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Lecture EZ6: Redox Potential

This document discusses redox reactions and redox potential. It defines redox reactions as those where reduction of one species and oxidation of another occur simultaneously. Redox potential is a measure of the tendency of a chemical species to acquire or lose electrons. Standard redox potentials are expressed for partial reactions involving an oxidant donating electrons and a reductant accepting electrons. ATP and electron carriers like NADH act as activated carriers that allow energy released from redox reactions to be stored and transferred. Examples are given of redox reactions involving pyruvate and oxygen that release energy which can be harnessed to drive endergonic processes like ATP synthesis.

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88 views23 pages

Lecture EZ6: Redox Potential

This document discusses redox reactions and redox potential. It defines redox reactions as those where reduction of one species and oxidation of another occur simultaneously. Redox potential is a measure of the tendency of a chemical species to acquire or lose electrons. Standard redox potentials are expressed for partial reactions involving an oxidant donating electrons and a reductant accepting electrons. ATP and electron carriers like NADH act as activated carriers that allow energy released from redox reactions to be stored and transferred. Examples are given of redox reactions involving pyruvate and oxygen that release energy which can be harnessed to drive endergonic processes like ATP synthesis.

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Cheng
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Lecture EZ6:

Redox potential
Redox
• A reduction-oxidation reaction: one where the
reduction of one chemical species and the
oxidation of another occur simultaneously.
• Oxidation

• Involves

When it comes to • Loss


electrons… • Reduction

• Involves

• Gain
Redox potential convention

• By convention, the standard redox


potential is expressed for the partial
reaction in the following form:



oxidant + e- → reductant
Activated carriers: an example of
modular design in metabolism
• ATP can be thought of as an ‘activated carrier’ of
phosphoryl groups (and hence chemical energy).

• Likewise, there are activated carrier molecules for


electrons (and hence chemical energy).

• Electron transfer potential can be thought of as


another important form of free energy transfer.
NAD(P)

nicotinamide adenine dinucleotide

nicotinamide

adenine
P

vectorstock.com/6229785
FAD 

flavin adenine dinucleotide
flavin 

(isoalloxazine)

adenine

vectorstock.com/6229792
The phosphoryl-transfer potential
• We defined the energy released in ATP
hydrolysis terms of the chemical equilibrium of
the reaction:



!!
ATP + H 2O ↽ ⇀
!! ADP + Pi

[ADP][Pi ]
ΔG°′ = −RT ln = −30.5 kJ . mol -1

[ATP][H2O]
The redox potential
• In a similar way, we can define the energy
released in a redox reaction in terms of the
redox potential (a.k.a. the reduction potential or
the oxidation-reduction potential)

ΔG°′ = −nFΔE°′
ΔE°′ standard redox potential at 25°C, pH= 7, in V
F Faraday constant: 96.48 kJ mol-1 V-1
n number of electrons involved
Going uphill again!
• Some reactions in metabolism (especially anabolic
ones) may not be spontaneous i.e. ΔG > 0

• These reactions cannot happen, even in the


presence of an enzyme

• Coupling a favourable redox reaction to the


unfavourable reaction makes the overall
reaction spontaneous, i.e. combined ΔG < 0
Redox potential convention

• By convention, the standard redox


potential is expressed for the partial
reaction in the following form:



oxidant + e- → reductant
Some example ΔE°′values
Oxidant Reductant n
ΔE°′
NAD+ NADH + H+ 2 -0.32 V

FAD FADH2 2 -0.22 V

Pyruvate Lactate 2 -0.19 V

½O2 H2O 2 +0.82 V

Remember that a positive value for ΔE°′


means a negative value for ΔG°′
Example 1:

Pyruvate to lactate
Lactate dehydrogenase
+ - !!!!!!!!!
Pyruvate + 2H + 2e ↽!!!!!!!!⇀
! Lactate

ΔE°′ = −0.19 V

ΔG°′ = −nFΔE°′
ΔG°′ = −2× 96.48 × −0.19
ΔG°′ = +36.7 kJ . mol -1
Lactate dehydrogenase
− !!!!!!!!!
Pyruvate + 2H + 2e ↽
+
!!!!!!!! ⇀
! Lactate
+ −
+NADH +NAD + H + 2e
+

ΔE°′ = −0.19 V + 0.32 V


ΔE°′ = +0.13 V

ΔG°′ = −nFΔE°′
ΔG°′ = −2 × 96.48 × 0.13
ΔG°′ = −25.1 kJ . mol -1
Example 2:

Oxygen as the terminal electron
acceptor of oxidative phosphorylation
Catabolism generates ATP
Mitochondrion

Chemotrophs
!!
ADP + Pi ↽ ⇀
!! ATP + H2O

ΔG°′ = +30.5 kJ . mol -1

ATP synthesis requires energy


+ −
1 O
2 2 + 2H + 2e → H2
O ΔE°′ = +0.82 V

NADH→ NAD + H + 2e ΔE°′ = +0.32 V


+ + −

+ +
1 O
2 2 + NADH+ H → NAD + H2
O

Reduction of oxygen provides energy


ΔG°′ = −nFΔE°′
ΔG°′ = (−2 × 96.48 × 0.82) + (−2 × 96.48 × 0.32)

ΔG°′ = −220.1 kJ . mol -1

Reduction of oxygen provides energy


Mitochondrion

Chemotrophs
Reading

• Appling Section 3.4 pp96-99

• Berg 9th Ed Section 18.2 pp576-579

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