enzymes that catalyze the removal of hydrogen from organic compounds. The coenzymes of dehydrogenases are usually dinucleotides: nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP), or flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), which accept hydrogen from the oxidizable substance. Dehydrogenases bring about the first stage of biological oxidation and play a large part in the Krebs cycle, glycolysis, and the pentose phosphate cycle. In the organism the NADP, reduced through the action of certain dehydrogenases, is used to synthesize fatty acids. Some dehydrogenases, unconnected with coenzymes, catalyze the direct oxidation of substances by oxygen. Many dehydrogenases contain metals (zinc or manganese) that are part of their active centers.