Ribose-5-phosphate isomerase B (RpiB), a crucial
enzyme of pentose phosphate pathway, was proposed to be a potential
drug target for
visceral leishmaniasis. In this study, we have analyzed the biophysical properties of Leishmania donovani RpiB (LdRpiB)
enzyme to gain insight into its unfolding pathway under various chemical and thermal denaturation conditions by using fluorescence and CD spectroscopy. LdRpiB inactivation precedes the structural transition at lower concentrations of both
urea and
guanidine hydrochloride (GdHCl). 8-Anilinonapthalene 1-sulfonic (ANS) binding experiments revealed the presence of molten globule intermediate at 1.5 M GdHCl and a nonnative intermediate state at 6-M
urea concentration. Acrylamide quenching experiments further validated the above findings, as
solvent accessibility of
tryptophan residues increased with increase in GdHCl and
urea concentration. The recombinant LdRpiB was completely unfolded at 6 M GdHCl, whereas the
enzyme molecule was resistant to complete unfolding even at 8-M
urea concentration. The GdHCl- and
urea-mediated unfolding involves a three-state transition process. Thermal-induced denaturation revealed complete loss of
enzyme activity at 65 °C with only 20 % secondary structure loss. The formation of the well-ordered β-sheet structures of
amyloid fibrils was observed after 55 °C which increased linearly till 85 °C as detected by
thioflavin T dye. This study depicts the stability of the
enzyme in the presence of chemical and thermal denaturants and stability-activity relationship of the
enzyme. The presence of the intermediate states may have major implications in the way the
enzyme binds to its natural
ligand under various conditions. Also, the present study provides insights into the properties of intermediate entities of this important
enzyme.