Cyclosporin A binding to Mycobacterium tuberculosis peptidyl-prolyl cis-trans isomerase A--investigation by CD, FTIR and fluorescence spectroscopy

Circular dichroism and resolution-enhanced Fourier transform infrared reveal induction of secondary structural elements on peptidyl-prolyl cis–trans isomerase A (PpiA) from Mycobacterium tuberculosis upon binding cyclosporin A (CsA). Thermal denaturation shows aggregation of PpiA at higher temperatures (>70 °C) and CsA fails to impart stabilization in protein structure. However, CsA stabilizes PpiA structure in urea denaturation. In presence/absence of CsA, urea-induced reversible unfolding of secondary and tertiary structures follows two-state and three-state transition, respectively. The chemical unfolding results also demonstrate that loss in the tertiary structure precedes the loss in secondary structure both in presence and absence of CsA at the initial stages. Fluorescence quenching suggests presence of a positive barrier around tryptophan microenvironment of PpiA.