scholarly journals Electron-paramagnetic-resonance measurements of the electron-transfer components of the reaction centre of Rhodopseudomonas viridis. Oxidation–reduction potentials and interactions of the electron acceptors

1979 ◽  
Vol 182 (2) ◽  
pp. 515-523 ◽  
Author(s):  
A. William Rutherford ◽  
Peter Heathcote ◽  
Michael C. W. Evans
Keyword(s):  
Reaction Centre ◽  
Double Reduction ◽  
Complex Situation ◽  
Paramagnetic Resonance ◽  
Rhodopseudomonas Viridis ◽  
Midpoint Potential ◽  
Reduction Potentials ◽  
Oxidation Reduction ◽  
Iron Form ◽  
Primary Acceptor

Oxidation–reduction potentiometry was carried out on Rhodopseudomonas viridis chromatophores. Measurements of e.p.r. signals of the semiquinone–iron type at g=1.82 have revealed a more complex situation than previously reported. The presence of three different components is indicated. The midpoint potential (Em) of the primary acceptor quinone/semiquinone couple was found to be approx. −165mV at pH10, with a pK being reached at around pH7.5. The primary acceptor also accepts a second electron with an Em of −525mV, but this redox transition exhibits a hysteresis effect. Interaction effects indicate the presence of another component with Em values at pH10 of approx. −165mV (pK reached at around pH7.5) for single reduction and −350mV (pK at pH10 or greater) for double reduction. It is suggested that this component is the secondary acceptor. Another semiquinone–iron-type component which gives a g=1.82 signal is also present. This component is distinguishable from the primary acceptor by its e.p.r. spectrum, which shows a double peak at g=1.82 and a gx line at g=1.76. This component has Em values at pH10 for single and double reduction of −15mV and approx. −150mV respectively. Both of these Em values are pH-dependent. The presence of an interaction between this component and the photoreduced primary acceptor indicates the close proximity of these components. However, the midpoint potential of this component indicates a function as a secondary electron-transport component rather than an electron acceptor in the reaction centre. The dependence of the bacteriopheophytin intermediate (I) doublet e.p.r. signal on the presence of the semiquinone–iron form of the primary acceptor is demonstrated. The midpoint potential of the I/I− couple is estimated to be lower than −600mV.

scholarly journals Oxidation--reduction potentials of turkey liver xanthine dehydrogenase and the origins of oxidase and dehydrogenase behaviour in molybdenum-containing hydroxylases

1977 ◽  
Vol 163 (2) ◽  
pp. 279-289 ◽  
Author(s):  
M J Barber ◽  
R C Bray ◽  
R Cammack ◽  
M P Coughlan
Keyword(s):  
Uric Acid ◽  
Electron Paramagnetic Resonance Spectroscopy ◽  
Xanthine Dehydrogenase ◽  
Resonance Spectroscopy ◽  
Redox Potentials ◽  
Paramagnetic Resonance ◽  
Reduction Potentials ◽  
Oxidation Reduction ◽  
Temperature Electron ◽  
Full Reduction

Redox potentials for the various centres in the enzyme xanthine dehydrogenase (EC 1.2.1.37) from turkey liver determined by potentiometric titration in the presence of mediator dyes, with low-temperature electron-paramagnetic-resonance spectroscopy. Values at 25 degrees C in pyrophosphate buffer, pH 8.2, are: Mo(VI)/Mo(V)(Rapid),-350 +/- 20mV; Mo(V) (Rapid)/Mo(IV), -362 +/- 20mV; Fe-S Iox./Fe-S Ired., -295 +/- 15mV; Fe-S IIox./Fe-S IIred., -292 +/- 15mV; FAD/FADH,-359+-20mV; FADH/FADH2, -366 +/- 20mV. This value of the FADH/FADH2 potential, which is 130mV lower than the corresponding one for milk xanthine oxidase [Cammack, Barber & Bray (1976) Biochem. J. 157, 469-478], accounts for many of the differences between the two enzymes. When allowance is made for some interference by desulpho enzyme, then differences in the enzymes' behaviour in titration with xanthine [Barber, Bray, Lowe & Coughlan (1976) Biochem. J. 153, 297-307] are accounted for by the potentials. Increases in the molybdenum potentials of the enzymes caused by the binding of uric acid are discussed. Though the potential of uric acid/xanthine (-440mV) is favourable for full reduction of the dehydrogenase, nevertheless, during turnover, for kinetic reasons, only FADH and very little FADH2 is produced from it. Since only FADH2 is expected to react with O2, lack of oxidase activity by the dehydrogenase is explained. Reactivity of the two enzymes with NAD+ as electron acceptor is discussed in relation to the potentials.

