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Nicotinamide adenine dinucleotide

Nicotinamide adenine dinucleotide (NAD⁺) is an important coenzyme found in cells. It plays key roles as carriers of electrons in the transfer of reduction potential.

NADH is the reduced form of NAD⁺, and NAD⁺ is the oxidized form of NADH.

Synthesis

Cells synthesize NAD⁺ from a form of vitamin B₃, (niacin) called nicotinamide. Nicotinamide is joined with Ribose and ADP to form NAD⁺. From NAD⁺, addition of a phosphate group to the 2' position of the adenyl nucleotide through an ester linkage forms NADP⁺.

NAD⁺ and NADH

Cells produce NAD⁺ from niacin, and use it to transport electrons in redox reactions. During this process NAD⁺ picks up a pair of electrons and a proton and is thus reduced to NADH, releasing one proton (H⁺).

MH₂ + NAD⁺ → NADH + H⁺ + M: + energy, where M is a metabolite.

Two hydrogen atoms (a hydride ion and a proton H⁺) are removed from the metabolite. The proton is released into solution. From the hydride electron pair, one electron is transferred to the positively-charged nitrogen, and one hydrogen attaches to the carbon atom opposite to the nitrogen.

The reducing potential stored in NADH can be converted to ATP through the aerobic electron transport chain or used for anabolic metabolism. ATP is the universal energy currency of cells, and the contribution of NADH to the synthesis of ATP under aerobic conditions is substantial. However, under certain conditions (e.g. hypoxia) the aerobic regeneration of oxidized NAD⁺ is unable to meet the cell's immediate demand for ATP. In contrast, glycolysis does not require oxygen, but it does require the anaerobic regeneration of NAD⁺. The oxidation of NADH to NAD⁺ in the absence of oxygen is called fermentation.

<center> <gallery> Image:NAD-3D-balls.png|<center>Ball-and-stick model of NAD⁺</center> Image:NADH-3D-balls.png|<center>Ball-and-stick model of NADH</center> Image:NADH-3D-vdW.png|<center>Space-filling model of NADH</center> </gallery> </center>

Relationship to NADPH

Though differing by only a single phosphate group, NAD⁺ and NADPH have distinct functions in biochemistry:

• NAD⁺ is used extensively in glycolysis and the citric acid cycle of cellular respiration.
• NADPH is generated by the pentose phosphate pathway and required for the biosynthesis of nucleic acids and fatty acids.

External links

• NAD and NADP
• Nicotinamide

Contents

• Synthesis
• NAD⁺ and NADH
• Relationship to NADPH
• External links