Reverse electron flow

Thermodynamics and Reverse Electron Flow Thermogenic Tissues... 

Synthesis of ATP by mitochondria is inhibited by oligomycin, which binds to the OSCP subunit of ATP synthase. On the other hand, there are processes that require energy from electron transport and that are not inhibited by oligomycin. These energy-linked processes include the transport of many ions across the mitochondrial membrane (Section E) and reverse electron flow from succinate to NAD+ (Section C,2). Dinitrophenol and many other uncouplers block the reactions, but oligomycin has no effect. This fact can be rationalized by the Mitchell hypothesis if we assume that Ap can drive these processes. 

Such reverse electron flow is a common process for many chemolithotrophic organisms. 

The carbocation intermediate in the Friedel-Crafts reaction (Chapter 22) is rather stable, being tertiary and benzylic, and the formation of the cation, normally the rate-determining step, with inevitably a negative p value, goes faster and faster as the electron-donating power of the substituents increases until it is faster than the cyclization which becomes the rate-determining step. The cycliza-tion puts electrons back into the carbocation and has a positive p value. As the two steps have more or less the reverse electron flow to and from the same carbon atom, it is reasonable for the size of p to be about the same but of opposite sign. 

Inhibitors binding at the Qo site exclusively to cytochrome b, e.g., myxothiazol as well as E-fi-methoxyacrylate (MOA) and related inhibitors. These inhibitors prevent the reduction of both the Rieske center and heme 6l through die Qo site but heme hn can still be reduced through the Qi site with hydroquinone acting as a reductant (reverse electron flow). 

Little is known about the electron flow pathways when Chloroflexus oxidizes to S° and fixes CO2, but it seems reasonable to suppose that NAD is reduced by reverse electron flow from the MQ pool in analogy to the mechanism used by the purple sulfur bacteria (Chapter 9). 

Those photosynthetic eubacteria with RC-2 centers (filamentous and purple bacteria) reduce NAD" for CO2 fixation by reverse electron flow from the quinone pool, whereas the green sulfur bacteria (RC-1 center) reduce ferredoxin and NAD directly from the secondary acceptor (Fe-S center) of the RC. In both cases an external reductant such as H2S is required. The mechanism of NAD reduction in the gram-positive line has not yet been investigated, but H. chlorum is a het-erotroph rather than an autotroph, and may not need to fix CO2. 

ATP, under similar experimental conditions, has also been shown to drive reverse electron flow, leading to the oxidation of an external electron donor, such as DTT or hydroquinone, and the reduction of Q. The reaction seems to involve. 

There is one bacterial system where such reversed electron transfer is of great importance. In Rps. sphaeroides the generated by cyclic electron flow through the reaction centre and cytochrome system is used to induce reversed electron flow from the level of the ubiquinone pool to the NADH/NAD" pool, in a manner analogous to that described for mitochondria. The role of this is to supply low potential electrons for the biosynthetic functions of the cell [38]. 

Since the ATP synthase is reversible, it is also possible in the absence of respiration to drive a proton circuit by ATP hydrolysis (Fig. 2.4). The + achievable by this means is identical to that supported by respiration [39] and can be utilized to drive ion transport, as well as to reverse electron flow through Complex I (Fig. 2.4). 

Yeast mitochondria depleted of ADP and ATP also have PPj synthesis coupled to electron transport as shown by Mansurova in preparations from Endomyces mag-nusii [19]. Also, recent work in this laboratory (unpublished) with mitochondria from Saccharomyces cerevisiae shows the existence of a membrane-bound PPase, the activity of which is stimulated by uncouplers and inhibited by fluoride. Earlier, we showed [4] PP-induced cytochrome redox changes, indicative of reversed electron flow in mitochondria from Saccharomyces cerevisiae. 

Fig. 36. Forward and reversed electron flow in T. denitrificans, after reference 396. Oxidation of thiosulfate at cai can either generate ATP at cytochrome or...

Sadler and Johnson [S96) claimed that reverse electron flow involving cytochrome c has not been established for T. neapolitanus and T. thio-parus and proposed the parallel pathways of Figs. 37a and 37b but offered no suggestion as to the source of the NADH. Yamanaka et al. (334) have found multiple cytochromes in T. novellus which they place in series as in Fig. 37c. They proposed that sulfite delivers its electrons to C551 by means of a sulfite. cytochrome c oxidoreductase enzyme. In all of these Thiobadllus species, there is at least one cytochrome c with a reduction potential near -j-280 mV and a molecular weight around 13,000, which probably is an evolutionary homolog of eukaryotic c. 

The addition of ATP to anaerobic or terminally inhibited mitochondria or submitochondrial particles containing succinate Eo = 0.03 V at pH 7) induces reduction of cjdiochrome bj 16,17,65 see also 6 6). The original concept of the possible mechanism of this phenomenon described by Wilson and Dutton 19) was that the Eo of cytochrome f T changes because of the formation of a high energy derivative which is the primary intermediate for site 2 energy conservation reaction in oxidative phosphorylation. However, there has been another possible mechanism presented in which ATP can induce reduction of cytochrome bx by the decrease in the effective redox potential Ek) of the cytochrome because of reversed electron flow 57) or of the abolition of an accessibility barrier between the substrate and the cytochrome 58). The former explanation would be favored by the chemical hypothesis of oxidative phosphorylation, while the latter is favorable for the chemiosmotic hypothesis. 

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