scholarly journals Oxidation-reduction potentials of molybdenum, flavin and iron-sulphur centres in milk xanthine oxidase

1976 ◽  
Vol 157 (2) ◽  
pp. 469-478 ◽  
Author(s):  
R Cammack ◽  
M J Barber ◽  
R C Bray
Keyword(s):  
Xanthine Oxidase ◽  
Electron Paramagnetic Resonance Spectroscopy ◽  
Bovine Milk ◽  
Resonance Spectroscopy ◽  
Paramagnetic Resonance ◽  
Reduction Potentials ◽  
Point Reduction ◽  
Oxidation Reduction ◽  
Mediator System ◽  
Electron Paramagnetic

1. The mid-point reduction potentials of the various groups in xanthine oxidase from bovine milk were determined by potentiometric titration with dithionite in the presence of dye mediators, removing samples for quantification of the reduced species by e.p.r. (electron-paramagnetic-resonance) spectroscopy. The values obtained for the functional enzyme in pyrophosphate buffer, pH8.2, are: Fe/S centre I, −343 +/- 15mV; Fe/S II, −303 +/- 15mV; FAD/FADH-; −351 +/- 20mV; FADH/FADH2, −236 +/-mV; Mo(VI)/Mo(V) (Rapid), −355 +/- 20mV; Mo(V) (Rapid)/Mo(IV), −355 +/- 20mV. 2. Behaviour of the functional enzyme is essentially ideal in Tris but less so in pyrophosphate. In Tris, the potential for Mo(VI)/Mo(V) (Rapid) is lowered relative to that in pyrophosphate, but the potential for Fe/S II is raised. The influence of buffer on the potentials was investigated by partial-reduction experiments with six other buffers. 3. Conversion of the enzyme with cyanide into the non-functional form, which gives the Slow molybdenum signal, or alkylation of FAD, has little effect on the mid-point potentials of the other centres. The potentials associated with the Slow signal are: Mo(VI)/Mo(V) (Slow), −440 +/- 25mV; Mo(V) (Slow)/Mo(IV), −480 +/- 25 mV. This signal exhibits very sluggish equilibration with the mediator system. 4. The deviations from ideal behaviour are discussed in terms of possible binding of buffer ions or anti-co-operative interactions amongst the redox centres.

scholarly journals The oxidation-reduction potential of the reaction-centre chlorophyll (P700) in Photosystem I. Evidence for multiple components in electron-paramagnetic-resonance signal 1 at low temperature

1977 ◽  
Vol 162 (1) ◽  
pp. 75-85 ◽  
Author(s):  
M C W Evans ◽  
C K Sihra ◽  
A R Slabas
Keyword(s):  
Electron Paramagnetic Resonance ◽  
Photosystem I ◽  
Reduction Potential ◽  
Potential Range ◽  
Large Contribution ◽  
Reaction Centre ◽  
Paramagnetic Resonance ◽  
Oxidation Reduction ◽  
Oxidation Reduction Potential ◽  
Electron Paramagnetic

