Reduced Electron Carriers Research Articles

Dependence of tetrachloroethylene dechlorination on methanogenic substrate consumption by Methanosarcina sp. strain DCM.

Tetrachloroethylene (perchloroethylene, PCE) is a suspected carcinogen and a common groundwater contaminant. Although PCE is highly resistant to aerobic biodegradation, it is subject to reductive dechlorination reactions in a variety of anaerobic habitats. The data presented here clearly establish that axenic cultures of Methanosarcina sp. strain DCM dechlorinate PCE to trichloroethylene and that this is a biological reaction. Growth on methanol, acetate, methylamine, and trimethylamine resulted in PCE dechlorination. The reductive dechlorination of PCE occurred only during methanogenesis, and no dechlorination was noted when CH4 production ceased. There was a clear dependence of the extent of PCE dechlorination on the amount of methanogenic substrate (methanol) consumed. The amount of trichloroethylene formed per millimole of CH4 formed remained essentially constant for a 20-fold range of methanol concentrations and for growth on acetate, methylamine, and trimethylamine. These results suggest that the reducing equivalents for PCE dechlorination are derived from CH4 biosynthesis and that the extent of chloroethylene dechlorination can be enhanced by stimulating methanogenesis. It is proposed that electrons transferred during methanogenesis are diverted to PCE by a reduced electron carrier involved in methane formation.

Quantum efficiency of Photosystem II in relation to ‘energy’-dependent quenching of chlorophyll fluorescence

The balance between light-dependent reactions and electron-consuming reactions in intact sunflower leaves was varied by changing the incident light-flux at constant intercellular CO 2 concentration. Measurements of fluorescence quenching were compared to measurements of the rate and apparent quantum yield of whole-chain electron transport at a number of steady-state conditions. The steady-state quantum yield declined with increasing light intensity, falling at the highest intensity to approx. 40% of the maximum value observed in low light. The coefficient for photochemical quenching, q Q, was near 1 in low light and only declined to 0.7 at the highest light, indicating that there was very little feedback from accumulation of reduced electron carriers. On the other hand, there was a large increase in q E, the coefficient for ‘energy’-dependent quenching, as the quantum yield fell. We found that these changes in the steady-state quantum yield, Φ s, could be related to the changes in fluorescence quenching by an empirical equation, Φ s = q Q(0.32 − 0.17 q E) which accounted for variation in Φ s resulting from light saturation or changes in CO 2 concentration. We develop a hypothesis that Photosystem (PS) II centers may be converted to an altered state (possibly mediated by the chloroplast ΔpH) which has very little variable fluorescence and a lowered photochemical yield. We develop a kinetic explanation for the properties of the altered form of PS II, and we propose that this mechanism (indicated by q E) functions together with the accumulation of reduced Q A (indicated by q Q) to regulate the rate of net photochemistry by PS II when — with increasing light or decreasing CO 2 — the potential rate of net photochemistry exceeds that for carbon metabolism. The latter mechanism apparently permits down-regulation of PS II to occur without strong accumulation of reduced Q A, except during transients or under the most extreme conditions.

Ubisemiquinone is the electron donor for superoxide formation by complex III of heart mitochondria

Much evidence indicates that superoxide is generated from O 2 in a cyanide-sensitive reaction involving a reduced component of complex III of the mitochondrial respiratory chain, particularly when antimycin A is present. Although it is generally believed that ubisemiquinone is the electron donor to O 2, little experimental evidence supporting this view has been reported. Experiments with succinate as electron donor in the presence of antimycin A in intact rat heart mitochondria, which contain much superoxide dismutase but little catalase, showed that myxothiazol, which inhibits reduction of the Rieske iron-sulfur center, prevented formation of hydrogen peroxide, determined spectrophotometrically as the H 2O 2-peroxidase complex. Similarly, depletion of the mitochondria of their cytochrome c also inhibited formation of H 2O 2, which was restored by addition of cytochrome c. These observations indicate that factors preventing the formation of ubisemiquinone also prevent H 2O 2 formation. They also exclude ubiquinol, which remains reduced under these conditions, as the reductant of O 2. Since cytochrome b also remains fully reduced when myxothiazol is added to succinate and antimycin A-supplemented mitochondria, reduced cytochrome b may also be excluded as the reductant of O 2. These observations, which are consistent with the Q-cycle reactions, by exclusion of other possibilities leave ubisemiquinone as the only reduced electron carrier in complex III capable of reducing O 2 to O 2 −.

A light-dependent oxygen consumption induced by photosystem II of isolated chloroplasts

The photooxidants accumulated on the water-splitting side of photosystem II in chloroplasts are destabilized by certain membrane active chemicals. In the light and in the presence of oxygen, this destabilization results in a consumption of oxygen and in a lowering of the fluorescence emission from the chloroplasts. It is shown that a close correlation exists between the oxygen uptake and the fluorescence lowering, and that with some of the destabilizing agents photosystem I activity is not required for either process. When electron flow through photosystem I is blocked, the oxygen consumption appears to occur without formation of free oxygen-derived radicals. It is concluded that, in the light, a disturbed water-splitting enzyme may initiate oxygen-dependent photooxidations which the superoxide dismutase of chloroplasts cannot protect against. The fluorescence lowering is attributed to either direct quenching actions of oxygenated reaction products or to a cyclic electron flow between reduced electron carriers and such intermediates.

Some effects of uncouplers and inhibitors on growth and electron transport in rumen bacteria

Uncouplers and inhibitors of electron transport affected growth and electron transport of rumen bacteria in various ways. Selenomonas ruminantium was not affected by inhibitor and uncoupler concentrations which affected growth and electron transport of Bacteroides ruminicola, B. succinogenes, and Butyrivibrio fibrisolvens. Inhibitors, when active, led to accumulation of reduced electron carriers before the site of action, but differences were found among organisms in the site of action of these inhibitors. Uncouplers reduced the glucose molar growth yields (Ygluc) of B. ruminicola, B. succinogenes, and B. fibrisolvens compared with those obtained without uncouplers. The extent of Ygluc reduction accompanying inhibitor exposure reflected electron transport chain structure. S. ruminantium appeared to obtain its adenosine 5'-triphosphate from substrate-level processes only. The other organisms studied appeared to obtain adenosine 5'-triphosphate both from substrate-level processes and from electron transport but differed in the amount of adenosine 5'-triphosphate obtained from glucose catabolism and in the proportions of adenosine 5'-triphosphate obtained from substrate-level reactions and electron transport.

Pyruvate: Ferredoxin oxidoreductase: II. Characteristics of the forward and reverse reactions and properties of the enzyme

Pyruvate:ferredoxin oxidoreductase, partially purified from extracts of Clostridium acidiurici , catalyzes the oxidation and the reductive synthesis of pyruvate in addition to the exchange of CO 2 with the carboxyl group of pyruvate. The oxidation of pyruvate requires CoA, high levels of a thiol, and an electron carrier such as clostridial ferredoxin, FAD, or 2,3,5-triphenyltetrazolium chloride. DPN + cannot replace these electron acceptors in the reaction catalyzed by the purified enzyme. The reductive synthesis of pyruvate from acetyl CoA and radioactive bicarbonate requires a reduced electron carrier of low potential such as ferredoxin or methyl viologen and is stimulated by a thiol. The enzyme, whose molecular weight has been estimated to be 215,000, contains a nonheme iron chromophore, equimolar amounts of iron and sulfide, thiamine, and trace amounts of flavin. When a stoichiometric amount of enzyme is incubated with pyruvate, CoA, and thiol, in the absence of added electron acceptor, the enzyme-bound chromophore is reduced.