Definition for View source for Nicotinamide adenine dinucleotide

An important, ubiquitous redox cofactor that functions as a carrier of electron pairs. The oxidized form of the cofactor actually carries a positive charge, and is denoted NAD+ while the reduced form is NADH. The nicotinamide portion of NAD+, which consists of a carbamylated pyridine ring (in red in the figure below), acts as the electron pair acceptor. The 4-position of the ring (para to the nitrogen atom) in effect becomes substituted with a hydride ion. Note that the "dinucleotide" part of the name is due to the fact that the other "half" of NAD is an adenine-containing nucleotide, AMP. The two "halves" of the molecule are linked by a phosphoanhydride bond.	 
	 	
A closely related cofactor, NADP+, (reduced form, NADPH) differs only in having a phosphate attached to the 2′ position of the ribose attached to adenine. NADPH generally functions in redox reactions in biosynthetic pathways (e.g. fatty acid synthesis) , whereas NAD+ predominates in catabolic processes, such as those associated with glycolysis and oxidative phosphorylation.

http://guweb2.gonzaga.edu/faculty/cronk/biochem/images/NAD.gif 

The NAD+/NADH redox half-reaction (see figure below) has a standard biochemical reduction potential, ΔE°′ of −0.315 V. The progress of reactions involving NAD+/NADH can be conveniently monitored spectrophometrically due to the appearance of a broad absorption with its peak at 340 nm when NADH is formed.	

http://guweb2.gonzaga.edu/faculty/cronk/biochem/images/NAD-redox.gif

The major source of NADH in oxidative metabolism is the citric acid cycle. The NADH produced is reoxidized to NAD+ when the former donates its electrons to the first component of the electron transport chain. Eventually these electrons reduce molecular oxygen to water.