BIOC 2300 Lecture Notes - Lecture 20: Oxidative Phosphorylation, Reduction Potential, Coenzyme Q10
3 May 2018
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Redox Reactions and Oxidative Phosphorylation
March 4-7, 2016
ETC and oxidative phosphorylation utilize the energy from the oxidation of NADH and QH2 to
generate ATP
Recall: an oxidant is reduced and gains electrons.
• Oxygen is strong oxidant and is reduced
FADH2 + Q FAD + QH2
• Q is oxidized
• Redox through transfer of hydride ion: NAD+ and NADH
• NAD+ accepts a proton and two electrons to become NADH
The reduction potential of a substance indicates its tendency to accept electrons:
• Reduction potential – tendency of a substance to accept electrons (in V)
• A higher reduction potential, means that the more easily it is reduced (strong oxidant)
• Volt = measures electric potential which reflects the energy that could be released
• The higher and more positive a reduction potential, the more energy is released during
the reaction
Standard Reduction potential – reduction potential under standard conditions (25C, pH 7, 1M)
• Characteristic of each redox active substrate and reflects its affinity for electrons
• Full oxidation-reduction reactions require a reductant and oxidant
• Each half reaction has a standard reduction potential(what is used)
• The oxidative potential = -‘ve reduction potential
• Indication of which direction the electrons will flow spontaneously
• Reactions with largest reduction potential have the greatest tendency to accept electrons
• If –‘ve: oxidation will release energy
The actual reduction potentials are calculated from the standard reduction potential:
• See the Nernst Equation * n usually equals 2
The difference between standard reduction potentials of the oxidant and reductant shows which
direction electrons will move
• (e- acceptor) – (e- donor)
• Ex. NADH + Q + H NAD+ +QH2
Acceptor: Q + 2H + 2e QH2 e=+0.045V
Donor: NAD+ + H+ + 2e NADH e=-0.315 V
therefore, change in reduction potential = +0.36 V ***spontaneously (positive)
find more resources at oneclass.com
find more resources at oneclass.com
The free energy change can be calculated from change in reduction potential:
• See standard free energy change and actual free energy change **memorize for exam
• Reaction is possible when the change of reduction potential is positive and the change in
free energy is negative
• Living organisms utilize redox coenzymes as high-energy electron carriers
Concepts:
• Standard reduction potential indicates substance’s tendency to become reduced; actual
reduction potential depends on concentrations of reactants
• Electrons are transferred from substance with lower to higher reduction potential
• Free energy change depends on the change in reduction potential, when it is –‘ve, G is
+’ve
ETC and Oxidative Phosphorylation
• NADH and FADH2 are produced by glycolysis, FA oxidation and citric acid cycle
• FADH2 reduces Q to QH2
• NADH and FADH2 each carry 2e
• Oxidative phosphorylation – inner membrane of mitochondrial
o Membrane is protein rich
o Impermeable to several metabolites and ions
o Fully permeable to O2, H2O and CO2
o Cristae increases SA
o ETC happens across inner membrane
o Protons accumulate in the inter membrane space
• ETC works due to limited permeability
o Compartmentalization allows pathway control
o NADH NAD+ in complex 1
Q in Complex III
cytochrome C
complex IV
O2 H2O
o Reduction potential increases while free energy decreases
Oxidative Phosphorylation a series of redox reactions generates a proton gradient to fuel ATP
synthesis
• Electrons are passed down to ETC of complexes I-IV and fuel pumping of H+ out of
mitochondrial matrix
• Protons flow back into mitochondrial matrix through complex V (ATP Synthase) and
fuels synthesis of ATP from ADP and Pi
• I,III, and IV are proton pumps creates concentration gradient
NADH from citric acid cycle is in mitochondrial matrix and directly available
• NADH from glycolysis must be transported to mitochondria = indirect transport by the
malate-aspartate shuttle
• Mitochondrial malate dehydrogenase is part of citric acid cycle
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Etc and oxidative phosphorylation utilize the energy from the oxidation of nadh and qh2 to generate atp. Recall: an oxidant is reduced and gains electrons: oxygen is strong oxidant and is reduced. Fadh2 + q fad + qh2: q is oxidized, redox through transfer of hydride ion: nad+ and nadh, nad+ accepts a proton and two electrons to become nadh. Indication of which direction the electrons will flow spontaneously. The actual reduction potentials are calculated from the standard reduction potential: see the nernst equation * n usually equals 2. The difference between standard reduction potentials of the oxidant and reductant shows which direction electrons will move (e- acceptor) (e- donor: ex. Nadh + q + h nad+ +qh2. Acceptor: q + 2h + 2e qh2 e=+0. 045v. Donor: nad+ + h+ + 2e nadh e=-0. 315 v therefore, change in reduction potential = +0. 36 v ***spontaneously (positive) O2 h2o: reduction potential increases while free energy decreases.
