Effects of cadmium on manganese peroxidase - Competitive inhibition of Mn-II oxidation and thermal stabilization of the enzyme

Inhibition of manganese peroxidase by cadmium was studied under steady-stat e and transient-state kinetic conditions. Cd-II is a reversible competitive inhibitor of Mn-II in the steady state with K-i approximate to 10 mu m. Cd -II also inhibits enzyme-generated Mn-III-chelate-mediated oxidation of 2,6 -dimethoxyphenol with K-i approximate to 4 mu m. Cd-II does not inhibit dir ect oxidation of phenols such as 2,6-dimethoxyphenol or guaiacol (2-methoxy phenol) in the absence of Mn-II. Cd-II alters the heme Soret on binding man ganese peroxidase and exhibits a K-d approximate to 8 mu m, similar to Mn ( K-d approximate to 10 mu m). Under transient-state conditions, Cd-II inhibi ts reduction of compound I and compound II by Mn-II at pH 4.5. However, Cd- II does not inhibit formation of compound I nor does it inhibit reduction o f the enzyme intermediates by phenols in the absence of Mn-II. Kinetic anal ysis suggests that Cd-II binds at the Mn-II-binding site, preventing oxidat ion of Mn-II, but does not impair oxidation of substrates, such as phenols, which do not bind at the Mn-II-binding site. Finally, at pH 4.5 and 55 deg rees C, Mn-II and Cd-II both protect manganese peroxidase from thermal dena turation more efficiently than Ca-II, extending the half-life of the enzyme by more than twofold. Furthermore, the combination of half Mn-II and half Cd-II nearly quadruples the enzyme half-life over either metal alone or eit her metal in combination with Ca-II.