Roles of manganese and calcium in the extracellular degradation system of white-rot fungi.

摘要

Lignin peroxidase (LiP) and manganese peroxidase (MnP) are integral components of the extracellular degradation system of the white-rot fungus, Phanerochaete chrysosporium. These peroxidases are able to directly or indirectly catalyze the one-electron oxidation of a wide range of compounds resulting in the production of free radicals. Indirect oxidation occurs via redox mediators which are directly oxidized by the peroxidases to species which oxidize other compounds. Veratryl alcohol (VA) and Mn{dollar}sp{lcub}2+{rcub}{dollar} are the two most well characterized, and physiologically relevant, substrates of LiP and MnP, respectively.; Due to the ability of Lip isozyme H2 to oxidize Mn{dollar}sp{lcub}2+{rcub}{dollar} at a high rate, the LiPH2-catalyzed oxidation of a number of compounds was much higher in the presence of Mn{dollar}sp{lcub}2+{rcub}{dollar} than VA (Chapter III). A novel, VA-dependent mechanism for the catalytic oxidation of Mn{dollar}sp{lcub}2+{rcub}{dollar} by the other five LiP isozymes was then elucidated (Chapter IV).; Manganese peroxidase was shown to be susceptible to thermal inactivation due to the release of calcium (Chapter V), a process which was prevented and reversed by calcium. The loss of the distal calcium resulted in alterations in the heme environment of MnP, binding of a histidine residue to the heme iron, and conversion of the heme iron from a high-spin to a low-spin complex (Chapter VI). The distal calcium bound with relatively high affinity to MnP during reactivation (Chapter VII). This process was associated with large decreases in entropy and enthalpy due to decreases in the structural flexibility of the protein upon calcium binding. This was consistent with the proposal that calcium served to maintain the structural stability of MnP.; The ability of P. chrysosporium to degrade synthetic superabsorbent polymers, such as those used in disposable diapers, was also demonstrated (Chapter VIII). The process was dependent on the production of the extracellular degradation system. The polymers were rapidly depolymerized and subsequently internalized and a mineralized. The involvement of the peroxidases was demonstrated by showing that degradation was stimulated in vivo by the addition of VA, calcium, and Mn{dollar}sp{lcub}2+{rcub}{dollar}, and that purified preparations of LiP and MnP catalyzed depolymerization in vitro (Chapter IX).