Nicotinamide adenine dinucleotide phosphate

Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a coenzyme used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent.

NADPH is the reduced form of NADP. NADP differs from NAD in the presence of an additional phosphate group on the 2' position of the ribose ring that carries the adenine moiety.

In plants
In photosynthetic organisms, NADPH is produced by ferredoxin-NADP reductase in the last step of the electron chain of the light reactions of photosynthesis. It is used as reducing power for the biosynthetic reactions in the Calvin cycle to assimilate carbon dioxide.

Synthesis
The oxidative phase of the pentose phosphate pathway is a major source of NADPH in cells, and in cells without mitochondria (such as red blood cells), it is the only source.

However there are several other lesser-known mechanisms of generating NADPH, all of which depend on the presence of mitochondria. The key enzymes in these processes are: NADP-linked malic enzyme, NADP-linked isocitrate dehydrogenase, and nicotinamide nucleotide transhydrogenase. The isocitrate dehydrogenase mechanism appears to be the major source of NADPH in fat and possibly also liver cells. Also in mitochondria, NADH kinase produces NADPH and ADP using NADH and ATP as substrate.

Biological functions
NADPH provides the reducing equivalents for biosynthetic reactions and the oxidation-reduction involved in protecting against the toxicity of ROS (reactive oxygen species), allowing the regeneration of GSH (reduced glutathione). NADPH is also used for anabolic pathways, such as lipid synthesis, cholesterol synthesis, and fatty acid chain elongation.

The NADPH system is also responsible for generating free radicals in immune cells. These radicals are used to destroy pathogens in a process termed the respiratory burst. It is the source of reducing equivalents for cytochrome P450 hydroxylation of aromatic compounds, steroids, alcohols, and drugs.