PubMed1993
Manganese peroxidase from the lignin-degrading fungus Phanerochaete chrysosporium catalyzes the H2O2-dependent oxidation of Mn2+ to Mn3+. Presteady-state methods were employed to characterize the reactions of free and chelated Mn2+ with the 2-electron and 1-electron oxidized forms of the enzyme, compounds I and II, respectively. At pH 4.5, the optimum pH for steady-state turnover, the reaction of compound I with Mn2+, either free or complexed, is too rapid to measure by stopped flow methods. The reactions of compound I with Mn2+ can only be monitored under non-optimal conditions of pH 2.5. The reaction of compound II with Mn2+ is much slower than compound I. Chelators such as oxalate, lactate, and malonate facilitated the reaction of Mn2+ with compound II. In contrast, succinate, which does not readily form a complex with Mn2+, and polyglutamate, which is polymeric, were ineffective in stimulating the reaction of Mn2+ with compound II. The 1:1 chelator-Mn2+ complex is the preferred substrate for compound II; this conclusion is based on known formation constants for the various Mn2+ complexes. Steady-state kinetics studies were performed by directly measuring the initial rate of Mn3+ formation. The kcat values for the formation of Mn(3+)-oxalate, Mn(3+)-lactate, and Mn(3+)-malonate are 308, 211, and 220 s-1, respectively. The Km values for Mn(2+)-oxalate, Mn(2+)-lactate, and Mn(2+)-malonate are 13, 41, and 18 microM, respectively. These results collectively indicate that manganese peroxidase does not readily oxidize free (hexa-aquo) Mn2+ as previously proposed (Wariishi, H., Valli, K., and Gold, M. H. (1992) J. Biol. Chem. 267, 23688-23695), but the Mn2+ has to be chelated to support steady-state turnover.