[Penguinone]

Question: Compound I (see picture below) is stable for many hours at 30°C. However, when incubated with lactate dehydrogenase (or alanine dehydrogenase and ammonia), the absorbance peak at A³⁴⁰ disappears. No such change is observed if alcohol dehydrogenase is present. Explain these findings.

Attempt: Compound I is stable in phosphate buffer for many hours at 30°C because the phosphodiester bond is kinetically stable and this low temperature. When compound I is incubated with LDH it binds covalently to the lactate dehydrogenase active site so that the absorbance peak at 340nm is lost. When alcohol dehydrogenase is present compound I non-covalently binds to it so the absorbance peak remains.

I am really struggling with this topic so any help will be greatly appreciated. 

[Babcock_Hall]

I think your charge is in the wrong place; it should be on each of the two phosphoryl groups (I agree that the linkage is inert).  I will ask you to re-check the structure of the alkyl group on the pyridine ring to make sure that you have drawn it correctly.  You must show your attempt to answer a question before we can help, according to forum rules.  Is what you have written after the word your attempt, or is it additional information that the problem gave you?  I will give you a small question to get you started.  What do you know about NADH and NAD as substrates or products of oxidoreductase enzymes, and what do you know about their spectral properties?

[Penguinone]

Thank you for your reply.
The structure is the drawn out as it was shown in the question sheet I was given. What is written after the word attempt is my attempt and not additional information given in the question. (There is, however, a mistake in my attempt when I say 'and this low temperature' I mean 'at this low temperature'.)
I know that oxidoreductase enzymes usually use NAD as a cofactor. I know NAD is an oxidizing agent and that NADH is a reducing agent. RH₂ + NAD⁺ → NADH + H⁺ + R. I also know that NAD and NADH strongly absorb ultraviolet light because of adenine. NADH absorbs at higher wavelengths, with a second peak in UC absorption at 339nm. Also when NAD is bound in the active site of an oxidoreductase, the nicotinamide ring of the coenzyme is positioned so that it can accept hydride from the other substrate.

[Babcock_Hall]

Try writing out the reaction catalyzed by lactate dehydrogenase, and see if it helps.