A search for ligninolytic peroxidases in the fungus Pleurotus eryngii involving alpha-keto-gamma-thiomethylbutyric acid and lignin model dimers

Because there is some controversy concerning the ligninolytic enzymes produ ced by Pleurotus species, ethylene release from alpha-keto-gamma-thiomethyl butyric, acid (KTBA), as described previously for Phanerochaete chrysospori um lignin peroxidase (LiP), was used to assess the oxidative power of Pleur otus eryngii cultures and extracellular proteins. Lignin model dimers were used to confirm the ligninolytic capabilities of enzymes isolated from liqu id and solid-state fermentation (SSF) cultures. Three proteins that oxidize d KTBA in the presence of veratryl alcohol and H2O2 were identified (two pr oteins were found in liquid cultures, and one protein was found in SSF cult ures). These proteins are versatile peroxidases that act on Mn2+, as well a s on simple phenols and veratryl alcohol. The two peroxidases obtained hom the liquid culture were able to degrade a nonphenolic beta-O-4 dimer, yield ing veratraldehyde, as well as a phenolic dimer which is not efficiently ox idized by P. chrysosporium peroxidases, The former reaction is characterist ic of Lip. The third KTBA-oxidizing peroxidase oxidized only the phenolic d imer (in the presence of Mn2+). Finally, a fourth Mn2+-oxidizing peroxidase was identified in the SSF cultures on the basis of its ability to oxidize KTBA in the presence of Mn2+. This enzyme is related to the Mn-dependent pe roxidase of P. chrysosporium because it did not exhibit activity with verat ryl alcohol and Mn-independent activity with dimers. These results show tha t P. eryngii produces three types of peroxidases that have the ability to o xidize lignin but racks a typical Lip. Similar enzymes (in terms of N-termi nal sequence and catalytic properties) are produced by other Pleurotus spec ies. Some structural aspects of P. eryngii peroxidases related to the catal ytic properties are discussed.