1. The P/O ratio for mitochondrial oxidative phosphorylation is the number of ATP molecules that are synthesized for each two electrons that enter the electron transport pathway. Under normal conditions, the P/O ratio is approximately 2.5. [The derivation of this number will be explained in class, though it is not necessary for you to understand the derivation to answer this question. Just know that the P/O ratio is the number of ATPs made for each NADH that is oxidized]. (a) Uncouplers cause the mitochondrial membrane to become permeable to protons, and so uncouple ATP synthesis from electron transport. What would be the effect of a low concentration of an uncoupler on the P/O ratio? Note that a low concentration of uncoupler will only partially dissipate the proton gradient. (b) DCCD is a reagent that inhibits ATP synthase by blocking the proton channel through the enzyme. What effect would DCCD have on the P/O ratio? Tip: it is important to understand that inhibition of ATP synthase also inhibits electron transport, because an increase in [H+] in the inter-membrane space exerts an inhibitory backpressure on the proton pumping complexes (ie remember that ATP synthesis and electron transport are coupled processes). (c) If mitochondria are inhibited with DCCD, and then an uncoupler is aIDed, what will be the effect of the uncoupler on ATP synthesis, oxygen consumption and the P/O ratio? Explain your reasoning behind all answers.
