Effect of calcium on the reversible thermal inactivation of lignin peroxidase

This study investigated the effects of calcium on the thermal inactivation of lignin peroxidase from Phanerochaete chrysosporium. The monophasic loss of veratryl alcohol oxidase activity corresponded to the loss of calcium when the enzyme was thermally inactivated. Addition of calcium slowed and oxalate and EGTA increased the apparent inactivation rate. The thermally inactivated lignin peroxidase could be readily reactivated by addition of Ca2+. The amount of activity recovered was dependent on temperature, Ca2+ concentration, and incubation conditions. Enzyme activity could be recovered up to 95% of its original value by addition of Ca2+ when lignin peroxidase was depleted of Ca2+ by incubation with EGTA. Although heme absorbance decreased when the enzyme was thermally inactivated, the amount of iron in the enzyme did not change. Changes in the heme environment of the inactivated enzyme were suggested by changes in the electronic absorption in which the Soret band shifted from 408 to 410 nm, the absorption at 502 nm shifted to 532 nm, and the absorption at 634 nm disappeared upon inactivation. Upon the addition of Ca2+, the bands returned to the original wavelength. Therefore, it is proposed that the inactivation mechanism of lignin peroxidase is that the loss of calcium causes heme environmental changes resulting in the loss of enzyme activity.