LAUSR

Abstract The [4Fe-4S]-containing enzyme quinolinate synthase, also called NadA, catalyzes the synthesis of quinolinic acid (QA) using what is probably the oldest pathway to generate this universal precursor of the biological cofactor nicotinamide dinucleotide (NAD). QA synthesis involves the condensation of dihydroxyacetone phosphate (DHAP) and iminoaspartate (IA), dephosphorylation, isomerization, cyclization and two dehydration steps. This remarkable series of reactions take place in a narrow active site defined by the convergence of the three homologous domains of NadA. The catalytically essential [4Fe-4S] cluster is found at one end of this site and is connected to the protein surface through a tunnel that can be open or closed depending on the nature (or absence) of the bound ligand. Crystal structures and X-ray diffraction data are available from the Protein Data Bank for complexes of NadA with inhibitors, substrate analogs, at least one substrate (DHAP), product and potential intermediates of QA synthesis. In this review, we have used this information to propose a coherent and comprehensive view of NadA catalysis from a structural viewpoint.