also, most residues of the core subunits (I, II, and III) of COX showed low accessibility, ∼35%, and compared to the oxidized subunits, the reduced subunits had higher energies. We concluded that substrate binding and dissociation is modulated by specific redox-dependent conformational changes. We further conclude that high energy and structural relaxation of reduced cytochrome c and core COX subunits drive their rapid electron transfer.

Cytochrome c release and mitochondria involvement in programmed cell death induced by acetic acid in Saccharomyces cerevisiae

Evidence is presented that mitochondria are implicated in the previously described programmed cell death (PCD) process induced by acetic acid in Saccharomyces cerevisiae. In yeast cells undergoing a PCD process induced by acetic acid, translocation of cytochrome c (CytC) to the cytosol and reactive oxygen species production, two events known to be proapoptotic in mammals, were observed. Associated with these events, reduction in oxygen consumption and in mitochondrial membrane potential was found. Enzymatic assays showed that the activity of complex bc(1) was normal, whereas that of cytochrome c oxidase (COX) was strongly decreased. This decrease is in accordance with the observed reduction in the amounts of COX II subunit and of cytochromes a+a(3). The acetic acid-induced PCD process was found to be independent of oxidative phosphorylation because it was not inhibited by oligomycin treatment. The inability of S. cerevisiae mutant strains (lacking mitochondrial DNA, heme lyase, or ATPase) to undergo acetic acid-induced PCD and in the ATPase mutant (knockout in ATP10) the absence of CytC release provides further evidence that the process is mediated by a mitochondria-dependent apoptotic pathway. The understanding of the involvement of a mitochondria-dependent apoptotic pathway in S. cerevisiae PCD process will be most useful in the further elucidation of an ancestral pathway common to PCD in metazoans.

Site-directed mutagenesis of cytochrome c oxidase reveals two acidic residues involved in the binding of cytochrome c

Site-directed mutagenesis in subunit II of the cytochrome c oxidase (haem aa3) from Paracoccus denitrificans reveals that two carboxylic residues, Glu-246 and Asp-206 (corresponding to 198 and 158 in the bovine subunit II), are involved in the binding of cytochrome c. Spectrophotometric and polarographic measurements with the isolated enzymes of both mutant strains show a strongly reduced activity compared to wild-type oxidase, with the overall catalytic capacity (kcat/KM) of both mutants decreased about 8-fold. EPR spectra reveal no significant differences between the wild-type and the mutant enzymes, indicating that neither residue contributes significantly to the structure of the CuA centre. We conclude that Glu-246 and Asp-206 constitute an essential part of the binding site for cytochrome c.

Isoforms of yeast cytochrome c oxidase subunit V affect the binuclear reaction center and alter the kinetics of interaction with the isoforms of yeast cytochrome c

Subunit V, one of the nuclear-coded subunits of yeast cytochrome c oxidase, has two isoforms, Va and Vb. These alter the in vivo intramolecular rates of electron transfer within the holoenzyme (Waterland, R. A., Basu, A., Chance, B., and Poyton, R. O. (1991) J. Biol. Chem. 266, 4180-4186). The isozyme with Vb has a higher turnover rate and a higher intramolecular transfer rate than the isozyme with Va. To determine how these isoforms affect catalysis, we have examined their effects on the binuclear reaction center and on the interaction between cytochrome c oxidase and the two isoforms, iso-1 and iso-2, of yeast cytochrome c. Infrared spectroscopy of carbon monoxide liganded to heme a3 has revealed a single conformer for the binuclear reaction center in the isozyme with Vb but two discrete conformers in the isozyme with Va. The kinetics of interaction for all four pairwise combinations of isozymes with each subunit V isoform and the two cytochrome c isoforms are biphasic, with high and low affinity electron transfer reactions. In general, the isoforms of cytochrome c and subunit V do not alter the Km but do affect the TNmax. The TNmax for isozymes carrying Vb are higher at both high and low affinity sites for each cytochrome c isoform. Iso-1-cytochrome c supports a higher TNmax than Iso-2-cytochrome c. Surprisingly, the combinatorial effect of both sets of isoforms on TNmax is minimized with the pairs of isoforms (iso-1-cytochrome c and subunit Va or iso-2 and subunit Vb) that are co-expressed in cells. Together, these findings support the conclusion that the subunit V isoforms modulate catalysis and suggest that they do so by affecting the environment or structure of the binuclear reaction center. They also suggest that the coexpression of the two cytochrome c isoforms with two subunit V isoforms serves to minimize differences in electron transfer rates brought about by the subunit V isoforms.

