What is the importance of NAD+/ NADH as a coenzyme and its role in metabolism?

Answer 1

NAD+ is a coenzyme which accepts electrons from a number of oxidation reactions. Some of these reactions are in glycolysis and in the citric acid cycle. When NAD+ gains a pair of electrons (and a proton) it is reduced to NADH. NADH, then, is able to be re-oxidized back to NAD+ by the electron transport system (ETS). The electrons are passed through the electron transport system, and are ultimately used to reduce oxygen to water. The movement of electrons through the ETS generates a gradient of protons across the inner membrane of the mitochondria which is used (in part) to drive the production of ATP.

So, NAD+/NADH form an electron shuttle which transfers electrons generated by catabolism to the ETS.