Docking studies on novel alkaloid tryptanthrin and its analogues against enoyl-acyl carrier protein reductase (InhA) of Mycobacterium tuberculosis.

Isoniazid resistance is a serious threat in the battle against the treatment of multi-drug resistant tuberculosis (MDR-TB) and extremely drug-resistant tuberculosis (XDR-TB). Isoniazid is an inhibitor of enoyl-acyl carrier protein reductase (InhA) of Mycobacterium tuberculosis, which is an important and functional enzyme of the type II fatty acid synthesis system and important therapeutic target. Natural alkaloid tryptanthrin and its analogues have shown anti-tubercular activity against MDR-TB, but their cellular target is unknown. In this work, in silico molecular docking was performed using docking server in order to see the interaction of tryptanthrin and its 15 analogues with InhA of M. tuberculosis. Results showed that among tryptanthrin and its 15 analogues, tryptanthrin and its two analogues exhibited good affinity to the binding site of InhA with free binding energy of -7.94 kcal/mol and inhibition constant (Ki) of 1.50 µm. Active site residues of InhA interacting with tryptanthrin were Ser13, Thr39, Phe41, Leu63, Asp64, Val65, Ile95, Phe97 and Ile122. In binding mode, polar bond were found between O1 (1) with Asp64 of bond length (3.34 Å) and hydrophobic bonds were found between Leu63 with C15 and C12, Val65 with C7, Val65 with C12 and C4, Ile95 with C6 and C7, Ile95 with C10, C12 and C14. Important pi-pi bonds were found between Phe41 with C2, C5, C7, C12, C4, C6, C8, C9, C13 and Phe97 with C9. These interactions indicated stability of tryptanthrin in active residue and suggested it as a potential drug candidate. Thus, good affinity of tryptanthrin to binding site of InhA may lead to synthesis of anti-tubercular drug capable of combating MDR strains of M. tuberculosis