Multifunctional Enzymes as Targets for the Treatment of Tuberculosis: Paving the Way for New Anti-TB Drugs

In spite of the medical and technological developments of the last centuries, Tuberculosis (TB) has remained a challenging disease, with a limited number of therapeutic options, particularly in light of the increase in drug-resistant cases. The search for new molecules continues, with several candidates currently in clinical testing and ongoing efforts to identify novel targets. This work summarizes recent developments on anti-TB therapy, starting by discussing the current epidemiologic status and presenting an overview of the history of anti-tuberculosis drug discovery. Special attention is dedicated to five multifunctional enzymes that are regarded as promising targets for new anti-TB drugs: 5-aminoimidazole4-carboxamide ribonucleotide transformylase/IMP cyclohydrolase (ATIC); 3,4-dihydroxy-2-butanone 4-phosphate synthase (DHBPS)/GTP cyclohydrolase II (GCHII); glutamine dependent NAD+ Synthetase (NadE); chorismate synthase (CS); and Tryptophan synthase (TS). These enzymes are involved in metabolic pathways critical for the M. tuberculosis survival, growth or replication, but that are not expressed in humans or have significant differences in terms of functionality, which makes them appealing targets. Their function, structure, possible catalytic mechanisms and current inhibition strategies and inhibitors are reviewed and discussed.

NAD+ biosynthesis in bacteria is controlled by global carbon/nitrogen levels via PII signaling

NAD+ is a central metabolite participating in core metabolic redox reactions. The prokaryotic NAD synthetase enzyme NadE catalyzes the last step of NAD+ biosynthesis, converting nicotinic acid adenine dinucleotide (NaAD) to NAD+. Some members of the NadE family use l-glutamine as a nitrogen donor and are named NadEGln. Previous gene neighborhood analysis has indicated that the bacterial nadE gene is frequently clustered with the gene encoding the regulatory signal transduction protein PII, suggesting a functional relationship between these proteins in response to the nutritional status and the carbon/nitrogen ratio of the bacterial cell. Here, using affinity chromatography, bioinformatics analyses, NAD synthetase activity, and biolayer interferometry assays, we show that PII and NadEGln physically interact in vitro, that this complex relieves NadEGln negative feedback inhibition by NAD+. This mechanism is conserved in distantly related bacteria. Of note, the PII protein allosteric effector and cellular nitrogen level indicator 2-oxoglutarate (2-OG) inhibited the formation of the PII-NadEGln complex within a physiological range. These results indicate an interplay between the levels of ATP, ADP, 2-OG, PII-sensed glutamine, and NAD+, representing a metabolic hub that may balance the levels of core nitrogen and carbon metabolites. Our findings support the notion that PII proteins act as a dissociable regulatory subunit of NadEGln, thereby enabling the control of NAD+ biosynthesis according to the nutritional status of the bacterial cell.

Cloning, expression, purification, crystallization and preliminary X-ray diffraction studies of NAD synthetase from methicillin-resistantStaphylococcus aureus

Staphylococcus aureusis an important human and animal pathogen that causes a wide range of infections. The prevalence of multidrug-resistantS. aureusstrains in both hospital and community settings makes it imperative to characterize new drug targets to combatS. aureusinfections. In this context, enzymes involved in NAD metabolism and synthesis are significant drug targets as NAD is a central player in several cellular processes. NAD synthetase catalyzes the last step in the biosynthesis of nicotinamide adenine dinucleotide, making it a crucial intermediate enzyme linked to the biosynthesis of several amino acids, purine and pyrimidine nucleotides, coenzymes and antibiotics.