Reconstitution of Respiratory Chain Enzyme Systems

The subunit composition and function of mammalian cytochrome c oxidase

Cytochrome c oxidase (COX) from mammals and birds is composed of 13 subunits. The three catalytic subunits I-III are encoded by mitochondrial DNA, the ten nuclear-coded subunits (IV, Va, Vb, VIa, VIb, VIc, VIIa, VIIb, VIIc, VIII) by nuclear DNA. The nuclear-coded subunits are essentially involved in the regulation of oxygen consumption and proton translocation by COX, since their removal or modification changes the activity and their mutation causes mitochondrial diseases. Respiration, the basis for ATP synthesis in mitochondria, is differently regulated in organs and species by expression of tissue-, developmental-, and species-specific isoforms for COX subunits IV, VIa, VIb, VIIa, VIIb, and VIII, but the holoenzyme in mammals is always composed of 13 subunits. Various proteins and enzymes were shown, e.g., by co-immunoprecipitation, to bind to specific COX subunits and modify its activity, but these interactions are reversible, in contrast to the tightly bound 13 subunits. In addition, the formation of supercomplexes with other oxidative phosphorylation complexes has been shown to be largely variable. The regulatory complexity of COX is increased by protein phosphorylation. Up to now 18 phosphorylation sites have been identified under in vivo conditions in mammals. However, only for a few phosphorylation sites and four nuclear-coded subunits could a specific function be identified. Research on the signaling pathways leading to specific COX phosphorylations remains a great challenge for understanding the regulation of respiration and ATP synthesis in mammalian organisms. This article reviews the function of the individual COX subunits and their isoforms, as well as proteins and small molecules interacting and regulating the enzyme.

Ribose 5-phosphate glycation reduces cytochrome c respiratory activity and membrane affinity

Spontaneous glycation of bovine heart cytochrome c (cyt c) by the sugar ribose 5-phosphate (R5P) weakens the ability of the heme protein to transfer electrons in the respiratory pathway and to bind to membranes. Trypsin fragmentation studies suggest the preferential sites of glycation include Lys72 and Lys87/88 of a cationic patch involved in the association of the protein with its respiratory chain partners and with cardiolipin-containing membranes. Reaction of bovine cyt c with R5P (50 mM) for 8 h modified the protein in a manner that weakened its ability to transfer electrons to cytochrome oxidase by 60%. An 18 h treatment with R5P decreased bovine cyt c's binding affinity with cardiolipin-containing liposomes by an estimated 8-fold. A similar weaker binding of glycated cyt c was observed with mitoplasts. The reversal of the effects of R5P on membrane binding by ATP further supports an A-site modification. A significant decrease in the rate of spin state change for ferro-cyt c, thought to be due to cardiolipin insertion disrupting the coordination of Met to heme, was found for the R5P-treated cyt c. This change occurred to a greater extent than what can be explained by the permanent attachment of the protein to the liposome. Turbidity changes resulting from the multilamellar liposome fusion that is readily promoted by cyt c binding were not seen for the R5P-glycated cyt c samples. Collectively, these results demonstrate the negative impact that R5P glycation can have on critical electron transfer and membrane association functions of cyt c.

Succinate cytochrome c reductase in schistosomiasis: in vitro inhibition by some schistosomicidal drugs

Enzymes in mitochondria play an important role in biological oxidation and energy production. To understand the effect of schistosomiasis on these important processes, succinate cytochrome c reductase (SCR) from control and Schistosoma-infected mice was subjected for investigation. In this article, we report that SCR from Schistosoma-infected mouse showed a significant decrease in its Vmax and Km compared to control using both cytochrome c and 2,6-dichlorophenolindophenol as substrates. Furthermore, the kinetic studies of the purified SCR in the absence and presence of the schistosomicidal drugs praziquantel and Commiphora extract reveal that both drugs have an inhibitory action on the enzyme from the control and Schistosoma-infected mice and praziquantel changes the type of inhibition of SCR towards cytochrome c from mixed type in control to a competitive one in the case of the infection.

