Quinolinate synthetase, an iron-sulfur enzyme in NAD biosynthesis.

Nicotinamide adenine dinucleotide (NAD) plays a crucial role as a cofactor in numerous essential redox biological reactions. NAD derives from quinolinic acid which is synthesized in Escherichia coli from L-aspartate and dihydroxyacetone phosphate (DHAP) as the result of the concerted action of two enzymes, L-aspartate oxidase (NadB) and quinolinate synthetase (NadA). We report here the characterization of NadA protein from E. coli. When anaerobically purified, the isolated soluble protein contains 3-3.5 iron and 3-3.5 sulfide/polypeptide chain. Mössbauer spectra of the 57Fe-protein revealed that the majority of the iron is in the form of a (4Fe-4S)2+ cluster. An enzymatic assay for quinolinate synthetase activity was set up and allowed to demonstrate that the cluster is absolutely required for NadA activity. Exposure to air leads to degradation of the cluster and inactivate enzyme.