The oxidation-reduction potential of the reaction-centre chlorophyll of Photosystem I (P700) in spinach chloroplasts was determined by using the ability of the reaction centre to photoreduce the bound ferredoxin and to photo-oxidize P700 on illumination at 20K as an indicator of the oxidation state of P700. This procedure shows that P700 is oxidized with Em (pH8.0)(mid-point redox potential at pH8.0)congruent to +375mV. Further oxidation of the chloroplast preparations by high concentrations of K3Fe(CN)6(10mM) in the presence of mediating dyes leads to the appearance of a large radical signal with an apparent Em congruent to +470mVA second, light-inducible, radical also appears over the same potential range. We propose that these signals are due to bulk chlorophyll oxidation and not, as was previously thought [Knaff & Malkin (1973) Arch. Biochem. Biophys. 159, 555-562], to reaction-centre oxidation. A number of optical techniques were used to determine Em of P700. Dual-wavelength spectroscopy (697-720nm) indicates Em congruent to +460-+480mV. The spectrum of the sample during the titration showed a large contribution to the signal by bulk chlorophyll oxidation, in agreement with the electron-paramagnetic-resonance results and those of Ke, Sugahara & Shaw [(1975) Biochim. Biophys. Acta 408, 12-25]. The light-induced absorbance change at 435 nm, usually attributed to P700, showed a potential dependence similar to that of bulk chlorophyll oxidation. Determination of Em of P700 on the basis of the appearance of the P700 signal in oxidized-versus-reduced difference spectra showed Em (pH8.0) congruent to +360mV. Measurements of the effect of potential on the irreversible photo-oxidation of P700 at 77K showed that P700 became oxidized in this potential range. We conclude that the reaction-centre chlorophyll of Photosystem I has Em (pH8.0) congruent to +375mV.

scholarly journals Oxidation-reduction studies of the Mo-(2Fe-2S) protein from Desulfovibrio gigas

1978 ◽  
Vol 173 (2) ◽  
pp. 419-425 ◽  
Author(s):  
J J G Moura ◽  
A V Xavier ◽  
R Cammack ◽  
D O Hall ◽  
M Bruschi ◽  
...  
Keyword(s):  
Reduction Potential ◽  
Type I ◽  
Midpoint Potential ◽  
Desulfovibrio Gigas ◽  
Reduction Potentials ◽  
Point Potential ◽  
Oxidation Reduction ◽  
Reducing Conditions ◽  
Oxidation Reduction Potential ◽  
Type Ia

Potentiometric titration followed by e.p.r. measurements were used to determine the midpoint reduction potentials of the redox centres of a molybdenum-containing iron-sulphur protein previously isolated from Desulfovibrio gigas, a sulphate-reducing bacterium (Moura, Xavier, Bruschi, Le Gall, Hall & Cammack (1976) Biochem. Biophys. Res. Commun. 728 782-789; Moura, Xavier, Bruschi, Le Gall & Cabral (1977) J. Less Common Metals 54, 555-562). The iron-sulphur centres could readily be distinguished into three types by means of g values, temperature effect, oxidation-reduction potential values and reduction rates. The type-I Fe-S centres are observed at 77 K. They show mid-point potential values of −260mV (Fe-S type IA) and −440 mV (Fe-S type IB). Centres of types IA and IB appear to have similar spectra at 77 K and 24 K. The Fe-S type-II centres are only observed below 65 K and have a midpoint potential of −28mV. Long equilibration times (30 min) with dye mediators under reducing conditions were necessary to observe the very slow equilibrating molybdenum signals. The potential values associated with this signal were estimated to be approx. −415 mV for Mo(VI)/Mo(V) and-530mV for Mo(V)/Mo(IV).

scholarly journals The oxidation-reduction potentials of cytochrome o + c4 and cytochrome o purified from Azotobacter vinelandii

1979 ◽  
Vol 181 (3) ◽  
pp. 763-766 ◽  
Author(s):  
T Yang ◽  
D O'Keefe ◽  
B Chance
Keyword(s):  
Azotobacter Vinelandii ◽  
Anaerobic Conditions ◽  
Midpoint Potential ◽  
Reduction Potentials ◽  
Cytochrome O ◽  
Oxidation Reduction ◽  
Ph Range

Oxidation-reduction titrations of Azotobacter vinelandii cytochrome o + c4 and cytochrome o were performed with simultaneous potential and absorbance measurements under anaerobic conditions. Cytochrome c4 has a midpoint potential (Em, 7.4) of 260mV and purified cytochrome o has an Em, 7.4 of −18mV. Little change in the midpoint potential of cytochrome o was observed when titrated in the pH range 6.2–9.8.

scholarly journals Measurement of the oxidation-reduction potentials for one-electron and two-electron reduction of electron-transfer flavoprotein from pig liver

