# NADH in Equilibrium constant

1. Jun 24, 2010

### soopo

What is the equilibrium constant of the reaction?

$$NO_{3}^{-} + NADH + H^{+} <=> NO_{2}^{-} + NAD+ + H_{2}O$$

I wrote in an exam that the [NADH] and [H2O] remain constant in the reaction.
So my answer is

$$K = \frac {[NO_{2}^{-}] [NAD^{+}]} {[NO_{3}^{-}] [H^{+}]}$$
which is wrong.

The right answer is

$$K = \frac {[NO_{2}^{-}] [NAD^{+}]} {[NO_{3}^{-}] [H^{+}] [NADH]}$$

So why is [NADH] not constant in the reaction?
NADH is apparently aques.
It is gains electrons in the reaction. It is probably not in excess so it is concentration is changing.

Is there any case where [NADH] is constant?

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