Different ways to transport ammonia in human and Mycobacterium tuberculosis NAD+ synthetases

Abstract

NAD⁺ synthetase is an essential enzyme of de novo and recycling pathways of NAD⁺ biosynthesis in Mycobacterium tuberculosis but not in humans. This bifunctional enzyme couples the NAD⁺ synthetase and glutaminase activities through an ammonia tunnel but free ammonia is also a substrate. Here we show that the Homo sapiens NAD⁺ synthetase (hsNadE) lacks substrate specificity for glutamine over ammonia and displays a modest activation of the glutaminase domain compared to tbNadE. We report the crystal structures of hsNadE and NAD⁺ synthetase from M. tuberculosis (tbNadE) with synthetase intermediate analogues. Based on the observed exclusive arrangements of the domains and of the intra- or inter-subunit tunnels we propose a model for the inter-domain communication mechanism for the regulation of glutamine-dependent activity and NH₃ transport. The structural and mechanistic comparison herein reported between hsNadE and tbNadE provides also a starting point for future efforts in the development of anti-TB drugs.

M. tuberculosis NAD⁺ synthetase (tbNadE) is of interest as a drug target. Here the authors present the actively trapped Homo sapiens NAD⁺ synthetase (hsNadE) and tbNadE structures and show key differences in the synthetase active site and in structural elements possibly involved in the allosteric regulation of catalysis to be leveraged for the development of M. tuberculosis selective inhibitors.