Stereospecificity of the Dihydroorotate‐Dehydrogenase Reaction

Dihydroorotate dehydrogenase from Zymobacterium oroticum is shown to catalyse the anti‐elimination of hydrogen from the substrate. In the presence of a catalytic amount of NAD+ the enzyme catalyses the exchange between the abstractable hydrogen atoms of dihydroorotate and solvent protons, the exchange of (5S)‐H atom being twice as fast as that of the 4‐H atom. Sodium‐ethoxide‐catalysed exchange affects both diastereotopic protons in the methylene group of dihydroorotate, the (5S)‐H atom being exchanged somewhat faster than the (5R)‐H atom. These findings are discussed in terms of stereospecificity and mechanism of flavin dependent dehydrogenase reactions.