Electron transport energetics

The energetics of the electron transport steps makes the process work. Overall there s a lot of free energy lost in the tranfer of electrons from NADH to oxygen—the overall reaction is very favorable, with an equilibrium constant that s overwhelmingly large. At the three sites where ATPs are made (labeled I, II, and III), the reaction is the most downhill. 

The membranes of most species of bacteria contain an electron transport system. This serves to carry electrons energetically downhill from reductants—which can include not only intracellular molecules such as NADH but also extracellular sources of electrons such as hydrogen... 

Biological Equilibria and Energetics. pH/bujfers, hinding/recognition, proton and electron transport, oxidation/reduction, macromolecular conformations. Some of these topics may be covered in laboratory courses. The experiments that are usedfor this purpose should emphasize techniques including error... 

Electron transport Comparing the energetics of the oxidation of NADH from NADH... 

There are two fermentative processes that at first appear to be quite similar to oxygen and nitrate-dependent respirations the reduction of C02 to methane and of sulfate to sulfide. However, on closer examination, it is clear that they bear little resemblance to the process of denitrification. In the first place, the reduction of C02 and of sulfate is carried out by strict anaerobes, whereas nitrate reduction is carried out by aerobes only if oxygen is unavailable. Equally important, nitrate respirers contain a true respiratory chain sulfate and C02 reducers do not. Furthermore, the energetics of these processes are very different. Whereas the free energy changes of 02 and nitrate reduction are about the same, the values are much lower for C02 and sulfate reduction. In fact, the values are so low that the formation of one ATP per H2 or NADH oxidized cannot be expected. Consequently, not all the reduction steps in methane and sulfide formation can be coupled to ATP synthesis. Only the reduction of one or two intermediates may yield ATP by electron transport phosphorylation, and the ATP gain is therefore small, as is typical of fermentative reactions. 

Fig. 9. Proposed function of electrochemical and Na potentials in energy conservation coupled to methanol disproportionation to CH4 and CO2- It is assumed that prior to oxidation methanol binds first to coenzyme M and that the oxidation is mechanistically and energetically the reversal of CO2 reduction to methyl-coenzyme M. The Na /H" antiporter is involved in the generation of A/iNa Ifom AjlVt. CHO-MFR, formyl-methanofuran CH2=H4MPT, methylene-tetrahydromethanopterin CH3-H4MPT, methyl-tetrahydromethanopterin CH3-S-C0M, methyl-coenzymeM. The hatched boxes indicate membrane-bound electron transport chains or membrane-bound methyltransferase catalyzing either Na or H translocation (see Figs. 5, 6 and 12). ATP is synthesized via membrane-bound H -translocating ATP synthase. The stoichiometries of Na and of translocation were taken from refs. [105,107,167]. x, y and z are unknown stoichiometric factors.

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