Question

For each of the following two-electron donors, state the number of protons translocated from the mitochondrion, the number of ATP molecules synthesized and the P/O ratio. Assume the electrons passed eventually to O2, and the electron transport and oxidative phosphorylation systems are fully functional. NADH succinate ascorbate tetramethyl-p-phenylethylenediamine (donates 2ē to Cytochrome c) (Ctrl)

Answer 1

Electrons from NADH is passed to terminal oxygen acceptor. For that a total of 10 protons are required (4 protons via complex I, 4 protons via complex III and 2 protons via complex IV) to transfer from the matrix across the inner mitochondrial membrane for every electron pair. Number of ATP molecules formed is 25 (10 NADH molecules enter into ETS and 1 NADH molecule produce 2.5 ATP). The P/O ratio is 10/4=2.5, as 4 protons are pumped from ATP synthase for 1 ATP molecule.

Since the succinate CoQreductase complex does not transport protons only 6 protons are transported across the membrane for every electron pair that is transferred from succinate to oxgen acceptor. Number of ATP produced from succinate is 8 ATP ( 3 ATP from FADH2 and 5 ATP from NADH). The P/O ratio is 6/4=1.5, as protons are pumped from ATP synthase for 1 ATP molecule.

Ascorbate/tetrametyl-p-phenylethylenediamine gives 2 electrons to cytochrome c. 4 protons are translocated from the mitochondrial matric across inner membrane for each pair of electron transferred. Number of ATP formed is 1 and P/O ratio is 4/4=1.