Other Double Bond-Cleaving Dioxygenases

There is another class of enzymes which may resemble mechanistically the extradiol dioxygenases. None of their substrates however can form quinones. They also show diversity in their co-factors and include cupro-and ferro-proteins as well as two heme-containing enzymes. 

---

Degradation of L-tryptophan in most organisms proceeds via L-kynurenine, 3-hydroxy-L-kynurenine, 3-hydroxyanthranilic acid and quinolinic acid to acetyl Co A and CO2 (Fig. 244). Anthranilic acid formed as an intermediate may be recycled to L-tryptophan (see above). The ring of 3-hydroxyanthranilic acid is cleaved by a dioxygenase (C 2.5). The x-amino-/3-carboxymuconic acid-e-semialdehyde formed either undergoes a cis trans isomerization of the Zl -double bond and cyclization to quinolinic acid, a compound synthesized in microorganisms and plants from aspartic acid and D-glyceraldehyde-3-phosphate (D 16.2). On the other hand o -amino-/3-carboxymuconic acid-e-aldehyde may be de-carboxylated and is then the immediate precursor of NH3, acetic acid and COg. 

---