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I've been reading a lot about the oxidative dissimilation etc, and often I see different sources use NADH and NADH2 in the same reactions. One source uses NADH and another uses NADH2 in the exact same way. Is there a difference or is it just a different name for the same substance?

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They are often used interchangeably to indicate the reduced form of $\ce{NAD+}$. The overall reaction when oxidizing some molecule $\ce{RH2}$ is: $\ce{RH2 + NAD+ -> NADH + H+ + R}$. The proper reduced $\ce{NAD+}$ is $\ce{NADH}$ (it accepts two electrons and one proton), but sometimes $\ce{NADH2}$ is used to account for that second hydrogen that gets removed from the substrate being oxidized. The notation $\ce{NADH2}$ doesn't really take into account the fact that the second hydrogen is charged, and not bound to the $\ce{NAD}$ in the same way that the first hydrogen is, so it is confusing. The notation: "$\ce{NADH + H+}$" is more correct and is also sometimes used.

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