Microcalorimetric characterization of the anion-exchange adsorption of recombinant cytochrome b5 and its surface-charge mutants.

The adsorption of recombinant soluble tryptic fragment of rat cytochrome b5 on the strong anion exchanger Mono Q was studied using isothermal titration calorimetry and differential scanning calorimetry (DSC). Titration calorimetry results obtained at low levels of adsorbed protein show increasingly endothermic (unfavorable) enthalpies of binding with increasing surface coverage, confirming the heterogeneous nature of binding. The enthalpy of adsorption declines toward zero at higher loadings. At low surface coverage, enthalpies increase linearly with temperature, giving rise to a positive value of delta Cp. Enthalpies of adsorption depend strongly on the history of the adsorbent. DSC is used to show that cytochrome b5 is stable in both free and adsorbed states at all temperatures used in the titration calorimetric experiments. Site-directed mutants of recombinant cytochrome b5 carrying single charge-neutralizing substitutions are used to test the contributions of particular residues to the thermodynamics of adsorption. Like those derived from van't Hoff analysis of equilibrium adsorption isotherms and HPLC retention data, calorimetric enthalpies of adsorption are positive, confirming the dominant role of entropic effects in ion-exchange adsorption in this system.