The kynurenine pathway of tryptophan metabolism in microorganisms.

In this chapter, the kynurenine pathway in bacteria, archaea, unicellular fungi and protists is reviewed. The kynurenine pathway is the major catabolic pathway for L-tryptophan metabolism in prokaryotes and eukaryotes. There are two different kynurenine pathways, sometimes called 'inducible' and 'constitutive', in microorganisms. The former of these pathways results in the 'complete' catabolism of L-tryptophan, while the latter leads to the biosynthesis of NAD(P). The enzymes in the first pathway are tryptophan dioxygenase, kynurenine formamidase and kynureninase. The second pathway requires two additional enzymes, kynurenine 3-monooxygenase and 3-hydroxyanthranilate dioxygenase, to produce quinolinate and thereby lead to the NAD(P)+ biosynthetic pathway. Catabolism via the latter pathway requires three further enzymes, 2-amino-3-carboxymuconate semialdehyde decarboxylase, 2-aminomuconate semialdehyde dehydrogenase and 2-aminomuconate deaminase. In addition, in some bacteria, metabolites of the kynurenine pathway are used for the biosynthesis of secondary metabolites.