Abstract

Examination of InhA mutants I16T, I21V, I47T, S94A, and I95P showed that direct and water mediated H-bond interactions between NADH and binding site residues reduced drastically. It allowed conformational flexibility to NADH, particularly at the pyrophosphate region, leading to weakening of its binding at dinucleotide binding site. The highly scattered distribution of pyrophosphate dihedral angles and chi1 side chain dihedral angles of corresponding active site residues therein confirmed weak bonding between InhA and NADH. The average direct and water mediated bridged H-bond interactions between NADH and mutants were observed weaker as compared to wild type. Further, estimated NADH binding free energy in mutants supported the observed weakening of InhA-NADH interactions. Similarly, per residue contribution to NADH binding was also found little less as compared to corresponding residues in wild type. This investigation clearly depicted and supported the effect of mutations on NADH binding and can be accounted for isoniazid resistance as suggested by previous biochemical and mutagenic studies. Further, structural analysis of InhA provided the crucial points to enhance the NADH binding affinity towards InhA mutants in the presence of direct InhA inhibitors to combat isoniazid drug resistance. This combination could be a potential alternative for treatment of drug resistant tuberculosis.

Highlights

  • The clinical isolates of mycobacterium tuberculosis bacilli obtained from the isoniazid (INH)resistant tuberculosis (TB) patients are observed to have mutations in the structural gene of InhA protein [1,2,3,4]

  • INH react with NADH to form INH-NAD adduct which binds into dinucleotide binding site (DBS) of InhA

  • The initial structural coordinates were obtained from protein data bank (PDB) for WT and MTs: I21V, I47T and S94A (PDB ID-2AQK, 2.30Å) [18,19,20]

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Summary

Introduction

The clinical isolates of mycobacterium tuberculosis bacilli obtained from the isoniazid (INH)resistant tuberculosis (TB) patients are observed to have mutations in the structural gene of InhA protein [1,2,3,4]. Dessen et al reported water molecules in the DBS of InhA to mediate H-bond interactions between NADH and binding site residues.

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