Flavoproteins: Flavoproteins are yellow proteins that serve as enzymes critical to the ability of cells in the body to respire, to breath or, to use oxygen. (The "flavo-" is borrowed from the Latin "flavus", yellow).
Flavoproteins are, in other words, respiratory enzymes. As enzymes they catalyze (speed) what are technically termed oxidation-reduction reactions.
The first yellow enzyme was discovered by the German biochemist Otto Heinrich Warburg (1883-1970), a pioneer in research on the respiration of cells, who won the Nobel Prize in physiology or medicine in 1931 "for his discovery of the nature and mode of action of the respiratory enzyme."