Describe what happens when nad+ is reduced. what enzyme is involved?

NAD+ is a co-enzyme which function as an electron carrier. When NAD+ is reduced it receive three electron from an electron donor. Two of the electrons are positive, one is negative. These three electrons neutralize NAD+ and reduce it to NADH. The enzyme that catalyses the reaction is dehydrogenase. 

Merlin in "the Sword Excalibur" placed Excalibur into an anvil atop a stone, saying that only the true King of Britain would have the power to pull it free. He advised Arthur how to officially become the King. It's a positive advice.

On the other hand, the pilgrim Shadow in "Eldorado" knew that the gallant knight was tired of the finding the land of Eldorado, he advised the knight to stop his journey. It's a negative advice.

Explanation: