Redox-dependent accessibility of subunit V of cytochrome oxidase. A novel use of ELISA as a probe of intact membranes

The effect of antibodies to subunit V of cytochrome oxidase on cyanide inhibition of electron transfer

Binding of antibodies raised against subunit V of mammalian cytochrome oxidase to the intact membranous enzyme is redox-sensitive, suggesting the existence of 'open' and 'closed' protein conformers (Freedman, J.A., Cooper, C.E., Leece, B., Nicholls, P. and Chan, S.H.P. (1988) Biochem. Cell Biol. 66, 1210-1217). Similar open and closed states for the oxygen-reacting site have been proposed to explain cyanide binding kinetics (Jensen, P., Wilson, M.T., Aasa, R. and Malmström, B.G. (1984) Biochem. J. 224, 829-837). We therefore examined cyanide inhibition of oxidase activity polarographically and spectrophotometrically using soluble oxidase preincubated with and without anti-subunit V or non-immune rabbit gamma-globulin. The subunit-specific antibody decreased the cyanide 'on' rate and essentially eliminated the rapid phase of cyanide binding. We conclude that (i), bound antibody blocks HCN binding; (ii), antibody and HCN probably bind to the same conformation of the oxidase and (iii), the 'open'-'closed' conformation change that modulates binding of HCN may be similar to that which modulates antibody binding. The results are consistent with some reciprocating models of electron transfer and energy transduction by the oxidase (cf., Wikström, M.K.F., Krab, K. and Saraste, M. (1981) Cytochrome Oxidase: A Synthesis).

Cytochrome oxidase immunohistochemistry in rat brain and dorsal root ganglia: visualization of enzyme in neuronal perikarya and in parvalbumin-positive neurons

Histochemical detection of cytochrome oxidase activity has been widely used to deduce patterns of neuronal electrical activity in the CNS. Here we investigated the utility of cytochrome oxidase localization by immunohistochemistry and compared immunostaining with histochemical staining patterns in dorsal root ganglia of the rat. In addition, a limited survey of cytochrome oxidase immunostaining density within what are thought to be highly active parvalbumin-immunoreactive neurons was conducted. The immunohistochemical approach produced granular cytoplasmic immunolabelling in neuronal cell bodies and allowed identification of individual labelled cells in all brain regions including those within dense immunoreactive networks of neuropil. Neuronal somata exhibited a wide range of staining densities which were particularly evident in the hippocampus and dorsal root ganglia. The distribution of neurons intensely immunoreactive for cytochrome oxidase within various structures was consistent with previous histochemical descriptions of enzyme activity. Densitometric measurements of immunohistochemical reaction product in individual neurons of hippocampus, substantia nigra, cerebellum and dorsal root ganglia showed that the rate of product deposition was linear with time under conditions chosen for comparisons of staining density. Quantitative analysis of cytochrome oxidase immunohistochemical and histochemical staining densities within the same cells in adjacent sections of dorsal root ganglion gave a correlation coefficient of r = 0.75 (P less than 0.001). In sections processed immunohistochemically for both cytochrome oxidase and parvalbumin, most but not all parvalbumin-containing cells displayed dense cytochrome oxidase immunolabelling. Conversely, many examples were found of neurons that were densely stained for cytochrome oxidase, but lacked parvalbumin. Immunohistochemistry for cytochrome oxidase reveals the enzyme in neuronal cell bodies with a clarity not usually seen with the histochemical method. Combination of this immunohistochemical approach with simultaneous immunolabelling of other neuronal markers, as shown here in the case of parvalbumin, is expected to assist the elucidation of patterns of activity in neurochemically identified cell types and anatomically defined neural systems.

Cytochrome oxidase from thermophilic bacterium PS3 contains a fourth protein subunit

Monoclonal antibodies prepared against subunits II and IV of beef heart cytochrome oxidase were found to cross-react with thermophilic bacterial PS3 oxidase. Each individual antibody affects the enzymatic activity. "Western" blot analyses showed that subunit II antibodies of beef heart recognized subunit II of PS3 and subunit IV antibody likewise recognized a fourth protein subunit on slab gels. This fourth subunit previously thought to be a contaminant or a degradation product has a molecular weight of about 10,500 on SDS-gels, and appears to exist in stoichiometric amount. We have extracted this subunit from slab gels and compared its amino acid composition with that of subunit III.

