NADH

NADH stands for the reduced form of nicotinamide adenine dinucleotide, a dinucleotide coenzyme that functions as an essential electron carrier in cellular metabolism. In its reduced state, NADH carries two electrons and one proton as a hydride ion, while NAD+ can accept a hydride to become NADH.

NADH is produced in several key metabolic pathways. In glycolysis, glyceraldehyde-3-phosphate dehydrogenase generates NADH. In the pyruvate dehydrogenase complex, NADH is produced as pyruvate is converted to acetyl-CoA. In the citric acid cycle, NADH is formed during reactions catalyzed by isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase. NADH then donates electrons to the mitochondrial electron transport chain, specifically to Complex I (NADH: ubiquinone oxidoreductase), which drives proton pumping and ATP synthesis through oxidative phosphorylation. Cells also use shuttle systems, such as the malate–aspartate shuttle and the glycerol-3-phosphate shuttle, to transfer cytosolic NADH reducing equivalents into mitochondria.

In anaerobic conditions, NADH must be recycled to NAD+ to sustain glycolysis. This occurs when lactate dehydrogenase converts pyruvate to lactate, regenerating NAD+, or when alcohol dehydrogenase converts acetaldehyde to ethanol in yeast, also regenerating NAD+.

NADH is often contrasted with NADPH. NADH primarily acts as an energy carrier, transferring electrons for ATP production, whereas NADPH provides a reducing power for biosynthetic reactions and antioxidant defense. The cellular NAD+/NADH ratio reflects the redox state and influences numerous metabolic pathways. NADH is a dinucleotide coenzyme composed of nicotinamide mononucleotide and adenine dinucleotide units, with redox cycling involving the nicotinamide ring.