A thermodynamic study of mesophilic, thermophilic, and hyperthermophilic L-arabinose isomerases: The effects of divalent metal ions on protein stability at elevated temperatures

To gain insight into the structural stability of homologous homo-tetrameric l-arabinose isomerases (AI), we have examined the isothermal guanidine hydrochloride (GdnHCl)-induced unfolding of AIs from mesophilic Bacillus halodurans (BHAI), thermophilic Geobacillus stearothermophilus (GSAI), and hyperthermophilic Thermotoga maritima (TMAI) using circular dichroism spectroscopy. The GdnHCl-induced unfolding of the AIs can be well described by a two-state reaction between native tetramers and unfolded monomers, which directly confirms the validity of the linear extrapolation method to obtain the intrinsic stabilities of these proteins. The resulting unfolding free energy (ΔG_U) values of the AIs as a function of temperature were fit to the Gibbs-Helmholtz equation to determine their thermodynamic parameters based on a two-state mechanism. Compared with the stability curves of BHAI in the presence and absence of Mn²⁺, those of holo GSAI and TMAI were more broadened than those of the apo enzymes at all temperatures, indicating increased melting temperatures (T_m) due to decreased heat capacity (ΔC_p). Moreover, the extent of difference in ΔC _p between the apo and holo thermophilic AIs is larger than that of BHAI. From these studies, we suggest that the metal dependence of the thermophilic AIs, resulting in the reduced ΔC_p, may play a significant role in structural stability compared to their mesophilic analogues, and that the extent of metal dependence of AI stability seems to be highly correlated to oligomerization.