Abstract
Abstract 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.
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