These comprises of coenzymes involved in hydrogen transfer reactions and form essential components of dehydrogenases taking part in fermentation and glycolysis. The coenzymes are known as nicotinamide adenine dinucleotide (NAD+), formerly known as coenzymes I isolated by H.von Euler in 1931, and nicotinamide and nicotinamide adenine dinucleotide phosphate (NADP+) also known as coenzyme II discovered by Warburg and Christian. Collectively these coenzymes are known as pyridine nucleotides.

Pyridine nucleotides are derived from vitamin B, nicotinic acid or niacin which is widely distributed in plant and animal tissues. The amide of nicotinic acid, called nocotinamide is the most active from present in attached to the ribose molecule through glycosidic linkage while the phosphate group provides a link between adenosine and nicotinamide riboside. In NADP, there is an additional phosphate group in C-2 position of the ribose molecule of adenosine component, hence it is called nicotinamide dinucleotide phosphate.
Biochemical function   The physiological functions and the behavior of these coenzymes as vitamins has been known for a long time. The pyridine ring of the coenzymes is positively charged and undergoes a change during oxidation reduction reactions the hydrogen can reversibly bind with the pyridine ring. The uptake of hydrogen causes reductions retaining two double bonds, accompanied by a loss of positive charge on the nitrogen.
These pyridine nucleotides, NAD⁺ and NADP and coenzymes of a number of dehydrogenases catalyzing oxidation reduction reactions. They have been sometimes called cosubstrates since the enzymes of which they form a prosthetic component are substrate specific. All reaction catalyzed by them are reversible.