The effects of calcium on the thermal stability and activity of manganese peroxidase.

The presence of micromolar Ca2+ efficiently prevented the thermal inactivation of manganese peroxidase from Phanerochaete chrysosporium. The amount of Ca2+ normally present in the enzyme decreased when the enzyme was thermally inactivated and EGTA increased the rate of inactivation. The inactivation kinetics were biphasic, suggesting a sequential two-step process. The rate of inactivation during the second, slower step corresponded to the rate of loss of heme from the enzyme. Thermally inactivated manganese peroxidase could be readily reactivated in the presence of excess Ca2+. However, as the time of thermal incubation increased and the amount of remaining heme decreased, the amount of enzyme activity recovered decreased. Therefore, while both steps of denaturation could be prevented by Ca2+, only one step could be reversed upon the addition of Ca2+. It is proposed that the first step of denaturation involves the loss of Ca2+ which causes conformational changes resulting in the loss of manganese peroxidase activity. The second step is believed to involve further structural loss and results in the loss of heme from the enzyme. It is concluded that manganese peroxidase is susceptible to thermal inactivation because it contains relatively labile Ca2+ ions required for stability and activity.