Microcalorimetric study of the effect of hexa-histidine tag and denaturant on the interaction mechanism between protein and metal-chelating gel

A recombinant protein, Schistosoma japonicum glutathione-S-transferase (SjGST), was fused with a C-terminal hexa-histidine tag to obtain SjGST/His. Both proteins were used to probe the interaction mechanisms with the metal ions immobilized on chromatography gels. Isothermal titration calorimetry was used to directly measure the adsorption enthalpies (Δ H_ads) of both proteins with Ni-NTA and TALON (Co²⁺) commercial affinity resins, under the conditions of with and without the presence of a denaturant. The result reveals that SjGST/His had a lower Δ H_ads value with Ni-NTA than did SjGST, mainly attributed to the formation of more coordination bonds with or a stronger binding with Ni-NTA. Furthermore, the difference between the Δ H_ads values of SjGST/His onto TALON under the nature and denaturing conditions were insignificant, implying that the binding topography of the hexa-histidine tail with immobilized Co²⁺ was not significantly changed with the presence of a denaturant. In addition, this study shows that the proposed binding models and the directly measured adsorption heat can be combined to elucidate the difference in the interaction mechanisms of SjGST/His adsorption onto those two adsorbents from a thermodynamic perspective.