Mechanisms of laccase-mediator treatments improving the enzymatic hydrolysis of pre-treated spruce

Background

The recalcitrance of softwood to enzymatic hydrolysis is one of the major bottlenecks hindering its profitable use as a raw material for platform sugars. In softwood, the guaiacyl-type lignin is especially problematic, since it is known to bind hydrolytic enzymes non-specifically, rendering them inactive towards cellulose. One approach to improve hydrolysis yields is the modification of lignin and of cellulose structures by laccase-mediator treatments (LMTs).

Results

LMTs were studied to improve the hydrolysis of steam pre-treated spruce (SPS). Three mediators with three distinct reaction mechanisms (ABTS, HBT, and TEMPO) and one natural mediator (AS, that is, acetosyringone) were tested. Of the studied LMTs, laccase-ABTS treatment improved the degree of hydrolysis by 54%, while acetosyringone and TEMPO increased the hydrolysis yield by 49% and 36%, respectively. On the other hand, laccase-HBT treatment improved the degree of hydrolysis only by 22%, which was in the same order of magnitude as the increase induced by laccase treatment without added mediators (19%). The improvements were due to lignin modification that led to reduced adsorption of endoglucanase Cel5A and cellobiohydrolase Cel7A on lignin. TEMPO was the only mediator that modified cellulose structure by oxidizing hydroxyls at the C6 position to carbonyls and partially further to carboxyls. Oxidation of the reducing end C1 carbonyls was also observed. In contrast to lignin modification, oxidation of cellulose impaired enzymatic hydrolysis.

Conclusions

LMTs, in general, improved the enzymatic hydrolysis of SPS. The mechanism of the improvement was shown to be based on reduced adsorption of the main cellulases on SPS lignin rather than cellulose oxidation. In fact, at higher mediator concentrations the advantage of lignin modification in enzymatic saccharification was overcome by the negative effect of cellulose oxidation. For future applications, it would be beneficial to be able to understand and modify the binding properties of lignin in order to decrease unspecific enzyme binding and thus to increase the mobility, action, and recyclability of the hydrolytic enzymes.

     

The laccase-catalyzed modification of lignin for enzymatic hydrolysis

The efficient use of cellulases in the hydrolysis of pretreated lignocellulosic biomass is limited due to the presence of lignin. Lignin is known to bind hydrolytic enzymes nonspecifically, thereby reducing their action on carbohydrate substrates. The composition and location of residual lignin therefore seem to be important for optimizing the enzymatic hydrolysis of lignocellulosic substrates. The use of lignin-modifying enzymes such as laccase may have potential in the modification or partial removal of lignin from the biomass. In this study, the effect of lignin modification by laccase on the hydrolysis of pretreated spruce (Picea abies) and giant reed (Arundo donax) was evaluated. The substrates were first treated with laccase and then hydrolyzed with commercial cellulases. Laccase modification improved the hydrolysis yield of spruce by 12%, but surprisingly had an adverse effect on giant reed, reducing the hydrolysis yield by 17%. The binding properties of cellulases on the untreated and laccase-treated lignins were further studied using isolated lignins. The laccase treatment reduced the binding of enzymes on modified spruce lignin, whereas with giant reed, the amount of bound proteins increased after laccase treatment. Further understanding of the reactions of laccase on lignin will help to control the unspecific-binding of cellulases on lignocellulosic substrates.     

The laccase-catalyzed modification of lignin for enzymatic hydrolysis

The efficient use of cellulases in the hydrolysis of pretreated lignocellulosic biomass is limited due to the presence of lignin. Lignin is known to bind hydrolytic enzymes nonspecifically, thereby reducing their action on carbohydrate substrates. The composition and location of residual lignin therefore seem to be important for optimizing the enzymatic hydrolysis of lignocellulosic substrates. The use of lignin-modifying enzymes such as laccase may have potential in the modification or partial removal of lignin from the biomass. In this study, the effect of lignin modification by laccase on the hydrolysis of pretreated spruce (Picea abies) and giant reed (Arundo donax) was evaluated. The substrates were first treated with laccase and then hydrolyzed with commercial cellulases. Laccase modification improved the hydrolysis yield of spruce by 12%, but surprisingly had an adverse effect on giant reed, reducing the hydrolysis yield by 17%. The binding properties of cellulases on the untreated and laccase-treated lignins were further studied using isolated lignins. The laccase treatment reduced the binding of enzymes on modified spruce lignin, whereas with giant reed, the amount of bound proteins increased after laccase treatment. Further understanding of the reactions of laccase on lignin will help to control the unspecific-binding of cellulases on lignocellulosic substrates.     

