Nearest neighbor relationships of the polypeptides in ubiquinone cytochrome c reductase (complex III)

Cross-linking of the electron-transfer flavoprotein to electron-transfer flavoprotein-ubiquinone oxidoreductase with heterobifunctional reagents

The mitochondrial electron-transfer flavoprotein (ETF) is a heterodimer containing only one FAD. In previous work on the structure-function relationships of ETF, its interaction with the general acyl-CoA dehydrogenase (GAD) was studied by chemical cross-linking with heterobifunctional reagents [D. J. Steenkamp (1987) Biochem. J. 243, 519-524]. GAD whose lysine residues were substituted with 3-(2-pyridyldithio)propionyl groups was preferentially cross-linked to the small subunit of ETF, the lysine residues of which had been substituted with 4-mercaptobutyramidine (MBA) groups. This work was extended to the interaction of ETF with ETF-ubiquinone oxidoreductase (ETF-Q ox). ETF-Q ox was partially inactivated by modification with N-succinimidyl 3-(2-pyridyldithio)propionate to introduce pyridyl disulphide structures. A similar modification of ETF caused a large increase in the apparent Michaelis constant of ETF-Q ox for modified ETF owing to the loss of positive charge on some critical lysines of ETF. When ETF-Q ox was modified with 2-iminothiolane to introduce 4-mercaptobutyramidine groups, only a minor effect on the activity of the enzyme was observed. To retain the positive charges on the lysine residues of ETF, pyridyl disulphide structures were introduced by treating ETF with 2-iminothiolane in the presence of 2,2'-dithiodipyridyl. The electron-transfer activity of the resultant ETF preparation containing 4-(2-pyridyldithio)butyramidine (PDBA) groups was only slightly affected. When ETF-Q ox substituted with MBA groups was mixed with ETF bearing PDBA groups, at least 70% of the cross-links formed between the two proteins were between the small subunit of ETF and ETF-Q ox. ETF-Q ox, therefore, interacts predominantly with the same subunit of ETF as GAD. Variables which affect the selectivity of ETF-Q ox cross-linking to the subunits of ETF are considered.

Complexity and tissue specificity of the mitochondrial respiratory chain

There is a renewed interest in the structure and functioning of the mitochondrial respiratory chain with the realization that a number of genetic disorders result from defects in mitochondrial electron transfer. These socalled mitochondrial myopathies include diseases of muscle, heart, and brain. The respiratory chain can be fractionated into four large multipeptide complexes, an NADH ubiquinone reductase (complex I), succinate ubiquinone reductase (complex II), ubiquinol oxidoreductase (complex III), and cytochrome c oxidase (complex IV). Mitochondrial myopathies involving each of these complexes have been described. This review summarizes compositional and structural data on the respiratory chain proteins and describes the arrangement of these complexes in the mitochondrial inner membrane. This biochemical information is provided as a framework for the diagnosis and molecular characterization of mitochondrial diseases.

Nearest-neighbor relationships of the constituent polypeptides in plastoquinol-plastocyanin oxidoreductase

The nearest-neighbor relationship among the constituent polypeptides of the isolated plastoquinol-plastocyanin oxidoreductase from spinach chloroplasts has been investigated. (1) The isolated plastoquinol-plastocyanin oxidoreductase (the b6/f complex) is treated with various concentrations of the cross-linker glutaraldehyde. The treated b6/f complexes are then analyzed by SDS-polyacrylamide gel electrophoresis coupled with the immunodecoration of cross-link products by specific antibodies for each of the four prominent constituent polypeptides. Cytochrome b6 is found to be most resistant to forming any intermolecular cross-link products. At low concentrations of glutaraldehyde, the 'Rieske' iron-sulfur (Fe-S) protein and subunit IV of the b6/f complex, however, appear to form cross-link products with a relative molecular weight of 35 000. Dimers of cytochrome f and cytochrome f/Rieske protein cross-link products can also be detected. (2) When a Rieske Fe-S protein-depleted b6/f complex is used in place of the control b6/f complex, cytochrome b6 is less resistant to intermolecular cross-linking, while subunit IV does not form any 35 kDa cross-link product, unlike the case in control b6/f complex. Subunit IV is concluded to be closely associated with the Rieske Fe-S protein. This provides evidence that subunit IV is a bona fide component of the cytochrome b6/f complex, although no function can yet be assigned to it. The results are discussed in relationship to the spatial and functional relationships among the components of the b6/f complex.

Is there sufficient experimental evidence to consider the mitochondrial cytochrome bc1 complex a proton pump? Probably no

The electron flow through the cytochrome bc1 complex of the mitochondrial respiratory chain is accompanied by vectorial proton translocation, though the mechanism of the latter phenomenon has not yet been clarified. Several proposed hypotheses are briefly presented and discussed here. Recently, a number of papers have appeared claiming the existence of a proton pump in the enzyme mainly on the basis of the interaction of the complex with N,N'-dicyclohexylcarbodiimide. These data are reviewed here with the aim of showing their ability to fit multiple interpretations. This together with some other arguments leads to the conclusion that a proton pump in the mitochondrial bc1 complex has not yet been demonstrated.

