Metals and activity of bovine heart cytochrome c oxidase are independent of polypeptide subunits III, VII, a, and b

Differential effects of 808-nm light on electron transport chain enzymes in isolated mitochondria: Implications for photobiomodulation initiation

Photobiomodulation is a term for using low-power red to near-infrared light to stimulate a variety of positive biological effects. Though the scientific and clinical acceptance of PBM as a therapeutic intervention has increased dramatically in recent years, the molecular underpinnings of the effect remain poorly understood. The putative chromophore for PBM effects is cytochrome c oxidase. It is postulated that light absorption at cytochrome c oxidase initiates a signaling cascade involving ATP and generation of reactive oxygen species (ROS), which subsequently results in improved cellular robustness. However, this hypothesis is largely based on inference and indirect evidence, and the precise molecular mechanisms that govern how photon absorption leads to these downstream effects remain poorly understood. We conducted low-power PBM-type light exposures of isolated mitochondria to 808 nm NIR light, at a number of irradiances. NIR exposure was found to enhance the activity of complex IV, depress the activity of complex III, and had no effect on the activity of complex II. Further, examining the dose-response of complex IV we found NIR enhancement did not exhibit irradiance reciprocity, indicating the effect on complex IV may not have direct photochemical basis. In summary, this research presents a novel method to interrogate the earliest stages of PBM in the mitochondria, and a unique window into the corresponding molecular mechanism(s) of induction.

Inhibition of the synthesis of a cytochrome-c-oxidase subunit isoform by antisense RNA

To investigate the role of subunit VIIe, an oxygen-regulated subunit isoform of Dictyostelium discoideum cytochrome-c oxidase, the full-length cDNA was inserted into an expression vector under the control of an actin promoter in the sense and antisense orientation. The DNA constructs were used for stable transformation of the slime mold amoebae. In most of the 28 antisense clones tested, the concentration of cytochrome-c oxidase was lowered compared to the wild type, while no significant changes were found in the sense mutants. Antisense RNA was abundantly expressed, leading to a drastic reduction of the steady-state level of the endogenous subunit VIIe mRNA, which was decreased up to 20-30% the level observed in parent cells. In these transformants, the amount of the target polypeptide and cytochrome c oxidase was 40-50% and 60-70% of control, respectively. A similar decrease was found in the level of the remaining nuclear and mitochondrial subunits. Unexpectedly, these changes affected neither basal nor uncoupled cell respiration suggesting an increase of the enzyme specific activity. Hypoxia completely relieved the cytochrome-c-oxidase deficit. These results indicate that subunit VII is needed for an efficient assembly of the protein complex and provide evidence for its involvement in the modulation of the enzyme activity.

The most conserved nuclear-encoded polypeptide of cytochrome c oxidase is the putative zinc-binding subunit: primary structure of subunit V from the slime mold Dictyostelium discoideum

A full-length 515 base pairs cDNA for cytochrome c oxidase subunit V of D. discoideum was isolated from a lambda gt11 expression library. The encoded polypeptide, whose identity was confirmed by partial protein sequencing, is 119 amino acids long (Mr = 13,352) and does not contain a cleavable presequence. The protein, which is homologous to human subunit Vb and yeast subunit IV, exhibits the highest degree of sequence conservation found among nuclear-encoded subunits of cytochrome c oxidase from distantly related organisms. All the invariant residues are clustered in two regions of the C-terminus which include the putative amino acids involved in the coordination of the Zn ion tightly associated to eukaryotic oxidase.

Interaction of Zn2+ with the bovine-heart mitochondrial bc1 complex

A study is presented of the effect of Zn2+ on the enzymatic properties of the bovine-heart cytochrome-bc1 complex. Micromolar concentrations of Zn2+ reversibly inhibit the cytochrome-c reductase activity of either the cholate-solubilized or liposome-reconstituted complex. Kinetic analysis of the redox reactions of the cytochromes indicate that Zn2+ affects the activity of the complex at the quinol oxidation site. The following have been determined: (a) Zn2+ inhibits the pre-steady-state reduction of cytochrome c1 by duroquinol either in the absence or in the presence of antimycin, (b) it does not inhibit the reduction of b cytochromes in the absence of antimycin or in the presence of myxothiazol, (c) it inhibits cytochrome-b reduction in the presence of antimycin. Furthermore Zn2+ inhibits the antimycin-promoted oxidant-induced extrareduction of b cytochromes. Addition of Zn2+ to reduced bc1 complex causes a red shift in the absorption spectrum of cytochrome b566 and a substantial decrease in the signal intensity of the EPR spectrum of the Fe-S protein. This is interpreted as an interaction of Zn2+ with the 2Fe-2S-cluster region of the Fe-S protein, thus giving rise to inhibition of the reductase activity and of the antimycin-insensitive reduction route of b cytochromes. A Scatchard-plot of 65Zn2+ binding to the native isolated complex gave a straight line from which a value of three binding sites and a single dissociation constant of 3 x 10(-6) M can be calculated, which is practically equal to the concentration causing 50% inhibition of electron flow.

Inhibition of the phosphate-stimulated cytochrome c oxidase activity by thiophosphate

Yeast and mammalian cytochrome c oxidase activity is inhibited by thiophosphate. This inhibition was observed when using either whole mitochondria or the isolated or reconstituted enzyme. The kinetics of the reduction reaction enabled us to demonstrate that thiophosphate acted on the electron transfer between hemes a and a3. With whole mitochondria, phosphate alone stimulated respiration. The inhibition induced by thiophosphate was suppressed by phosphate only in mitochondria, but not when the isolated enzyme was used. The possibility of a kinetic regulation is discussed.

Crystalline cytochrome c oxidase of bovine heart mitochondrial membrane: composition and x-ray diffraction studies

The integral mitochondrial membrane protein cytochrome c oxidase (ferrocytochrome-c:oxygen oxidoreductase, EC 1.9.3.1) was crystallized from solutions of the protein from bovine heart isolated as described earlier [Yoshikawa, S. & Caughey, W. S. (1982) J. Biol. Chem. 257, 412-420]. Crystallinity was demonstrated by x-ray diffraction. Microcrystals (tetragonal prisms, 0.02 mm in the largest dimension) were obtained in high yield with retention of activity and contained Fe, Cu, Zn, and Mg in approximate atom ratios of 1.0:1.25:0.5:0.5, respectively. Analysis of the amino acid residues and the tightly bound detergent support an apparent molecular mass of about 200 kDa, of which 150 kDa is protein (1316 +/- 66 amino acids) and 50 kDa is detergent (Brij 35). Seven major polypeptides are evident by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Adjustments in buffer concentration and other conditions have yielded much larger green crystals, hexagonal bipyramids; a crystal 0.3 X 0.5 X 0.7 mm gave x-ray diffractions as high as 8-A resolution and a space group of P6(2) or P6(4), and cell dimensions of a = b = 174.5 A, c = 282.2 A, alpha = beta = 90 degrees, and gamma = 120 degrees were obtained. A reasonable value of 3.1 A3/Da for Vm, the average space per dalton of protein in the crystal, was obtained for the asymmetric unit, which contains four irons and is a dimer of two minimal catalytic units. Cylindrical dimers (80 X 100 A) estimated from two-dimensional electron diffraction studies [Fuller, S. D., Capaldi, R. A. & Henderson, R. (1979) J. Mol. Biol. 134, 305-327] pack well in the crystal lattice with the symmetry of the space group of the crystal. The crystallization procedure developed is useful in purification of the enzyme and shows promise for the production of crystals of sufficiently high order to gain improved structural information from x-ray diffraction.