Fig. 2: Nicotinate catabolic route in A. nidulans.

HxnP and HxnZ are transporters (represented by blue and green transmembrane domains, respectively) that transport the indicated compounds. HxnS hydroxylates nicotinic acid (NA) to 6-hydroxynicotinic acid (6-NA). HxnX operates in peroxisomes and converts 6-NA to 2,5-dihydroxypyridine (2,5-DP), which is subsequently hydroxylated by HxnV to 2,3,6-trihydroxypyridine (2,3,6-THP). HxnT and a yet-unknown alkene reductase (UE1) partially saturate the pyridine ring of 2,3,6-THP to (5S,6R)-( + )-dihydroxypiperidine-2-one (5,6-DHPip-2-O), which is then converted to 3-hydroxypiperidine-2,6-dione (3-HPip-2,6-DO) by HxnW, a NAD-dependent polyol dehydrogenase type enzyme. The ring of 3-HPip-2,6-DO is opened by the cyclic imidase HxnM between N-C2 resulting in (S)-( + )-α-hydroxyglutaramate (α-HGA) formation. The nitrogen is salvaged by HxnN amide hydrolase and results in α-hydroxyglutarate (α-HG) formation. This reaction can also be catalyzed by other amide hydrolases (UE2). NA can be formed endogenously by the hydrolytic cleavage of amide group of nicotinamide (NAA) by a non-HxnR regulated deamidase. Cellular components such as cell membrane, cytoplasm, and peroxisome are shown and indicated by pictograms. Reaction in the peroxisome pictogram indicates the spatial separation of the referred catabolic step in the peroxisomes. The compound in square brackets denotes a predicted intermediate that was not detected by the UHPLC-HRMS method but deduced from the structure of the identified upstream and downstream metabolites. The structure of the compound in the dashed square brackets was deduced by the exact m/z value and MS/MS fragmentation pattern of the compound obtained by UHPLC-HRMS (Supplementary Table 1), the UHPLC-HRMS and NMR confirmed structures of the upstream and downstream metabolites (Supplementary Tables 1, 2) in line with the proposed ketoreductase activity of the HxnW. UE: unidentified enzyme. PI: physiological metabolite inducer of the pathway-related hxn genes; Compound names in red lettering denote metabolites, which have never been detected before neither in eukaryotic nor in prokaryotic organisms. Compound names in blue lettering denote metabolites not detected in prokaryotic NA catabolic pathways. (Created with BioRender.com).