Modulation of cytochrome oxidase kinetics by indirect antibody action

Polyclonal antibodies raised against isolated subunit V from beef heart cytochrome oxidase or against the intact enzyme increase its apparent affinity for the substrate cytochrome c at the high-affinity site while diminishing the turnover at that site. At the low-affinity site the major action of both types of antibody is to reduce the apparent affinity for cytochrome c. At high ionic strengths the kinetic effect of anti-subunit V is very small although it still binds to the enzyme. The results are interpreted in terms of a model for the enzyme in which antibodies can modulate cytochrome oxidase kinetics by affecting the binding of cytochrome c, even if the antibody-binding site is on a subunit not directly involved in substrate binding.

Two monoclonal antibody lines directed against subunit IV of cytochrome oxidase: a study of opposite effects

Two monoclonal lines of antibodies were isolated with specificities against the amino half of Subunit IV of beef heart cytochrome oxidase. The lines had nonoverlapping epitopes. Both bound to the matrix face of membranous oxidase, neither bound to the cytoplasmic face. One line (QA4/C4) stimulated electron transfer in soluble or membranous oxidase, while the other (QA4) inhibited that activity by both oxidase preparations. These effects on electron transfer activity were not altered by the inclusion or omission of detergent. ATP depressed the binding of either antibody to either soluble or membranous oxidase. In the absence of ATP, QA4/C4 stimulated electron transfer only in the high affinity phase of cytochrome c oxidation (with decreased KM and increased Vmax), causing slight inhibition in the low affinity phase (with decreased KM). In the presence of ATP, QA4/C4 abolished the high affinity phase, but did not alter the ATP influence on the low affinity phase. In the absence of ATP, antibodies of line QA4 abolished the low affinity phase, leaving a high affinity phase similar to that induced by ATP. In the presence of ATP, QA4 abolished the high affinity phase, leaving a low affinity phase similar to that seen with ATP alone. This behavior is consistent with the dissection of two catalytic sites for cytochrome c and more than one ATP affector site.

The random collision model and a critical assessment of diffusion and collision in mitochondrial electron transport

This review focuses on our studies over the past ten years which reveal that the mitochondrial inner membrane is a fluid-state rather than a solid-state membrane and that all membrane proteins and redox components which catalyze electron transport and ATP synthesis are in constant and independent diffusional motion. The studies reviewed represent the experimental basis for the random collision model of electron transport. We present five fundamental postulates upon which the random collision model of mitochondrial electron transport is founded: All redox components are independent lateral diffusants; Cytochrome c diffuses primarily in three dimensions; Electron transport is a diffusion-coupled kinetic process; Electron transport is a multicollisional, obstructed, long-range diffusional process; The rates of diffusion of the redox components have a direct influence on the overall kinetic process of electron transport and can be rate limiting, as in diffusion control. The experimental rationales and the results obtained in testing each of the five postulates of the random collision model are presented. In addition, we offer the basic concepts, criteria and experimental strategies that we believe are essential in considering the significance of the relationship between diffusion and electron transport. Finally, we critically explore and assess other contemporary studies on the diffusion of inner membrane components related to electron transport including studies on: rotational diffusion, immobile fractions, complex formation, dynamic aggregates, and rates of diffusion. Review of all available data confirms the random collision model and no data appear to exist that contravene it. It is concluded that mitochondrial electron transport is a diffusion-based random collision process and that diffusion has an integral and controlling affect on electron transport.

Isolation and properties of cytochrome c oxidase from rat liver and quantification of immunological differences between isozymes from various rat tissues with subunit-specific antisera

