In-silico identification of critical residues in the mannose-transfer mechanism of phosphatidyl-myo-inositol mannosyltransferase B'.

The complex cellular envelope is one of the major reasons behind the survival in hostile conditions and the emergence of the drug-resisting properties of mycobacteria. Phosphatidyl-myo-inositol hexamannoside (PIM), Lipomannan (LM), and Lipoarabinomannan (LAM) are important structural constituents of the cell envelope and have roles in modulating host immune functions. Phosphatidyl-myo-inositol (PI) is first mannosylated at the 2-position of the inositol group by phosphatidyl-myo-inositol mannosyltransferase A (PimA) to produce phosphatidyl-myo-inositol monomannoside (PIM). This PIM is then further mannosylated at the 6-position of the inositol group by phosphatidyl-myo-inositol mannosyltransferase B' (PimB') utilizing GDP-mannose as the mannose-donor to synthesize phosphatidyl-myo-inositol dimannoside (PIM) and GDP. Further mannosylation and acylation on PIM produce AcPIM, which can then be converted to either AcPIM or LM/LAM. Detailed functional mechanism of how PimB' transfers the mannose sugar to PIM is not understood. Using molecular docking, the interactions of PimB' with the substrate PIM and the product PIM are analyzed here. Molecular dynamics (MD) simulations of PimB' with the substrates and the products were performed for 300ns to find out critical residues involved in the mannose-transfer reaction. Docking and MD analyses indicated the residues R206 and R210 bind both PIM and PIM and are critical in the mannose-transfer reaction. The residues HEVGWSMLPGS and RTRGGGL were involved in the transfer of PIM from the active site. The residues IGG, K211, E290, G291, IV, and E298 were also important in the mannosylation reaction. The crucial residues obtained from this study may help design novel drugs against mycobacterial PimB'.