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