In silico studies on nicotinamide analogs as competitive inhibitors of nicotinamidase in methicillin-resistant Staphylococcus aureus.

Nicotinamidase/PncA is a member of the hydrolase enzyme family, catalyzing the de-amidation of nicotinamide (NM) to nicotinic acid (NA) via salvage pathway. Products are fed into Preiss-Handler pathway for NAD⁺ biosynthesis which is an important enzyme cofactor and crucial for redox balance in microorganisms. Pathogens like methicillin-resistant Staphylococcus aureus (MRSA) are NAD⁺ auxotroph and rely on their host environment for NAD⁺ precursors to synthesize NAD⁺. Mutations in nicotinamidase/PncA have been reported to be associated with resistance to pyrazinamide (PZA), a front-line anti-tubercular drug, underlying its importance as an important link in NAD⁺ biosynthesis network in pathogenic organisms such as MRSA. The conserved features of PncA and essentiality of salvage route in MRSA and the absence of this enzyme in humans and other eukaryotes are attractive options to explore therapeutics against this target. In this work, we have screened novel substrate analogs from the PubChem database using virtual screening approaches employing fingerprint tanimoto-based 2D similarity search against Staphylococcus aureus PncA (SaPncA). Identified compounds were further assessed using molecular dynamics simulations to investigate conformational stability and structural integrity. We propose two analogs, namely L28 and L33 with greater stability, favorable binding and strong binding free energies in MM-PBSA calculations. The strategy could provide an important clue in developing similar compound scaffolds as potent drug-like molecules against MRSA and other pathogenic species harboring this enzyme. Smaller scaffolds of these molecules could be attractive options for fragment-based derivatization for inhibitor discovery.