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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
D- = donor; More negative E’o
A+ = acceptor; More positive E’o
+
Coupled ½ reactions:
A reducedD oxidized
Spontaneous (-ΔGo’ or +ΔE’o):½ O2 + NADH → H2O + NAD+
ΔE’o = 0.815V – (-0.42V) = 1.235V
BetterDonors
Better Acceptors
Endergonic (+ΔGo’ or -ΔE’o):H2O + NADP+ → ½ O2 + NADPHΔE’o = -0.42V – 0.815V = -1.235V
Electron Transport Chains
NO3-
O2
• 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)
*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.
*
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.
• 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.
Electron Transport Chain (Prokaryote)
E.g. Parococcus denitrificans;facultative anaerobe
Denitrification (nitrate to N-gases)
• Nitrate Reductase like E.coli.• Further reduction of toxic nitrite
• Nitric Oxide to Nitrous Oxide to N2
Other Anaerobic Respirations