NADPH

NADPH, or nicotinamide adenine dinucleotide phosphate in the reduced form, is a coenzyme that donates electrons for reductive biosynthesis and antioxidant defense. It serves as the reducing equivalent in many anabolic pathways and helps maintain the cellular redox balance. It is distinct from NADH, which is primarily involved in energy production.

The primary cellular source of NADPH is the oxidative branch of the pentose phosphate pathway, with glucose-6-phosphate dehydrogenase as a key enzyme. Other NADPH-generating enzymes include malic enzyme and cytosolic isocitrate dehydrogenase; in photosynthetic organisms, chloroplasts generate NADPH during light reactions via ferredoxin-NADP+ reductase.

NADPH provides the reducing power for fatty acid, cholesterol, and nucleotide biosynthesis, among other anabolic processes. It also fuels antioxidant systems, notably glutathione reductase and thioredoxin reductase, helping to keep cellular components in a reduced state. Additionally, NADPH supplies electrons to NADPH oxidase in phagocytes, contributing to the respiratory burst that generates reactive oxygen species to kill pathogens, and it serves as a cofactor for various detoxifying enzymes such as cytochrome P450 reductase.

Clinical relevance includes disorders that diminish NADPH production or utilization. Glucose-6-phosphate dehydrogenase deficiency reduces NADPH availability in red blood cells, compromising redox defense and predisposing individuals to oxidative hemolysis.

In plants and other photosynthetic organisms, light reactions drive the production of NADPH, which is then used in carbon fixation and assimilation processes, linking energy capture to biosynthesis.