Cytochrome c oxidase was isolated from rat liver either by affinity chromatography on cytochrome-c--Sepharose 4B or by chromatography on DEAE-Sepharose. Dodecyl sulfate gel electrophoresis of both preparations showed the same subunit pattern consisting of 13 different polypeptides. Kinetic analysis of the two preparations gave a higher Vmax for the enzyme isolated by chromatography on DEAE-Sephacel. Specific antisera were raised in rabbits against nine of the ten nuclear endoded subunits. A monospecific reaction of each antiserum with its corresponding subunit was obtained by Western blot analysis, thus excluding artificial bands in the gel electrophoretic pattern of the isolated enzyme due to proteolysis, aggregation or conformational modification of subunits. With an antiserum against rat liver holocytochrome c oxidase a different reactivity was found by Western blot analysis for subunits VIa and VIII between isolated cytochrome c oxidases from pig liver or kidney and heart or skeletal muscle. For a quantitative analysis of immunological differences a nitrocellulose enzyme-linked immunosorbent assay was developed. Monospecific antisera against 12 of the 13 subunits of rat liver cytochrome c oxidase were titrated with increasing amounts of total mitochondrial proteins from different rat tissues dissolved in dodecyl sulfate and dotted on nitrocellulose. The absorbance of a soluble dye developed by the second peroxidase-conjugated antibody was measured. From the data the following conclusions were obtained: (a) The mitochondrial encoded catalytic subunits I-III of cytochrome c oxidase are probably identical in all rat tissues. (b) All nine investigated nuclear encoded subunits of cytochrome c oxidase showed immunological differences between two or more tissues. Large immunological differences were found between liver, kidney or brain and heart or skeletal muscle. Minor but significant differences were observed for some subunits between heart and skeletal muscle and between liver, kidney and brain. (c) Between corresponding nuclear encoded subunits of cytochrome c oxidase from fetal and adult tissues of liver, heart and skeletal muscle apparent immunological differences were observed. The data could explain cases of fatal infantile myopathy due to cytochrome c oxidase deficiency.

Cytochrome oxidase: structural insights from electron microscopy and from secondary structure prediction

Electron microscopic images of selectively contrasted cytochrome oxidase dimer crystals are interpreted in a manner consistent with the structure of monomers determined by Fuller et al. (J. Molec. Biol. 134, 305-327). The arms of the y-shaped monomers lie within and perpendicular to the lipid bilayer protruding approximately 25 A on the matrix side of the membrane. The cytoplasmic-side tails of two monomers spread apart in a dimer forming a large cleft. Decoration of the exposed matrix side of vesicle crystals with antisubunit IV antibody fragments indicates that subunit IV lies along the a-crystal axis roughly 20 A from the center of the dimer. A membrane propensity algorithm applied to the sequences of cytochrome oxidase subunits predicts a total of 19 transmembrane alpha-helices per monomer.

Structure of the cytochrome c oxidase complex of rat liver. 2. Topological orientation of polypeptides in the membrane as studied by proteolytic digestion and immunoblotting

The orientation of the thirteen polypeptides of rat-liver cytochrome c oxidase in the inner mitochondrial membrane was studied by proteolytic digestion of mitoplasts and sonicated particles. After separation by sodium dodecylsulfate gel electrophoresis proteins were transferred on nitrocellulose, and individual polypeptides were identified by incubation with polypeptide-specific antisera, followed by fluorescein-isothiocyanate-conjugated protein A. The three catalytic polypeptides I-III and seven nuclear coded polypeptides (IV, Vb, VIa, VIc, VIIa, VIIb and VIII) were found accessible to proteases from the cytoplasmic phase. Polypeptides II, IV, Va, Vb and VIa were accessible from the matrix phase, indicating a transmembraneous orientation of polypeptides II, IV, Vb and VIa. Together with data on cross-linking and on cytochrome-c-protected labeling of polypeptides, a model of the cytochrome c oxidase complex was developed. It is suggested that the cytochrome c binding site on polypeptide II is surrounded by several nuclear-coded polypeptides, which may modulate the affinity of the enzyme towards cytochrome c.

The mechanism of antibody inhibition of proton pumping by cytochrome oxidase vesicles

Antibodies previously shown to inhibit vectorial proton translocation through cytochrome oxidase vesicles were converted to F(ab')2 and Fab'. Neither fragment inhibited proton pumping, although binding capacity was present. However, when a surrogate Fc was added to F(ab')2, inhibition of proton translocation was restored; indicating that the inhibition is due to steric hindrance. These results provide insight into mechanisms of energy transduction by oxidase.

Interactions in cytochrome oxidase: functions and structure

Mitochondrial cytochrome c oxidase is an exceedingly complex multistructural and multifunctional membranous enzyme. In this review, we will provide an overview of the many interactions of cytochrome oxidase, stressing developments not covered by the excellent monograph of Wikström, Krab, and Saraste (1981), and continuing into early 1983. First we describe its functions (both in the nominal sense, as a transporter of electrons between cytochrome c and oxygen, and in its role in energy transduction). Then we describe its structure, emphasizing the protein (its structure as a whole, the number and stoichiometry of its subunits, their biosynthetic origin, and their interactions with each other, with other components of the enzyme complex, and with the membrane as a whole). Finally, we present a model in which the protein conformation serves as the focus for the dynamic interaction of its two major functions.