dehydrogenase

Meaning: A dehydrogenase catalyzes oxidation of a substrate by removing two hydrogens, usually as a hydride ion (H⁻) plus a proton. The electrons go to a coenzyme: NAD⁺ → NADH, NADP⁺ → NADPH, or FAD → FADH₂. The reaction is reversible, so the same enzyme can reduce a substrate when NADH is abundant. Lactate dehydrogenase does this in fermentation, turning pyruvate into lactate and regenerating NAD⁺ so glycolysis can continue.

On the MCAT: Expect to spot which steps of glycolysis, pyruvate oxidation and the citric acid cycle make NADH or FADH₂, such as glyceraldehyde-3-phosphate dehydrogenase, pyruvate dehydrogenase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, succinate dehydrogenase and malate dehydrogenase. Passages may ask what happens when one is inhibited or when NAD⁺ runs short: the pathway slows at that step and intermediates before it build up. Succinate dehydrogenase is also complex II, bound to the inner mitochondrial membrane, and uses FAD. Glucose-6-phosphate dehydrogenase makes NADPH in the pentose phosphate pathway, which supports biosynthesis and keeps glutathione reduced.

Common confusion: Students treat the coenzyme as the thing being oxidized. In these reactions the substrate is oxidized and NAD⁺ or FAD is reduced, so NAD⁺ is the oxidizing agent. Also keep dehydrogenase (removes hydrogen) separate from dehydratase (removes water).