Differential labeling of the subunits of respiratory complex III with [3H]succinic anhydride, [14C]succinic anhydride, and p-diazobenzene-[35S]sulfonate

Exposure of antimycin-treated Complex III (ubiquinol-cytochrome c reductase) purified from bovine heart mitochondria to [3H]succinic anhydride plus [35S]p-diazobenzenesulfonate (DABS) resulted in somewhat uniform relative labeling of the eight measured subunits of the complex by [3H]succinic anhydride. In contrast, relative labeling by [35S]DABS was similar to [3H]succinic anhydride for the subunits of high molecular mass, i.e., core proteins, cytochromes, and the iron-sulfur protein, but greatly reduced for the polypeptides of molecular mass below 15 kDa. With Complex II depleted in the iron-sulfur protein the relative labeling of core protein I by exposure of the complex to [3H]succinic anhydride was significantly enhanced, whereas labeling of the polypeptides represented by SDS-PAGE bands 7 and 8 was significantly inhibited. Dual labeling of the subunits of Complex III by 14C- and 3H-labeled succinic anhydride before and after dissociation of the complex by sodium dodecyl sulfate, respectively, was measured with the complex in its oxidized, reduced, and antimycin-inhibited states. Subunits observed to be most accessible or reactive to succinic anhydride were core protein II, the iron-sulfur protein, and polypeptides of SDS-PAGE bands 7,8, and 9. Two additional polypeptides of molecular masses 23 and 12kDa, not normally resolved by gel-electrophoresis, were detected. Reduction of the complex resulted in a significant change of 14C/3H labeling ratio of core protein only, whereas treatment of the complex with antimycin resulted in decreases in 14C/3H labeling ratios of core proteins I and II, cytochrome c1, and a polypeptide of molecular mass 13kDa identified as an antimycin-binding protein.

Chemical cross-linking of mitochondrial NADH dehydrogenase from bovine heart

The structure of bovine heart mitochondrial NADH dehydrogenase was investigated by using two cleavable cross-linking agents, disuccinimidyl tartrate and (ethylene glycol)yl bis-(succinimidyl succinate). Cross-linking was analysed primarily by immunoblotting to detect products containing subunits of the iron-protein fraction from chaotropic resolution of the enzyme, namely those of 75, 49, 30 and 13 kDa. By using both the isolated iron-protein fraction and the intact dehydrogenase, cross-links were identified between these four subunits, from these subunits to the largest subunit of the flavoprotein fraction, which contains the active site for NADH, and from these subunits to polypeptides in the hydrophobic shell, which surrounds the hydrophilic iron-protein and flavoprotein fractions.

Molecular conversion between monomeric and dimeric states of the mitochondrial cytochrome b-c1 complex: isolation of active monomers

Bovine heart cytochrome b-c1 complex dispersed in 0.1% dodecylmaltoside, 10 mM Tris-HCl (pH 7.4), was subjected to filtration on Ultrogel AcA 34 columns. Apparent Mr values of about 400,000 and 170,000 were estimated for the enzyme-detergent complex in the presence and absence of 50 mM KCl, respectively. Similar Mr values (about 390,000 and 160,000) were obtained after sucrose gradient centrifugation of the b-c1 complex species isolated using Ultrogel filtration. Both species contained eight polypeptides, as in the original cytochrome b-c1 complex. The experiments suggest that the two species represent a dimer and a monomer of the b-c1 complex. The molecular conversion between the monomeric and dimeric state of the enzyme was found to be reversible. Both monomers and dimers of the b-c1 complex were competent to catalyze QH2:cytochrome c reductase activity with approximately the same maximal velocity. The finding that both molecular forms of the enzyme appear equally active does not support functional models based exclusively on a dimeric b-c1 complex.

Cross-linking of ubiquinone cytochrome c reductase (complex III) with periodate-cleavable bifunctional reagents

Two novel cross-linkers, disuccinimidyl tartarate (DST) and N,N'-bis(3-succinimidyloxycarbonylpropyl)tartaramide (SPT), have been synthesized. These reagents span 6 and 18 A, respectively, between functional groups and contain a vic-glycol bond which can be cleaved with periodate under mild reaction conditions. Both DST and SPT have been used to examine the near-neighbor relationships of polypeptides in ubiquinone cytochrome c reductase (complex III) from beef heart mitochondria. Among the cross-linked products resolved were pairs containing I + II, II + VI, I + V, and VI + VII. Polypeptides III and IV, a cytochrome b aproprotein, and the cytochrome c1 hemoprotein, respectively, were also resolved in several cross-linked products.

Polypeptides in the succinate-coenzyme Q reductase segment of the respiratory chain

Complex II (succinate-coenzyme Q reductase) was resolved into ten different polypeptides by polyacrylamide gel electrophoresis. Four polypeptides, CII-1, CII-2, CII-3, and CII-4 with molecular weights of 70 000, 24 000, 13 500, and 7000, were present in large amounts in all preparations examined. CII-1 and CII-2 are the flavoprotein and iron-sulfur protein, respectively, of succinate dehydrogenase; CII-3 and CII-4 have not been functionally indentified. Six polypeptides were present in much smaller amoumts as judged by staining intensity, and each of these comigrated with components in complex III. The amino acid compositions of several of the minor components in complex II were identical with that of an equivalently migrating polypeptide in complex III. We conclude that succinate-coenzyme Q reductase contains four different polypeptides and is contaminated with variable amounts of complex III when isolated as complex II.