Reorganization Energy for Internal Electron Transfer in Multicopper Oxidases.

We have calculated the reorganization energy for the intramolecular electron transfer between the reduced type 1 copper site and the peroxy intermediate of the trinuclear cluster in the multicopper oxidase CueO. The calculations are performed at the combined quantum mechanics and molecular mechanics (QM/MM) level, based on molecular dynamics simulations with tailored potentials for the two copper sites. We obtain a reorganization energy of 91-133 kJ/mol, depending on the theoretical treatment. The two Cu sites contribute by 12 and 22 kJ/mol to this energy, whereas the solvent contribution is 34 kJ/mol. The rest comes from the protein, involving small contributions from many residues. We have also estimated the energy difference between the two electron-transfer states and show that the reduction of the peroxy intermediate is exergonic by 43-87 kJ/mol, depending on the theoretical method. Both the solvent and the protein contribute to this energy difference, especially charged residues close to the two Cu sites. We compare these estimates with energies obtained from QM/MM optimizations and QM calculations in a vacuum and discuss differences between the results obtained at various levels of theory

Understanding Lignin-Degrading Reactions of Ligninolytic Enzymes: Binding Affinity and Interactional Profile

Previous works have demonstrated that ligninolytic enzymes mediated effective degradation of lignin wastes. The degrading ability greatly relied on the interactions of ligninolytic enzymes with lignin. Ligninolytic enzymes mainly contain laccase (Lac), lignin peroxidase (LiP) and manganese peroxidase (MnP). In the present study, the binding modes of lignin to Lac, LiP and MnP were systematically determined, respectively. Robustness of these modes was further verified by molecular dynamics (MD) simulations. Residues GLU460, PRO346 and SER113 in Lac, residues ARG43, ALA180 and ASP183 in LiP and residues ARG42, HIS173 and ARG177 in MnP were most crucial in binding of lignin, respectively. Interactional analyses showed hydrophobic contacts were most abundant, playing an important role in the determination of substrate specificity. This information is an important contribution to the details of enzyme-catalyzed reactions in the process of lignin biodegradation, which can be used as references for designing enzyme mutants with a better lignin-degrading activity

Harnessing the potential of ligninolytic enzymes for lignocellulosic biomass pretreatment

Abundant lignocellulosic biomass from various industries provides a great potential feedstock for the production of value-added products such as biofuel, animal feed, and paper pulping. However, low yield of sugar obtained from lignocellulosic hydrolysate is usually due to the presence of lignin that acts as a protective barrier for cellulose and thus restricts the accessibility of the enzyme to work on the cellulosic component. This review focuses on the significance of biological pretreatment specifically using ligninolytic enzymes as an alternative method apart from the conventional physical and chemical pretreatment. Different modes of biological pretreatment are discussed in this paper which is based on (i) fungal pretreatment where fungi mycelia colonise and directly attack the substrate by releasing ligninolytic enzymes and (ii) enzymatic pretreatment using ligninolytic enzymes to counter the drawbacks of fungal pretreatment. This review also discusses the important factors of biological pretreatment using ligninolytic enzymes such as nature of the lignocellulosic biomass, pH, temperature, presence of mediator, oxygen, and surfactant during the biodelignification process

Electrochemistry and kinetics of fungal laccase mediators

The screening of potential redox mediators for laccase was performed using homogeneous Trametes hirsuta laccase. Heterogeneous (electrochemical) and homogeneous (oxidation by laccase) reactions of the different types of the enhancers (mediators) of the enzyme were investigated. It was discovered that derivatives of phenyl-methyl-pyrazolones and benzoic acid, as well as N-hydroxynaphthalimide were efficient substrates for the laccase. The characterization of several representatives from each class was carried out using electrochemical and enzyme kinetics methods. The kinetic parameters for the oxidation of phenyl-methyl-pyrazolones and 3-(6-hylroxy)-aminobenzoic acid were comparable to those for 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) oxidation by the laccase, whereas the rate of enzymatic oxidation of N-hydroxynaphthalimide was sufficiently lower. Electrochemical experiments demonstrated that only oxidation of phenyl-methyl-pyrazolones and N-hydroxynaphthalimide yielded several high-potential intermediates capable of oxidizing veratryl alcohol, which was used as a lignin model substrate, whereas derivatives of benzoic acid showed low-potential intermediate, which was not able to oxidized lignin model compound. Phenyl-methyl-pyrazolones was about 50% as effective in degrading veratryl alcohol compared to ABTS as judged from HPLC kinetic studies, whereas N-hydroxynaphthalimide showed the same efficiency as ABTS. Phenyl-methyl-pyrazolones and hydroxynaphthalimides may be of commercial interest for oxidoreductase-catalyzed biodegradation of different xenobiotics. (c) 2005 Elsevier B.V. All rights reserved