Yellow enzyme, Warburg's: A key respiratory enzyme discovered by the German biochemist Otto Heinrich Warburg (1883-1970), a pioneer in research on the respiration of cells and the metabolism of tumors. Warburg's yellow enzyme is a flavoprotein that catalyzes an oxidation-reduction reaction necessary for normal breathing (respiration) of cells.

Warburg won the Nobel Prize in physiology or medicine in 1931 "for his discovery of the nature and mode of action of the respiratory enzyme."