Thermodynamics of binding of the distal calcium to manganese peroxidase.

We previously demonstrated that manganese peroxidase from Phanerochaete chrysosporiumwas susceptible to thermal inactivation due to release of the distal calcium, which maintained the distal heme environment of the enzyme [Sutherland, G. R. J., Zapanta, L. S., Tien, M., & Aust, S. D. (1997) Biochemistry 36, 3654-3662]. In this investigation the binding of calcium to the distal calcium binding site of manganese peroxidase was studied by optical absorption spectroscopy and isothermal titration calorimetry. The dissociation constant for the distal calcium binding site was 11 +/- 1 microM and the Hill coefficient was 1.1 +/- 0.1. The binding of calcium was accompanied by decreases in enthalpy and entropy that were large compared to those of other calcium binding proteins. The decreases were consistent with the large conformational changes proposed to occur in manganese peroxidase as a result of the binding and release of the distal calcium. Studies involving binding of the hydrophobic fluorescent probe, 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid, dipotassium salt (bis-ANS), to manganese peroxidase indicated that the active, calcium-containing form of the enzyme had less exposed hydrophobic surface area, which would contribute to an increase in enthalpy and entropy upon calcium binding. Therefore, the negative changes in enthalpy and entropy associated with calcium binding were attributed to a large increase in the structural rigidity and compactness of the enzyme. The dissociation constant for calcium decreased and the rate of thermal inactivation decreased with decreasing pH. However, both the ability of calcium to prevent thermal inactivation of manganese peroxidase and the rate of calcium binding decreased as the pH decreased. Therefore it was proposed that, at lower pH, calcium binding to manganese peroxidase was more thermodynamically favorable, but the rate of calcium binding decreased because the flexibility of the calcium binding site, and in turn exposure of the ligands to the incoming ion, decreased.