Oxidative Phosphorylation Making ATP (i.e. phosphorylation of ADP) from energy released during oxidation of an e- donor. 4 main components: 1) Reduced compound donates high energy e- to ETC. 2) Oxidized compound accepts low energy e- from ETC. 3) Energy released in ETC does the work of pumping H+ across a membrane to establish the PMF. 4) PMF fuels ATP Synthase to phosphorylate ADP.
Oxidation-Reduction (Redox) ΔEo’ = Change in standard reduction potential (Eo’)ΔE’o = (E’o acceptor - E’o donor)ΔGo’ = -nF·ΔE’o D + e-→ D- A+ + e-→ A Coupled ½ reactions: + A+ = acceptor; More positive E’o A reduced D- = donor; More negative E’o D oxidized
Spontaneous (-ΔGo’ or +ΔE’o): ½ O2 + NADH → H2O + NAD+ ΔE’o = 0.815V – (-0.42V) = 1.235V Endergonic (+ΔGo’ or -ΔE’o): H2O+ NADP+→ ½ O2 + NADPH ΔE’o = -0.42V – 0.815V = -1.235V BetterDonors Better Acceptors
Electron Transport Chains • NADH from Glycolysis (or E-DP) and Krebs Cycle has a very negative E’o. • It’s electron get transferred in a cascade of membrane associate electron carriers of increasing E’o values. • E’o of the terminal electron acceptor will set the upper limit of potential energy yield. • Aerobic Respiration uses O2 with a E’o = 0.812V. • Dissimilatory Nitrate Reduction* (a type of Anaerobic Respiration) uses NO3- with E’o=0.42V. (FADH2) * NO3- *NOTE: This specific example is for eukaryote mitochondria, which would NOT use nitrate as a terminal electron acceptor. It’s shown assuming this as a “generic” ETC of a facultative anaerobe. O2
Chemiosmotic Hypothesis for Oxidative Phosphorylation(eukaryote mitochondria) Energy release during electron transport pumps protons across the inner membrane. A favorable proton gradient establishes from outside to inside the inner membrane (proton motive force = PMF) drives the F1F0 Complex (ATP synthese) for ATP synthesis. 3 ATP per NADH; 2 ATP per FADH2; based on P/O ratios.
Electron Transport Chain (Prokaryote) • Prokaryote ETC are structurally different and typically less efficient (lower P/O ratios) • Escherichia coli has a branched chain dependent on oxygen supply. The cyt bd branch has a P/O of 0.67 and that for cyt bo is 1.3 (half that for mitochondria). • E. coli yields even less ATP per glucose consumed when there is no oxygen, and anaerobic respiration is by dissimilatory nitrate reduction (nitrate to nitrite). • When nitrate is exhausted, ATP yield decreases even more as the cell solely relies on fermentation.
E.g. Parococcus denitrificans;facultative anaerobeDenitrification (nitrate to N-gases) • Nitrate Reductase like E.coli. • Further reduction of toxic nitrite • Nitric Oxide to Nitrous Oxide to N2