NAD/NADH reduction potential

[Jasmin Bauer]

Dear eQuilibrator User support forum,

I was comparing literature values of dG'0 and dG' (doi.org/10.1038/nrmicro3365 ; Box 1) with eQuilibrator values.

I found that dG'0 values were differing.

While trying to find a reason for this, I saw that the E'm [NAD/NADH] in the literature was -280 mV and in eQuilibrator probably -330 mV (see https://equilibrator.weizmann.ac.il/static/classic_rxns/classic_reactions/glycolysis.html#nadh-as-an-electron-carrier).

Now, I wonder why there are different E'm values for [NAD/NADH]. I did not find any publication where the E'm value was truly measured. Only wikipedia says -280 mV as well but I don't know from where they got that information.

So I was hoping to get an answer here.

Thanks in advance and kind regards,

Jasmin

[Elad Noor]

Hi Jasmin,

First of all, I am not sure what you mean by literature values for ΔG', as they are concentration dependent and therefore not a physical constant.

Even for the standard Gibbs energy (ΔG'0) there can be significant differences depending on conditions (pH, ionic strength, temperature, etc.), which could explain the different numbers you find from various sources.

Regardless, the only value I found on Wikipedia is -320mV (from here):
>  The midpoint potential of the NAD+/NADH redox pair is −0.32 volts, which makes NADH a moderately strong reducing agent.

The value in eQuilibrator is derived mainly from Robert Alberty's handbook (Thermodynamics of Biochemical Reactions - see attachment). At pH 7, the difference between the formation energies

of NAD+ (nadox) and NADH (nadred) is ~61 kJ/mol, which translates to about -320 mV (using the equation ΔG' = n*F*E' with n=2).

Indeed, there is still a some gap left between -320 and -330, but that can be attributed to the more holistic approach taken in eQuilibrator, which tries to

compile together many sources of Keq measurements.

Kind regards,

Elad

[Jasmin Bauer]

Thanks for the fast response Elad.

Those values from Alberty are under standard conditions.

The standard E'0 for [NAD/NADH] is −0.320 mV, but I was referring to the physiological E'm where I found the value  -0.280 mV (here my wikipedia page: https://en.wikipedia.org/wiki/Table_of_standard_reduction_potentials_for_half-reactions_important_in_biochemistry   ; sorry for not providing it in my first post). And this -0.280 mV value was also used in the paper I posted.

And in eQuilibrator the E'm seems to be -330 mV.

Besides this, I noticed that I mixed up standard Gibbs energy (ΔG'0) and physiological Gibbs energy (ΔG'm) in the paper so yes you are probably right that different numbers come from this mixup. 

Kind regards,

Jasmin

[Elad Noor]

Hi,

Thanks for the reference, I didn't know about it.

I think there might be a slight confusion between two types of physiological definitions. E'm is the potential where all concentrations in solution are 1 mM rather than 1M. It is often more "physiological" than E'°, but not the same as what you have in the wikipedia entry, where they used an estimate for the NAD/NADH ratio. This is necessarily context dependent (I think it comes from a type of acetogen, according to the citation provided).

Note that for all reactions and half-reactions that have the same number of substrates and products (as is the case here), then E'm = E'°.