Study of respiratory cytochromes in liposomes

Important findings regarding the structure and function of respiratory cytochromes have been made from the study of these hemeproteins associated to liposomes. These studies contributed to the comprehension of the biological role of these proteins in the electron transfer process, the regulatory mechanisms, the energy transduction mechanisms, the protein sites that interact with mitochondrial membranes and the role played by the non-redox subunits present in the protein complexes of the respiratory chain of eukaryotes. In this chapter, the protocols developed to study cytochrome bc (1) activity in liposomes and the binding of cytochrome c to lipid bilayers is presented . The former protocol was developed to study the mechanism of energy transduction related to the topology of the components of bc (1) complex in the mitochondrial membrane. These studies were done with purified cytochrome bc (1) complexes reconstituted into potassium-loaded vesicles. The latter protocol was developed to study the influence of pH, DeltapH, and DeltaPsi on the interaction of cytochrome c with liposomes that mimic the inner mitochondrial membrane.

Effect of some cardiac and respiratory drugs on succinate-cytochrome c reductase

Succinate-cytochrome c reductase was inhibited in vitro and in vivo by phenobarbitone, aminophylline and neostigmine using both 2,6-dichlorophenolindophenol (DCIP) and cytochrome c (cyt c) as substrates. The enzyme was also activated by gallamine towards both substrates. In vitro, phenobarbitone and aminophylline inhibited the enzyme with respect to the reduction of DCIP and cyt c in a non-competitive manner with Ki values of 1.5 x 10(-5) and 5.7 x 10(-5)M, respectively. Moreover, neostigmine competitively inhibited the enzyme towards both substrates with Ki values of 1.36 x 10(-5) and 1.50 x 10(-5)M, respectively.

Inhibition of succinate-cytochrome C reductase by a ferromacrocyclic complex

Succinate-cytochrome c reductase (SCR) from mouse liver was inhibited strongly and reversibly by an iron (II) macrocyclic complex 3. The inhibition was observed for the enzyme toward the reduction of both 2,6-dichlorophenol indophenol (DCIP) and cytochrome c (cyt c). The inhibition was a mixed type and noncompetitive with respect to the reduction of DCIP and cyt c, respectively. Values of the inhibition constant ranged from 6.6 to 8.3 microM. The IC50 for the complex 3 was found to be 16.6 +/- 0.8 and 12.1 +/- 0.5 microM for the enzyme toward DCIP and cyt c, respectively. The reduced form of complex 3 also exhibited enzyme inhibition but to a less extent. Complex 3, at a lower level, equal to 25% of its LD50 showed about 50% inhibition of the enzyme through in vivo dose-dependent effect. These findings suggested that the structure of the equatorial benzoquinoid macrocyclic ligand of the Fe(II) complex is involved in the enzyme inhibition.

The sites of interaction of triphenyltetrazolium chloride with mitochondrial respiratory chains

The inability of cells and microorganisms to reduce the colourless electron acceptor triphenyltetrazolium chloride (TTC) to a red formazan precipitate is commonly used as a means of screening for cells that have a dysfunctional respiratory chain. The site of reduction of TTC is often stated to be at the level of cytochrome c oxidase where it is assumed to compete with oxygen for reducing equivalents. However, we show here that TTC is reduced not by cytochrome c oxidase but instead by dehydrogenases, particularly complex I, probably by accepting electrons directly from low potential cofactors. The reduction rate is fastest in coupled membranes because of accumulation in the matrix of the positively charged TTC+ cation. However, the initial product of TTC reduction is rapidly reoxidised by molecular oxygen, so that generation of the stable red formazan product from this intermediate occurs only under strictly anaerobic conditions. Colonies of mutants defective in cytochrome oxidase do not generate sufficiently anaerobic conditions to allow the intermediate to form the stable red formazan. This revision of the mode of interaction of TTC with respiratory chains has implications for the types of respiratory-defective mutants that might be detected by TTC screening.

Characterisation of 'fast' and 'slow' forms of bovine heart cytochrome-c oxidase

We have prepared cytochrome-c oxidase from bovine heart (using a modification of the method of Kuboyama et al. (1972) J. Biol. Chem. 247, 6375-6383) which binds cyanide rapidly, shows no kinetic distinction between the two haems on reduction by dithionite, has a Soret absorption maximum above 424 nm, and has a negligible 'g' = 12' EPR signal. On incubation at pH 6.5 this 'fast' oxidase reverts to the 'slow' ('resting') form characterised by slow cyanide binding, slow reduction of haem a3 by dithionite, a blue-shifted Soret maximum and a large 'g' = 12' signal. Incubation of 'fast' oxidase with formate produces a form of the enzyme with properties almost identical to those of 'slow' oxidase. The kinetics of formate binding to 'fast' oxidase are found to be biphasic, revealing the presence of at least two 'fast' subpopulations in our preparations. Evidence is presented that there is an equilibrium mixture of high-spin and low-spin forms of haem a3 in both 'fast' subpopulations at room temperature. Incubation of 'fast' oxidase with chloride or bromide at pH 6.5 produces forms of oxidase with much lower rates of cyanide binding. Our working hypothesis is that formate mimics a binuclear centre ligand which is present in the 'slow' form of cytochrome oxidase. Although we show that chloride and bromide can also be ligands of the binuclear centre, possibly onto CuB, we can rule out either of these being the ligand present in the 'slow' enzyme. We will argue that the 'fast' and 'slow' forms of oxidase are equivalent to the 'pulsed' and 'resting' forms of oxidase, respectively.

Inhibition of succinate-ubiquinone reductase by nitrosalicyl-N-alkylamides

The abilities of eight nitrosalicyl-N-alkylamides to inhibit succinate-ubiquinone reductase were compared. These compounds possess both a nitro group at 3- or 5-position of the benzyl ring, and a methyl, decyl, dodecyl or tetradecyl group subsituted for the amide hydrogen of salicylamide. The result shows the following: (a) 3-Nitrosalicyl-N-alkylamides are stronger inhibitors than 5-nitrosalicyl-N-alkylamides when they have the same alkyl group. (b) The inhibitory abilities of both 3-and 5-nitrosalicyl-N-alkylamides increase with increasing alkyl group length. (c) Kinetic data show that the inhibition by 3-nitrosalicyl-N-tetradecylamide is noncompetitive with respect to the electron acceptor Q2.

The effect of pH on redox titrations of haem a in cyanide-liganded cytochrome-c oxidase: experimental and modelling studies

Isolated cytochrome-c oxidase ligated with cyanide was titrated by Flash-Induced chemical photoREduction (FIRE) (Moody, A.J. and Rich, P.R. (1988) EBEC Short Rep. 5, 69) using cytochrome c as a redox indicator. Haem a is found to titrate in a complex manner consistent with its interacting anticooperatively with at least two other components. We assign CuB as the major interactant at neutral pH, and CuA as the minor interactant. In the pH range 7.0-8.1 the strength of the interaction with CuB is found to decrease with increasing pH, while the interaction with CuA remains essentially constant. The decrease in the interaction with CuB appears to continue above pH 8.1 such that at pH 9.2 the titration curve for haem a is only slightly distorted from an 'n = 1' shape, although it is not possible from the titration data to assess the relative contributions of CuB and CuA to the total interaction observed at pH values greater than 8.1. Haem a and CuB show similar pH-dependence and, to account for this, we present a model in which the oxidoreductions of both haem a and CuB are linked to the (de)protonation of a common acid/base group. The model predicts a pH-dependent indirect cooperative interaction between haem a and CuB in addition to the direct anticooperative interaction, thereby explaining the observed pH-dependence of the redox interaction between haem a and CuB.

Electron conduction between b cytochromes of the mitochondrial respiratory chain in the presence of antimycin plus myxothiazol

The b haems of the bc1 complex of bovine heart mitochondria were poised with succinate and fumarate so that only the high-potential haem (b-562) was reduced, and then isolated from further redox exchange with the ubiquinone pool by adding antimycin and myxothiazol. A transmembrane electric potential difference was then developed, either by electron flow from [Ru(NH3)6]Cl2 to oxygen or by ATP hydrolysis. The small difference spectrum, caused by the electric field, indicated 32-55% oxidation of b-562 with concomitant reduction of b-566. No lag greater than 0.1 s was detectable between the initiation of respiration and the development of the difference spectrum, thus providing a direct demonstration of (fairly) rapid electron transfer between the b haems.

