Determinants for arginine-mediated modulation of thermodynamic stability and conformational dynamics of horse cytochrome c

Arginine and its salts enhance protein refolding and suppress protein aggregation. The present work delineates the molecular basis by which the arginine alters the thermodynamic stability and conformational dynamics of horse cytochrome c (h-cyt c) in aqueous and denaturant media. The analysis of effect of arginine on the thermally and guanidinium chloride (GdmCl)-induced unfolding of ferrocytochrome c (h-cyt c^II) showed that the arginine reduces the thermodynamic stability of h-cyt c^II. The MD simulation was applied to quantitatively estimate the preferential interaction coefficient of h-cyt c^II at different arginine concentrations. The increase in the preferential interaction coefficient of h-cyt c^II with arginine concentration reveals that interaction of arginine molecules with protein contributes to the arginine-mediated decrease in thermodynamic stability of h-cyt c^II. Further analysis showed that the efficacy of arginine in exhibiting additive or counteracting effects on the denaturing efficiency of GdmCl is dependent upon arginine concentration. The analysis of MD simulation of h-cyt c^II to understand the effect of arginine on [GdmCl] denaturation shows that (i) arginine increases the conformational-fluctuations and decreases the structural stability of h-cyt c^II (ii) arginine increases the helical content with a simultaneous decrease in the β-sheet of protein, and (iii) arginine alters the [GdmCl]-dependent conformational change of h-cyt c^II.