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Dehydrogenases are enzymes that catalyze oxidation–reduction reactions by removing hydrogen from substrates and transferring electrons to redox cofactors such as NAD+, NADP+, or FAD. They belong to the oxidoreductases, EC class 1, and occur in many organisms from bacteria to humans.

Most dehydrogenases operate by hydride transfer, a two-electron transfer from the substrate to the cofactor. They are typically reversible, and in cells the direction of the reaction is governed by the relative concentrations of substrates and cofactors, notably the NAD+/NADH or NADP+/NADPH pools. Importantly, most dehydrogenases do not incorporate molecular oxygen into their substrates; that role is performed by oxidases or oxygenases.

Common examples include lactate dehydrogenase and malate dehydrogenase, which use NAD+/NADH in glycolysis and the tricarboxylic acid cycle, and glyceraldehyde-3-phosphate dehydrogenase, a key glycolytic enzyme. Alcohol dehydrogenases catalyze the oxidation of alcohols (often to aldehydes or ketones) using NAD+. Other dehydrogenases use FAD (such as some forms of succinate dehydrogenase) or NADP+ (as in several enzymes of the pentose phosphate pathway and fatty acid synthesis).

Many dehydrogenases are soluble cytosolic enzymes, while others are mitochondrial, chloroplast, or peroxisomal. Structurally, many bind dinucleotides via a Rossmann fold. Regulation occurs at the level of gene expression, allosteric control, and cofactor availability.

Physiological roles include energy metabolism, redox balance, and anabolic biosynthesis. Biotechnologically, dehydrogenases are used as biocatalysts in asymmetric synthesis and biosensors. Defects or imbalances in dehydrogenase activity can contribute to metabolic disorders and are studied in clinical biochemistry.