Question

In class we discussed the calculation of the P/O ratio assuming a c-ring comprised of 9 subunits. How would the P/O ratio change (for both NADH and succinate/FADH2) if a particular mitochondrion had FOF1-ATP-synthase molecules whose c-ring were composed of 12 subunits rather than 9? The best answer will explain your reasoning and show the calculation for each. (5 points)

Answer 1

In view of the difficulties with P/O determination, there was a time when “the bigger the better” was the motto of some, but it became widely accepted that the P/O ratio was 3 or 2 depending on whether matrix NADH or succinate was the electron donor. These values thus gave rise to the notion of three “sites” of ATP synthesis associated with the mitochondrial respiratory chain. This notion has turned out to be incorrect because around 1960 Mitchell introduced his chemiosmotic mechanism, which envisaged that there was a separate ATP synthase (now F1Fo), and that there were three sites of proton translocation rather than ATP synthesis. Each site was envisaged to translocate 2H+/2e, thus generating a proton electrochemical gradient, or protonmotive force in volts which would drive ATP synthesis by using 2H+ for each ATP made. Arithmetic shows that P/O ratios of 3 and 2 were retained. The idea of nonintegral P/O ratios was not envisaged although the chemiosmotic mechanism would allow such values.