Distinct oligomerization and NADPH binding modes observed between L. donovani and human quinone oxidoreductases

Electron-driven process helps the living organism in the generations of energy, biomass production and detoxification of synthetic compounds. Soluble quinone oxidoreductases (QORs) mediate the transfer of an electron from NADPH to various quinone and other compounds, helping in the detoxification of quinones. QORs play a crucial role in cellular metabolism and are thus potential targets for drug development. Here we report the crystal structure of the NADPH-dependent QOR from Leishmania donovani (LdQOR) at 2.05 Å. The enzyme exists as a homo-dimer, with each protomer consisting of two domains, responsible for binding NADPH cofactor and the substrate. Interestingly, the human QOR exists as a tetramer. Comparative analysis of the oligomeric interfaces of LdQOR with HsQOR shows no significant differences in the protomer/dimer assembly. The tetrameric interface of HsQOR is stabilized by salt bridges formed between Arg 169 and Glu 271 which is non-existent in LdQOR, with an Alanine replacing the glutamate. This distinct feature is conserved across other dimeric QORs, indicating the importance of this interaction for tetramer association. Among the homologs, the sequences of the loop region involved in the stabilization and binding of the adenine ring of the NADPH shows significant differences except for an Arginine & glycine residues. In dimer QORs, this Arginine acts as a gate to the co-factor, while the NADPH binding mode in the human homolog is distinct, stabilized by His 200 and Asn 229, which are not conserved in LdQOR. These distinct features have the potential to be utilized for therapeutic interventions.