Bounds on rate constants and relative potentials in electron transport chains.

Minimization of free-energy losses requires that the rate constants for reaction in both forward and reverse directions be several times the net rate of a reaction. In an electron transfer between wound molecules, the forward and reverse rate constants contribute separate factors to the free-energy drop across the reaction. If such a reaction has a reverse rate constant which is much greater than the net rate of reaction, then the midpoint potential of the acceptor may economically be more negative than the midpoint potential of the donor.