THE EFFECTS OF CALCIUM ON THE THERMAL-STABILITY AND ACTIVITY OF MANGANESE PEROXIDASE

The presence of micromolar Ca2+ efficiently prevented the thermal inac tivation of manganese peroxidase from Phanerochaete chrysosporium. The amount of Ca2+ normally present in the enzyme decreased when the enzy me was thermally inactivated and EGTA increased the rate of inactivati on. The inactivation kinetics were biphasic, suggesting a sequential t wo-step process. The rate of inactivation during the second, slower st ep 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 in creased and the amount of remaining heme decreased, the amount of enzy me activity recovered decreased. Therefore, while both steps of denatu ration could be prevented by Ca2+, only one step could be reversed upo n the addition of Ca2+. It is proposed that the first step of denatura tion involves the loss of Ca2+ which causes conformational changes res ulting in the loss of manganese peroxidase activity. The second step i s 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 lab ile Ca2+ ions required for stability and activity. (C) 1996 Academic P ress, Inc.