Description of a Versatile Peroxidase Involved in the Natural Degradation of Lignin That Has Both Manganese Peroxidase and Lignin Peroxidase Substrate Interaction Sites

Two major peroxidases are secreted by the fungus Pleurotus eryngii in lignocellulose cultures. One is similar to Phanerochaete chrysosporium manganese-dependent peroxidase. The second protein (PS1), although catalyzing the oxidation of Mn 2+ to Mn 3+ by H 2 O 2 , differs from the above enzymes by its manganese-independent activity enabling it to oxidize substituted phenols and synthetic dyes, as well as the lignin peroxidase (LiP) substrate veratryl alcohol. This is by a mechanism similar to that reported for LiP, as evidenced by p -dimethoxybenzene oxidation yielding benzoquinone. The apparent kinetic constants showed high activity on Mn 2+ , but methoxyhydroquinone was the natural substrate with the highest enzyme affinity (this and other phenolic substrates are not efficiently oxidized by the P. chrysosporium peroxidases). A three-dimensional model was built using crystal models from four fungal peroxidase as templates. The model suggests high structural affinity of this versatile peroxidase with LiP but shows a putative Mn 2+ binding site near the internal heme propionate, involving Glu 36 , Glu 40 , and Asp 181 . A specific substrate interaction site for Mn 2+ is supported by kinetic data showing noncompetitive inhibition with other peroxidase substrates. Moreover, residues reported as involved in LiP interaction with veratryl alcohol and other aromatic substrates are present in peroxidase PS1 such as His 82 at the heme-channel opening, which is remarkably similar to that of P. chrysosporium LiP, and Trp 170 at the protein surface. These residues could be involved in two different hypothetical long range electron transfer pathways from substrate (His 82 -Ala 83 -Asn 84 -His 47 -heme and Trp 170 -Leu 171 -heme) similar to those postulated for LiP.