Question

Overall electron flow in a cell using reverse electron transport Consider a cell that is using reverse electron transport to generate NADH and is not (for the moment) using the proton gradient for anything else. Assume the cell is not a phototroph (so electrons are not being recycled by light-driven pumping). Also assume that the proton gradient is produced solely by a Q-cycle like the one we've discussed, which pumps 2 protons per electron transferred. (This would imply that the bc1 complex passes electrons on to a terminal acceptor in a process that does not pump protons.) Also assume that the NADH dehydrogenase is like the one described today, coupling the movement of 4 H+ to the transfer of 2 electrons. How many electrons would need to be provided to this cell's electron transport machinery at Q (from $2,50, or some other donor that can reduce Q) to generate one NADH, in a sustainable process? Here,'sustainable' means in a way that maintains the proton gradient, by pumping as many H+as are used. Sketch the overall process that you needed to consider to answer this question, including the action needed to maintain the proton gradient. Write the chemical equation describing this overall process. Next, assume that the Q-cycle is able to pump only one H+ per electron transferred, as might be the case if the available electron acceptor is a poorer oxidant. Answer the same questions-how many input electrons would be needed to generate one NADH? What is the chemical equation describing this overall process? Summarize your answers on a page to turn in. Note from TA - For this, be sure to write out your work/thinking process as much as possible. The process, rather than the correct answer, is more important for your grade.

Answer 1

Reversed electron transport

The system normally runs with the redox gradient of the electron transport chain oxygen/NADH as the driving force, and ATP synthesis as the sink for the energy released.

All the reactions of the system are reversible.

With the exception of cytochrome oxidase, each of the other complexes can be driven in reverse by a driving force provided either directly by the proton gradient, or indirectly by ATP hydrolysis through the proton gradient generated.
The production of NADH from NAD+ for reduction of metabolites in the synthetic reactions of the cell occurs through this "reversed electron flow", with the cyclic photosynthetic chain provided the driving force through generation of a proton gradient

Succinate acting as the source of electrons, delivered via the quinone pool to the NADH:UQ oxidoreductase (Complex I), with the proton gradient driving the latter in reverse, ie. toward reduction of NAD+.