Abstract

The redox cycle of lignin peroxidase (LiP) is discussed in terms of the Marcus theory of electron transfer. The difference in kinetic behaviour of the two redox couples LiP-Compound I/LiP-Compound II (LiPI/LiPII), respectively LiPH/LiP, in the oxidation of veratryl alcohol is attributed to an estimated increase in reorganization energy of about 0.5 eV for the conversion of LiPII to native enzyme compared to the reduction of LiPI to LiPII. Whereas LiPI/LiPII involves a transition from a low-spin oxyferryl porphyrin radical cation to a low-spin oxyferryl porphyrin system, the conversion of LiPII to native enzyme involves a change in spin-state to high-spin ferric, accompanied by a conformational change of the protein. In addition, a molecule of water is formed after protonation of the oxyferryl porphyrin system by the distal His-47 and Arg-43. Furthermore, the reduction of LiPI to LiPII is observed as an irreversible process. Since the oxidation of veratryl alcohol by oxidized LiP will occur in the endergonic region of the driving force, it is postulated that the thermodynamic unfavourable formation of veratryl alcohol radical cation is facilitated by reaction of a nucleophile with the incipient radical cation. It is further postulated that the ordered carbohydrate residues found near the entrance to the active site channel in the LiP crystal structure play a role in this process.

Graphical Abstract

The redox cycle of lignin peroxidase is described in terms of Marcus theory of electron transfer. The oxidation of veratryl alcohol occurs in the endergonic region of the driving force, still the reduction of LiP Compound I is irreversible. A reversible reaction of the incipient veratryl alcohol radical cation with a glucose residue located at the entrance of the active site channel is postulated.

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