Preparation of a one-subunit cytochrome oxidase from Paracoccus denitrificans: spectral analysis and enzymatic activity

Cytochrome c oxidase was isolated from Paracoccus denitrificans as a two-subunit enzyme. Chymotrypsin-catalyzed proteolysis reduced the molecular weight of each subunit by about 8000. The spectral properties of this preparation, as well as its Km for cytochrome c(1.7 muM), remained unchanged with respect to the native enzyme. Vmax was reduced by about 55% when assayed in Triton X-100 or in Triton X-100 supplemented with asolectin. Following further proteolysis by Staphylococcus aureus V8 protease, subunit I remained unchanged as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, whereas subunit II was split into small peptides. These were removed by ion-exchange high-performance liquid chromatography. The one-subunit enzyme had an apparent molecular weight of 43,000. The reduction of molecular weight was also confirmed by the diminution of the ultraviolet/Soret absorption ratio. This value was 1.8-2.1 for the native enzyme and 1.3-1.5 for the one-subunit enzyme. The spectral properties (including the spectrum CO reduced minus reduced) were not modified by the proteolytic treatment, indicating that cytochromes a and a3 were present in equal amounts. The lack of spectral alteration and the known close association of the copper B atom with cytochrome a3 suggest that copper B is also contained within the one-subunit enzyme. The Km of the one-subunit oxidase was similar to that of the two-subunit enzyme; Vmax was decreased by about 50%. The activity of the one-subunit oxidase had a salt-dependent maximum at 30 mM KCl, almost identical with that of the undigested enzyme, and was inhibited by micromolar concentrations of KCN.

Cytochrome c oxidase from Paracoccus denitrificans: both hemes are located in subunit I

The two-subunit cytochrome c oxidase from Paracoccus denitrificans has been sequentially digested with chymotrypsin and Staphylococcus aureus V8 protease. The smaller subunit of the enzyme (apparent Mr 32,000) was split into numerous peptides that were removed by anion-exchange HPLC. The larger subunit was only digested to a limited extent (from an apparent Mr 45,000 to Mr 43,000), and the spectral properties were preserved relative to the native enzyme (a reduced minus oxidized difference spectrum with maxima at 447 and 607 nm in the Soret and alpha region, respectively). As judged from CO-reduced spectra this proteolytically digested, one-fragment oxidase was found to contain an equal amount of cytochromes a and a3. The enzymatic activity with reduced cytochrome c as substrate in the presence of Triton X-100 proceeded with equal affinity (apparent Km = 0.5-1.0 microM) and with a Vmax of approximately 20% (40 s-1) of that found with the native enzyme (200 s-1). When the assay system was supplemented with soybean phospholipids, the Km became 2 microM for both enzymes and the Vmax became 730 and 170 s-1 for the native and the digested enzyme, respectively. Thus subunit I of P. denitrificans oxidase, and most probably of the other cytochrome c oxidases as well, contains both hemes and at least one Cu atom and has significant enzymatic activity.

Lateral mobility of cytochrome c on intact mitochondrial membranes as determined by fluorescence redistribution after photobleaching

Lateral mobility of an active fluorescent derivative of cytochrome c on the membranes of giant mitochondria was measured by fluorescence redistribution after photobleaching. A diffusion coefficient of 1.6 X 10(-10) cm2/sec was determined for the labeled cytochrome c on inner mitochondrial membranes under conditions where succinate oxidase activity was demonstrated. This relatively low rate of diffusion, together with results of other investigators, is explained in terms of a model involving a dynamic equilibrium between freely diffusing and associated forms of electron-transfer components.