Manganese peroxidase (MnP) from Phanerochaete chrysosporium undergoes a pH-dependent conformational change evidenced by changes in the electronic absorption spectrum. This high-to lowspin alkaline transition occurs at ∼2 pH units lower in an F190I mutant MnP when compared to the wild-type enzyme. Herein, we provide evidence that these spectral changes are attributable to the formation of a bis(histidyl) heme iron complex in both proteins at high pH. The resonance Raman (RR) spectra of both ferric proteins at high pH are similar, indicating similar heme environments in both proteins, and resemble that of ferric cytochrome b 558 , a protein that contains a bis-His iron complex. Upon reduction with dithionite at high pH, the visible spectra of both the wild-type and F190I MnP exhibit absorption maxima at 429, 529, and 558 nm, resembling the absorption spectrum of ferrous cytochrome b 558 . RR spectra of the reduced wild-type and F190I mutant proteins at high pH are also similar to the RR spectrum of ferrous cytochrome b 558 , further suggesting that the alkaline low-spin species is a bis(histidyl) heme derivative. No shift in the low-frequency RR bands was observed in 75% 18 O-labeled water, indicating that the low-spin species is most likely not a hydroxo-heme derivative. Electronic and RR spectra also indicate that addition of Ca 2+ to either the ferric or ferrous enzymes at high pH completely restores the high-spin pentacoordinate species. Other divalent metals, such as Mn 2+ , Mg 2+ , Zn 2+ , or Cd 2+ , do not restore the enzyme under the conditions studied.Manganese peroxidase (MnP) 1 is an extracellular heme protein produced by virtually all lignin-degrading white-rot fungi (1-3). Sequences have been determined for mnp cDNA (4, 5) and genomic clones (6-8) encoding three MnP isozymes from Phanerochaete chrysosporium, the beststudied lignin-degrading fungus. These sequences and spectroscopic studies of the native and oxidized enzyme intermediates indicate that the heme environment of MnP is similar to that of other plant and fungal peroxidases (4,(9)(10)(11)(12)(13)(14). Kinetic and spectroscopic studies of the purified enzyme indicate a typical peroxidase reaction catalytic cycle:However, MnP is unique in its ability to efficiently oxidize Mn 2+ to Mn 3+ (9,15,16). The released Mn 3+ is stabilized by organic acid chelators, such as oxalate and malonate, which are secreted by the fungus (16, 17). The Mn 3+ -chelator complex is capable of diffusing from the enzyme to oxidize terminal substrates including lignin substructures, phenolic compounds, and pollutants (2, 3 and references therein, 18).The three amino acid residues believed to bind Mn 2+ , D179, E35, and E39, were investigated by site-directed mutagenesis of recombinant MnP homologously expressed in P. chrysosporium (19)(20)(21)(22). These studies, combined with X-ray crystal structure analyses of the proteins (23,24), confirmed that the side-chain carboxylates of D179, E35, and E39 form the only apparent Mn 2+ -binding site in MnP from P. chry...