Information

Oxidation and reduction: Redox reactions
Submitted by: danr June 23, 2023

Cellular energy, ATP and phosphorylation << Oxidation and reduction: Redox reactions >> Alternative inorganic terminal electron acceptors

When electrons move from one molecule (donor) to another (acceptor), energy is being transferred in the form of discrete energy packets. Electrons move because they are attracted to atoms that are more 'positively charged', or electronegative, and in the process the system loses energy (heat) and becomes more stable. The donor element or molecule becomes oxidised, while the recipient species becomes reduced. Some important biologically relevant 'half reactions' are shown in Table 1.

Table 1. Biologically relevant half reactions and apparent potentials. Standard reduction potentials (Eo´, Volts) are measured at 25°C, one atmosphere pressure and at pH 7 in a 1 M aqueous solution (see Wikipedia for an excellent explanation of biologically important half-reactions).

Half Reaction Standard Reduction Potential (Eo´, Volts) ΔG°' from NADH oxidation (kJ/2e-) Significance
Li+ + e− → Li −3.0   Extreme example of an endogenic reaction. Not biologically relevant; relative to 2H+ + 2e− ⇆ H2 being = 0 V.
CH3COOH + 2H+ + 2e− → CH3CHO + H2O −0.58   Many carboxylic acid: aldehyde redox reactions have a potential near this value (from Wikipedia).
2H+ + 2e− → H2 −0.41   Costly loss of energy since H2 is usually excreted. Non-zero value for the hydrogen potential because at pH = 7, [H+] = 10−7 M and not 1 M as in the standard hydrogen electrode (SHE), and that: Ered = -0.059 V × 7 = -0.41 V (from Wikipedia).
CO2 + H2O+ 2e− → HCOO− + OH− −0.45, −0.50    
6CO2 + 24e−→ glucose −0.43    
Ferredoxin (Fe3+) + e− → Ferredoxin (Fe2+) −0.42   High energy electron acceptor and storage system. Used for ATP synthesis.
NADP+  → NADPH −0.32 (−0.37 pysiological cond.)   High energy electron acceptor and storage system. The ratio of NADP+
:NADPH is maintained at around 1:50. This allows NADPH to be used to reduce organic molecules (from Wikipedia).
NAD+ + H+ + 2e− → NADH −0.32 (−0.28 pysiological cond.)   High energy electron acceptor and storage system. The ratio of NAD+
:NADH is maintained at around 30:1. This allows NAD+
 to be used to oxidise organic molecules (from Wikipedia).
S + 2H+ + 2e−→ H2S −0.27   Rare in gut bacteria (e.g., Oscillibacter ruminantium).
CO2 + 8H+ + 8e− → CH4 + 2H2O
 −0.24 −15  
SO42− → HS− −0.22 −20  
Acetaldehyde + 2H+ + 2e− → ethanol −0.20    
Pyruvate− + 2H+ + 2e− → lactate2− −0.19    
FAD + 2H+ + 2e−→ FADH2 −0.18 to −0.22   Depending on the protein involved, the potential of the flavine can vary widely (from Wikipedia).
SO32− → HS− −0.17    
Oxaloacetate2− + 2H+ + 2e− → malate2− −0.17   While under standard conditions malate cannot reduce the more electronegative NAD+:NADH couple, in the cell the concentration of oxaloacetate is kept low enough that Malate dehydrogenase can reduce NAD+ to NADH during the citric acid cycle (from Wikipedia). 
Fumarate2− + 2H+ + 2e− → succinate2− 0.03 −86 (from glucose)  
2CO2 + 12H+ + 12e− → CH3CH2OH + 3H2O 0.08    
Cytochrome b (Fe3+) + e− → Cytochrome b (Fe2+) 0.08    
Ubiquinone + 2H+ + 2e− → UbiquinoneH2 0.10    
Cytochrome c (Fe3+) + e− → Cytochrome c (Fe2+) 0.25    
Cytochrome a (Fe3+) + e− → Cytochrome a (Fe2+) 0.29    
O2 + 2H+ + 2e− → H2O2 0.30   Formation of hydrogen peroxide from oxygen (from Wikipedia).
Cytochrome a3 (Fe3+) + e− → Cytochrome a3 (Fe2+) 0.35    
NO3− + 2H+ + 2e− → NO2− + H2O 0.42 −163 (from glucose)  
NO2− + 8H+ + 6e− → NH4+ + 2H2O 0.44    
2NO3− → N2 0.74 −206  
Fe3+ + e− → Fe2+ 0.77 −209 Ferric ion is a strong electron acceptor.
½ O2 + 2H+ + 2e− → H2O 0.82 −219, −237 (from glucose) Strong electron acceptor and high net energy release. In classical electrochemistry, E° for O2 = +1.23 V with respect to the standard hydrogen electrode (SHE). At pH = 7, Ered = 1.23 – 0.059 V × 7 = +0.82 V (from Wikipedia).
O2 + 4H+ + 4e− → H2O 0.82   Strong electron acceptor and high net energy release. In classical electrochemistry, E° for O2 = +1.23 V with respect to the standard hydrogen electrode (SHE). At pH = 7, Ered = 1.23 – 0.059 V × 7 = +0.82 V (from Wikipedia).
F2 + 2e− → 2F− 2.9   Extreme example of an exogenic reaction. Not biologically relevant; relative to 2H+ + 2e− ⇆ H2 being = 0 V.

 

Half reactions are named as such because they focus on either the reduction or oxidation reaction component of the redox reaction, but not both. Redox potential, measured in volts, quantifies how willingly a molecule gives up an electron (or picks one up). A negative value means energy is needed to drive the process, whereas a more positive value results in a release of energy, which means substrates with negative potential would prefer to be donors and those with positive potential, acceptors.

Of relevance to eukaryotic cells, and aerobic, facultatively aerobic and microaerobic bacterial cells is the use of oxygen as the terminal electron acceptor in a metabolic cascade. For example, if NADH transfers its electrons to oxygen, the overall equation will be: NADH + H+ + ½ O2 → NAD+ + H2O. The net difference in reduction potential is 1.14 volts for this single electron transfer, and double that for the complete reduction of an oxygen molecule. This amounts to a hefty release of free energy (ΔG°´ = -110 kJ/mol/NADH) which can be harvested incrementally by the transfer of electrons through a series of carriers (quinones, cytochromes, flavins). This electron transport chain restores between 2.5 to 3 molecules of ADP to the equivalent number of ATP molecules for each NADH consumed.

But using oxygen as an electron acceptor is of little consequence for the vast array of anaerobic bacteria and archaea in the gut for whom oxygen is potentially lethal. Instead, anaerobes must find other terminal electron acceptors that ideally rival oxygen in their reduction potential. See next section: Alternative inorganic terminal electron acceptors.

Tags: