NAD: oxidized and reduced forms

NAD oxidized form

As NAD is reduced, one electron is added at the Nitrogen atom (removing the + charge), and one (electron + proton = H atom) is added at the upper position of the nicotinamide ring. NAD reduced form

NAD reduced form

NAD in the oxidized form contains the elements of ADP, with an additional ribose molecule and a nicotinamide ring. The latter is the critical element in redox reactions. When a pair of H atoms (2 protons + 2 electrons) are removed from organic substrates in an oxidation reaction, NAD⁺ accepts 2 electrons and 1 proton; the remaining proton is released as free H⁺ ion.