Thermogravimetry study of xylanase- and laccase/mediator-treated eucalyptus pulp fibres

Thermogravimetric analyses (TGA) was applied to study the effects of enzymatic bleaching of eucalyptus pulp with xylanase and a laccase-mediator system. The thermal degradation profile of the pulps was sensitive to the enzymatic treatments. Xylanase treatment produced an ordered and clean microfibril, whereas laccase oxidized surface cellulose chains and increased the amorphous (paracrystalline) cellulose content. In this case, pulp viscosity decreased from 972 to 859 mL/g and apparent pulp crystallinity calculated from TGA data decreased almost 50%. Alkaline extraction was necessary to recover pulp crystallinity and to remove oxidized lignin in the laccase-treated samples. TGA data allowed differentiating and quantifying crystalline and amorphous cellulose. This thermogravimetric approach is a simple method in order to monitor superficial changes in cellulosic microfibrils.     

Inhibition of cellulose enzymatic hydrolysis by laccase‐derived compounds from phenols

The presence of inhibitors compounds after pretreatment of lignocellulosic materials affects the saccharification and fermentation steps in bioethanol production processes. Even though, external addition of laccases selectively removes the phenolic compounds from lignocellulosic prehydrolysates, when it is coupled to saccharification step, lower hydrolysis yields are attained. Vanillin, syringaldehyde and ferulic acid are phenolic compounds commonly found in wheat‐straw prehydrolysate after steam‐explosion pretreatment. These three phenolic compounds were used in this study to elucidate the inhibitory mechanisms of laccase‐derived compounds after laccase treatment. Reaction products derived from laccase oxidation of vanillin and syringaldehyde showed to be the strongest inhibitors. The presence of these products causes a decrement on enzymatic hydrolysis yield of a model cellulosic substrate (Sigmacell) of 46.6 and 32.6%, respectively at 24 h. Moreover, a decrease in more than 50% of cellulase and β‐glucosidase activities was observed in presence of laccase and vanillin. This effect was attributed to coupling reactions between phenoxyl radicals and enzymes. On the other hand, when the hydrolysis of Sigmacell was performed in presence of prehydrolysate from steam‐exploded wheat straw a significant inhibition on enzymatic hydrolysis was observed independently of laccase treatment. This result pointed out that the other components of wheat‐straw prehydrolysate are affecting the enzymatic hydrolysis to a higher extent than the possible laccase‐derived products. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 31:700–706, 2015     

Energy requirements and process design considerations in compression-milling pretreatment of cellulosic wastes for enzymatic hydrolysis

The effectiveness of compression-milling pretreatment of lignocellulosics for enzymatic hydrolysis has been demonstrated for a wide variety of substrate sources. Reductions in the degree of crystallinity and the degree of polymerization of cellulose and partial destruction of the structural integrity of lignocellulosics brought about by compression milling significantly increase the susceptibility of cellulose to enzymatic hydrolysis. The enzymatic hydrolysis yield was found to be directly related to the specific energy input to the cellulosic substrate (kWh/1b substrate) by compression milling, and the energy input can be controlled by the milling time. The enzymatic hydrolysis yeilds from cellulosic materials pretreated by compression milling also vary significantly depending on the source and kind, the composition milling also vary significantly depending on the source and kind, the composition (contents of lignin and other components), and the structure. The power requirements for compression milling which renders equivalent hydrolysis yields also depend on the source and kind of lignocellulosics to be pretreated. For newspaper, the specific energy input required for 55% sugar yield is estimated as 0.3 kWh/lb substrate including 15% power loss. The additional sugar yield gained from the enzymatic hydrolysis of compression-milled newspaper (over and above the sugar yield of untreated substrate) is determined as 453 g sugar/kWh energy input.     

六种白腐菌预处理对毛白杨木材酶水解效果的影响

通过化学分析和酶水解试验,研究了不同的白腐菌对毛白杨的预处理效果及不同组分的降解对酶水解的影响。毛白杨木片经6种白腐菌预处理30d后,各组分都发生了降解,其中半纤维素的损失最为显著,Trametes ochracea C6888引起半纤维素降解率高达47.19%,其次是纤维素和酸不溶木素的降解。在后续酶水解过程中,6种白腐菌处理后的样品显示出不同的水解模式,菌株Trametes ochracea C6888、T. pubescens C7571和T. versicolor C6915预处理效果最为显著,还原糖得率在整个酶水解过程中一直高于对照,其中T. ochracea C6888在水解96h后还原糖得率达到15.93%,比未处理样品提高了25%。分析酸不溶木素降解率及半纤维素降解率与还原糖得率的关系发现,不同菌株在作用同一种基质时,预处理效果差异显著,木质素和半纤维素的脱除都会影响木质纤维素的酶水解。     

