How is this H+ ion bonded when NAD becomes NADH +H+ ?

[John421]

First of all: I keep finding two equations that are supposedly showing the same reaction:
1) NAD⁺ +2e +H⁺ -> NADH
2) NAD⁺ +2e +2H⁺ -> NADH +H⁺

The first one suggests NAD carries around 2 electrons and one hydrogen ion
The second one suggests NAD carries around 2 electrons and 2 hydrogen ions

I imagine that the second equation shows an extra H⁺ ion (when compared to the first one) because for some reason a H+ ion follows the NADH around. Is it because a H+ ion forms a co-dative bond with the NADH (allowing it to 'carry' around 2 H+ ions?).

If the second H+ doesn't form a codative bond with the NADH, then what kind of bond does it form that allows it to follow the NADH around instead of just diffusing away?

 

[Borek]

The latter contains an additional H⁺ on both sides. You can freely remove it and get the first equation, as technically in the overall reaction H⁺ is just a spectator.

 

[John421]

In the context of cellular respiration though - I think the H+ ion follows the NADH around when the NADH moves physically. I was wondering what type of bond this would be called.

 

[Borek]

I think the H+ ion follows the NADH around when the NADH moves physically.

Chemically it doesn't make much sense. NADH can be protonated (it has several positions which can accept a proton), but it doesn't have to be "followed" by H⁺ - they are present anywhere in the solution. To make things worse in physiological pH NAD is actually NAD^-, we just ignore the real charge.

I bet these two equations (from the first post) stem from the fact there are different conventions used to symbolize the molecule present in the solution. Neither of them refers to the real entity (of a real charge/composition) and these equations try to put emphasis on some different aspects of the NAD behavior in the solution. But just because these conventions ignore the chemistry behind doesn't mean this real chemistry doesn't exist. Sadly, this is something often forgotten.

 

[epenguin]

I cannot remember ever having seen the second, though it is also quite possible that if it had been in front of my eyes I would still have seen the first! What I have seen is the representation of the couple as NAD and NADH₂, in fact that used to be more usual.

Edit: Or (I remembered this morning) maybe I haven't. Before they were called NAD⁺ (nicotinamide adenine dinucleotide) and NADH they were called by the (even) less transparent names DPN (diphosphopyridine nucleotide) and DPNH₂. NADP⁺ and NADPH were TPN (triphosphopyridine nucleotide) and TPNH₂. Before that they were also called Coenzyme I and coenzyme II. You will find these terms in old textbooks and papers. I think around the early sixties one of these International Union terminology bodies decreed the present terminology.