Resolution and reconstitution of succinate-cytochrome c reductase: preparations and properties of high purity succinate dehydrogenase and ubiquinol-cytochrome c reductase

An improved method was developed to sequentially fractionate succinate-cytochrome c reductase into three reconstitutive active enzyme systems with good yield: pure succinate dehydrogenase, ubiquinone-binding protein fraction and a highly purified ubiquinol-cytochrome c reductase (cytochrome b-c1 III complex). An extensively dialyzed succinate-cytochrome c reductase was first separated into a succinae dehydrogenase fraction and the cytochrome b-c1 complex by alkali treatment. The resulting succinate dehydrogenase fraction was further purified to homogeneity by the treatment of butanol, calcium phosphate gel adsorption and ammonium sulfate fractionation under anaerobic condition in the presence of succinate and dithiothreitol. The cytochrome b-c1 complex was separated into chtochrome b-c1 III complex and ubiquinone-binding protein fractions by careful ammonium acetate fractionation in the presence of deoxycholate. The purified succinate dehydrogenase contained only two polypeptides with molecular weights of 70 000 anbd 27 000 as revealed by the sodium dodecyl sulfate polyacrylamide gel electrophoretic pattern. The enzyme has the reconstitutive activity and a low Km ferricyanide reductase activity of 85 mumol succinate oxidized per min per mg protein at 38 degrees C. Chemical composition analysis of cytochrome b-c1 III complex showed that the preparation was completely free of contamination of succinate dehydrogenase and ubiquinone-binding protein and was 30% more pure than the available preparation. When these three components were mixed in a proper ratio, a thenoyltrifluoroacetone- and antimycin A-sensitive succinate-cytochrome c reductase was reconstituted.

The dynamics of membrane structure

The membranes of living organisms are involved in many aspects of the life, growth and development of all cells. The predominant structural elements of these membranes are lipids and proteins and the basic strucvture of these molecules has been reviewed. The physical properties of the lipid constituents particularly their behavior in aqueous systems has led to the concepts of thermotropic and lyotropic mesomorphism; the interaction between different types of lipid molecules modulate this behavior. Interaction of phospholipids in aqueous systems with cholesterol, ions and drugs have been examined in this context. In addition a variety of model lipid-protein systems have been investigated and the implications of interactions between lipids and different proteins in biological membranes has been evaluated. This leads to a detailed consideration of the way lipids and proteins ae organized in cell membranes and contains an appraisal of the evidence supporting contemporary views of membrane structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Finally the biosynthesis, turnover and modulation of the properties of interacting membrane constituents is critically reviewed and possible ways of controlling the behavior of cells and organisms by altering the structural parameters of different membranes has been considered.

Subunit structure of the reconstitutively active cytochrome b-c1 complex. Determination of amino acids and molar distribution of subunit fractions from gel electrophoresis

A quantitative method has been developed to analyze the amino acid composition of protein subunits directly from the Coomassie Blue-stained band of polyacrylamide gel columns after electrophoresis. It is an improved method originally reported by Houston (Houston, L. L. (1971) Anal. Biochem. 44, 81--88). The results obtained can be thus used for the calculation of the molar ratios of subunit components of protein. The manipulation of the method and computation of the results are illustrated by a very complicated lipoprotein complex. The subunit molar ratios of the reconstitutively active cytochrome b-c1 complex were determined to be 2, 2, 2, 3, 2, 2, and 5 among the seven bands of the corresponding molecular weights of 53 000, 50 000, 37 000, 30 000, 28 000, 17 000, and 15 000, from gel electrophoretic columns. The amino acid composition of each subunit fraction determined directly from hydrolysis of gel was comparable with that obtained by actual isolation of each subunit.