oxidoreductase

Meaning: Oxidoreductases make up EC class 1, one of the six enzyme classes (oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases). They catalyze redox reactions, in which one substrate is oxidized (loses electrons) while another is reduced (gains them). Electrons often travel as hydride ions or hydrogen atoms on carriers such as NAD⁺, NADP⁺ and FAD, which become NADH, NADPH and FADH₂. Names give hints: dehydrogenase, reductase, oxidase, oxygenase, peroxidase.

On the MCAT: Expect to classify an enzyme from its name or reaction, then track the electrons. If NAD⁺ becomes NADH, the substrate was oxidized and the enzyme is an oxidoreductase. Examples include lactate dehydrogenase (pyruvate + NADH + H⁺ ⇌ lactate + NAD⁺), glyceraldehyde-3-phosphate dehydrogenase in glycolysis, and the complexes of the electron transport chain. Like any enzyme, it lowers activation energy and does not change ΔG or Keq.

Common confusion: Students sort enzymes by a single clue, such as ATP being involved, or assume a dehydrogenase only oxidizes. A redox reaction is a pair: when one substrate is oxidized, the other (often the cofactor) is reduced, and many of these enzymes can run in either direction depending on concentrations. A kinase transfers a phosphate group, so it is a transferase, not an oxidoreductase.