1984 ◽  
Vol 219 (3) ◽  
pp. 1043-1047 ◽  
Author(s):  
M Husain ◽  
M T Stankovich ◽  
B G Fox
Keyword(s):  
Electron Transfer ◽  
Pig Liver ◽  
Electron Transfer Flavoprotein ◽  
Midpoint Potential ◽  
Potentiometric Titrations ◽  
Reduction Potentials ◽  
Oxidation Reduction ◽  
Electron Reduction ◽  
Reducing Equivalents

Potentiometric titrations of pig liver electron-transfer flavoprotein (ETF) were performed at pH 7.5 and 4 degrees C, both in the reductive and oxidative directions. Reduction of ETF to the hydroquinone form required a total of two reducing equivalents/mol of ETF with the formation of sub-stoichiometric amounts of anionic semiquinone as an intermediate. The oxidation-reduction potentials for the two one-electron couples, oxidized ETF/ETF semiquinone and ETF semiquinone/fully reduced ETF, are +4 mV and -50 mV respectively. The overall midpoint potential for the two-electron couple (oxidized ETF/fully reduced ETF) is -23 mV.

Potentiometric and spectroscopic properties of the cytochrome o complex of Escherichia coli

1990 ◽  
Vol 68 (1) ◽  
pp. 83-90 ◽  
Author(s):  
H. Keith Withers ◽  
Philip D. Bragg
Keyword(s):  
Escherichia Coli ◽  
Absorption Maximum ◽  
Distinct Species ◽  
Spectroscopic Properties ◽  
Alpha Band ◽  
Difference Spectra ◽  
Midpoint Potential ◽  
Reduction Potentials ◽  
Cytochrome O ◽  
Oxidation Reduction

Cytochrome o purified from cell membranes of Escherichia coli shows two potentiometrically distinct species with midpoint oxidation–reduction potentials of + 265 ± 5 and + 140 ± 15 mV. The component with the higher potential reacted with carbon monoxide and so likely is the oxygen-reacting heme of the cytochrome o complex. It appears to be responsible for the absorption maximum at 564 nm in reduced minus oxidized difference spectra measured at 77 K. The midpoint potential of the other component was sensitive to oxidation by ferricyanide. This latter component had an absorption maximum at about 554 nm. The inhibitor 2-heptyl-4-hydroxyquinoline N-oxide inhibited reoxidation of reduced cytochrome o by oxygen and modified the spectroscopic behaviour of the 564 nm component. The ratio of the heights of the maxima in the alpha-band region of the absorption spectrum differed in cytochrome o prepared from cloned material from that found in cytochrome o from noncloned sources, in spite of the similar polypeptide compositions of the two preparations.Key words: cytochrome o, potentiometry, HOQNO (2-heptyl-4-hydroxyquinoline N-oxide), oxidase, respiratory chain.

Structural analysis of the photosynthetic membrane from Ectothiorhodospira halochloris

1983 ◽  
Vol 41 ◽  
pp. 740-741
Author(s):  
H. Engelhardt ◽  
R. Guckenberger ◽  
W. Baumeister
Keyword(s):  
Lattice Structure ◽  
Bacterial Species ◽  
Hexagonal Lattice ◽  
Reaction Centre ◽  
Photosynthetic Membrane ◽  
Rhodopseudomonas Viridis ◽  
Photosynthetic Membranes ◽  
Ectothiorhodospira Halochloris ◽  
Main Components

Bacterial photosynthetic membranes contain, apart from lipids and electron transport components, reaction centre (RC) and light harvesting (LH) polypeptides as the main components. The RC-LH complexes in Rhodopseudomonas viridis membranes are known since quite seme time to form a hexagonal lattice structure in vivo; hence this membrane attracted the particular attention of electron microscopists. Contrary to previous claims in the literature we found, however, that 2-D periodically organized photosynthetic membranes are not a unique feature of Rhodopseudomonas viridis. At least five bacterial species, all bacteriophyll b - containing, possess membranes with the RC-LH complexes regularly arrayed. All these membranes appear to have a similar lattice structure and fine-morphology. The lattice spacings of the Ectothiorhodospira haloohloris, Ectothiorhodospira abdelmalekii and Rhodopseudomonas viridis membranes are close to 13 nm, those of Thiocapsa pfennigii and Rhodopseudomonas sulfoviridis are slightly smaller (∼12.5 nm).

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