Do carbohydrates play a role in the lignin peroxidase cycle? Redox catalysis in the endergonic region of the driving force.

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 LiPII/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 prophyrin 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.