Comparing the efficiency of the laccase–NHA and laccase–HBT systems in eucalyptus pulp bleaching

Laccase–mediator systems have the disadvantage that the mediator is expensive and potentially toxic. In this work, we used N-hydroxyacetanilide (NHA) in combination with laccase for the first time to bleach eucalypt pulp and found it to be a very promising, advantageous alternative to 1-hydroxybenzotriazole (HBT) as mediator. Thus, NHA is efficiently oxidized by laccase to a radical that absorbs light at 350 nm. Also, NHA is a better substrate for laccase than is HBT. An innovative result is that the enzyme is inactivated to a similar extent by both mediators under the typical treatment conditions of the bleaching step (L). This adverse effect, however, is strongly reduced in the presence of pulp. Moreover, the laccase–NHA system is as efficient as the laccase–HBT system in reducing the kappa number of eucalyptus pulp. Using a xylanase pretreatment or unbleached pulp boosts kappa number reduction and bleaching with the laccase–mediator system. Based on the results of cyclic voltammetry tests, NHA has a slightly lower redox potential than HBT, which further supports use of the former; also, unlike HBT, NHA is oxidized in a reversible, pH-dependent manner. Interestingly, the laccase–NHA system provides more efficient bleaching of eucalyptus pulp at pH 5 than it does at pH 4.     

Oxidative degradation of model lipids representative for main paper pulp lipophilic extractives by the laccase–mediator system

Different model lipids-alkanes, fatty alcohols, fatty acids, resin acids, free sterols, sterol esters, and triglycerides-were treated with Pycnoporus cinnabarinus laccase in the presence of 1-hydroxybenzotriazole as mediator, and the products were analyzed by gas chromatography. The laccase alone decreased the concentration of some unsaturated lipids. However, the most extensive lipid modification was obtained with the laccase-mediator system. Unsaturated lipids were largely oxidized and the dominant products detected were epoxy and hydroxy fatty acids from fatty acids and free and esterified 7-ketosterols and steroid ketones from sterols and sterol esters. The former compounds suggested unsaturated lipid attack via the corresponding hydroperoxides. The enzymatic reaction on sterol esters largely depended on the nature of the fatty acyl moiety, i.e., oxidation of saturated fatty acid esters started at the sterol moiety, whereas the initial attack of unsaturated fatty acid esters was produced on the fatty acid double bonds. In contrast, saturated lipids were not modified, although some of them decreased when the laccase-mediator reactions were carried out in the presence of unsaturated lipids suggesting participation of lipid peroxidation radicals. These results are discussed in the context of enzymatic control of pitch to explain the removal of lipid mixtures during laccase-mediator treatment of different pulp types.     

Pretreatment of Miscanthus with biomass-degrading bacteria for increasing delignification and enzymatic hydrolysability

Biomass recalcitrance is still a main challenge for the production of biofuels and high-value products. Here, an alternative Miscanthus pretreatment method by using lignin-degrading bacteria was developed. Six efficient Miscanthus-degrading bacteria were first cultured to produce laccase by using 0.5% Miscanthus biomass as carbon source. After 1–5 days of incubation, the maximum laccase activities induced by Miscanthus in the six strains were ranged from 103 to 8091 U l⁻¹. Then, the crude enzymes were directly diluted by equal volumes of citrate buffer and added Miscanthus biomass to a solid concentration at 4% (w/v). The results showed that all bacterial pretreatments significantly decreased the lignin content, especially in the presence of two laccase mediators (ABTS and HBT). The lignin removal directly correlated with increases in total sugar and glucose yields after enzymatic hydrolysis. When ABTS was used as a mediator, the best lignin-degrading bacteria (Pseudomonas sp. AS1) can remove up to 50.1% lignin of Miscanthus by obtaining 2.2-fold glucose yield, compared with that of untreated biomass. Therefore, this study provided an effective Miscanthus pretreatment method by using lignin-degrading bacteria, which may be potentially used in improving enzymatic hydrolysability of biomass.     

Modification phenomena of solid-state lignin caused by electron-abstracting oxidative systems

Oxidative treatments of wood pulp lignin by one-electron-abstracting enzymatic or chemical systems result in modification phenomena which are not fully described in terms of those known from lignin model compound studies. The generation of, e.g., long-lived radicals necessitates nondestructive spectroscopic analysis of the lignin polymer for a proper characterization of these. The present work exposes a complexity of spectroscopic modification phenomena, which has not previously been realized. This is achieved by a laccase-mediator system, where the mediator is an aromatic low-molecular-weight compound, which mediates the electron abstraction between the lignin and the enzyme laccase. It is demonstrated that the modification generated exhibits qualitatively different temporal phases. The mechanisms are partly explained in terms of Marcus electron transfer theory, and it is suggested that these may play a role in the in vivo synthesis and degradation of lignin.     

