Nicotinic acid (niacin) is a precursor of NAD and NADP and thus an essential metabolite.
Its degradation has been previously described only in bacteria.
We describe, for the first time, a complete eukaryotic pathway of nico- tinic acid
(NA) catabolism. The genes are organised in three co-regu- lated gene clusters, which
encode eight enzymes, two transporters and the pathway specific transcription factor.
The pathway is conserved; and its variable organisation in the Pezizomycotina illustrates
cluster evolution and horizontal gene transfer.
Reverse genetics was coupled with state-of-the-art chemical charac- terisation of
each intermediate. The first step, the conversion of NA to 6-hydroxynicotinic acid
(6-NA) is common to prokaryotic and eukary- otic pathways, even if catalysed by independently
evolved enzymes. While two downstream metabolites, 2,5-dihydroxypyridine and 2,3,6-tri-
hydroxypyridine, are common to various prokaryotic routes, other steps and metabolites
are unprecedented: 3-hydroxypiperidine-2,6-dione and 5,6-dihydroxypiperidine-2-one
intermediate metabolites have not been identified previously in any organism, the
latter being a completely nov- el chemical compound. Furthermore, the hydrolytic N-C
ring opening results in α-hydroxyglutaramate, a compound not detected in analo- gous
prokaryotic pathways. Remarkably, the physiological inducer of the whole pathway is
a near-terminal intermediate metabolite: 5,6-di- hydroxypiperidine-2-one.
While most steps are cytosolic, two steps take place in the peroxi- somes. 6-NA monooxygenase
(HxnX) enters peroxisomes through a canonical PTS-1 signal, while HxnW, a polyol dehydrogenase,
is co-transported by piggybacking HxnX.
The genomic organisation and phylogeny of the pathway cognate genes and proteins,
showed that this catabolic pathway is of fungal (Ascomycota) origin and thus it exemplifies
convergent evolution of catabolic pathways between fungi and bacteria, where at least
four different pathways occur.
