Dual activity of quinolinate synthase: triose phosphate isomerase and dehydration activities play together to form quinolinate.

Quinolinate synthase (NadA) is an Fe4S4 cluster-containing dehydrating enzyme involved in the synthesis of quinolinic acid (QA), the universal precursor of the essential coenzyme nicotinamide adenine dinucleotide. The reaction catalyzed by NadA is not well understood, and two mechanisms have been proposed in the literature that differ in the nature of the molecule (DHAP or G-3P) that condenses with iminoaspartate (IA) to form QA. In this article, using biochemical approaches, we demonstrate that DHAP is the triose that condenses with IA to form QA. The capacity of NadA to use G-3P is due to its previously unknown triose phosphate isomerase activity.