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In our curriculum biology textbook the reduction of NAD+ is depicted as follows:

NAD+ + 2 H+NADH + H+

If this proton in the products was not present in the reactants, then where does it come from? Alternatively, if it does not take part in the reaction, then why is it depicted?

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    $\begingroup$ Can you reference the book? It's a bit weird, usually, NAD reduction is written simply like NAD+ + H+ >> NADH, you don't need the extra proton. Having some more info from the book paragraph you have found this reaction may help. $\endgroup$ – alec_djinn Mar 7 at 9:37
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In a very simple way the equation describes the reaction. NAD is reduced using 2 hydrogen atoms. The two hydrogen atoms can come from one reactant (lactic acid fermentation) or from two (in a reaction of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate; as shown in the figure below).


First of all, I will pick a particular example from glycolysis, catalyzed by glyceraldehyde-3-phosphate dehydrogenase.


enter image description here

Phosphate by its own has one hydrogen at cytosolic pH (because of the pKA). Thus in this reaction, there are two hydrogen atoms. One hydrogen atom is in the form of a hydride (a proton and two electrons) from the aldehyde group. This hydrogen atom is used to reduce NAD+ to NADH. A second hydrogen atom (in the form of a proton) comes from the phosphate. In the bisphosphoglycerate form, the proton dissociates from the phosphate group and is in solution.

The eventual fate of the electrons that were used to reduce NAD+ to NADH is the electron transport chain (in the mitochondria). In some cases, though, for example muscle cells in an anaerobic environment, or other species that don't have an electron transport chain, the electrons are used in fermentation of pyruvate to alcohol or lactic acid. When NADH donates its electrons in either case, it is then oxidized back to NAD+.


References:

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    $\begingroup$ Hi LDiago. I've edited your post to make it a little more readable, and added a little more explanation in a few parts. I hope you find it helpful! Please feel free to rollback if I've inadvertently changed the meaning, or you disagree with me on style. $\endgroup$ – De Novo supports GoFundMonica Mar 13 at 5:55
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A quick search on Wikipedia showed the same reaction you book is presenting.

In metabolism, the compound accepts or donates electrons in redox reactions.[2] Such reactions (summarized in the formula below) involve the removal of two hydrogen atoms from the reactant (R), in the form of a hydride ion (H), and a proton (H+). The proton is released into solution, while the reductant RH2 is oxidized and NAD+ reduced to NADH by transfer of the hydride to the nicotinamide ring.

RH2 + NAD+ → NADH + H+ + R

This should answer your question.

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