Effect of oxygen delignification on the rate and extent of enzymatic hydrolysis of lignocellulosic material

In this study, we examined the effect of oxygen delignification on the rate and extent of enzymatic hydrolysis (using commercial cellulase and beta-glucosidase) of a number of lignocellulosic substrates, including kraft pulp (model substrate), pulp mill primary clarifier sludge (PCS) and steam-exploded Douglas fir chips. Oxygen delignification removed up to 67% of the lignin from softwood pulp and improved the rate of, and yield from, hydrolysis by up to 111% and 174%, respectively. Glucose yield varied linearly with fractional lignin removal. Oxygen delignification of primary clarifier sludge improved hydrolysis yield by up to 90%. However steam-exploded Douglas fir was very resistant to hydrolysis at low enzyme loading, and oxygen delignification decreased hydrolysis rate and yield.     

Influence of operation conditions on laccase-mediator removal of sterols from eucalypt pulp

The way how sterols, the main lipophilic compounds present in eucalypt kraft pulp, are eliminated by an enzymatic stage using the laccase-mediator system was evaluated. With this purpose laccase-mediator stage (L) was applied on an Eucalyptus globulus pulp under different operation conditions following a three-variable (laccase dose, mediator dose and reaction time) sequential statistical plan, to optimise the removal of sterols. The decrease in pulp sterol content during the enzymatic treatment was related to the decrease in kappa number and to brightness increase, as well as with the increase in some oxidation products of sitosterol (namely 7-oxositosterol and stigmasta-3,5-dien-7-one). The increase in reaction time from 1 to 5 h strongly reduced the sterol content, while no more sterols were eliminated during the 5–7 h period. Increasing the laccase dose from 1 to 20 U g⁻¹ of pulp produced a high reduction in pulp sterols, whereas the increase in mediator (1-hydroxybenzotriazole) dose (from 0.5 to 2.5% of pulp weight) had only a slight influence in removing sterols. Therefore, at 16 U g⁻¹ laccase dose, 0.5% mediator dose, 4 h of reaction, practically all the sterols were removed. Finally, it was demonstrated that sterols were more sensitive to a L stage (practically 100% of sterols were eliminated) than to a chlorine dioxide stage (54% of sterols eliminated).     

Structural modification of lignocellulosics by pretreatments to enhance enzymatic hydrolysis

In this work an evaluation was made of a wide variety of single and multiple pretreatment methods for enhancing the rate of enzymatic hydrolysis of wheat straw. A multiple pretreatment consisted of a physical pretreatment followed by a chemical pretreatment. The structural features of wheat straw, including the specific surface area, crystallinity index, and lignin content, were measured to understand the mechanism of the enhancement in the hydrolysis rate upon pretrement. It has been found that, in general, multiple pretreatments were not promising, since the hydrolysis rates rarely exceeded those achieved by single pretreatments. Ballmilling pretreatment was found to be effective in increasing the specific surface area and decreasing the crystallinity index. Treatment with ethylene glycol was highly effective in increasing the specific surface area, in addition to a high degree of delignification. Peracetic acid pretreatment was highly effective in delignifying substrate. Among multiple pretreatments, those involving peracetic acid treatment generally had lower crystallinity indices and lignin content values. The relationship between the hydrolysis rate and the set of structural features indicated that an increase in surface area and a decrease in the crystallinity and lignin content enhance the hydrolysis; the specific surface area is the most influential of the structural features, followed by the lignin content.     

Inhibition of enzymatic hydrolysis by residual lignins from softwood--study of enzyme binding and inactivation on lignin-rich surface

Lignin-derived inhibition is a major obstacle restricting the enzymatic hydrolysis of cell wall polysaccharides especially with softwood lignocellulosics. Enzyme adsorption on lignin is suggested to contribute to the inhibitory effect of lignin. The interaction of cellulases with softwood lignin was studied in the present work with commercial Trichoderma reesei cellulases (Celluclast) and lignin-rich residues isolated from steam pretreated softwood (SPS) by enzymatic and acid hydrolysis. Both lignin preparations inhibited the hydrolysis of microcrystalline cellulose (Avicel) and adsorbed the major cellulases present in the commercial cellulase mixture. The adsorption phenomenon was studied at low temperature (4°C) and at the typical hydrolysis temperature (45°C) by following activities of free and lignin-bound enzymes. Severe inactivation of the lignin-bound enzymes was observed at 45°C, however at 4°C the enzymes retained well their activity. Furthermore, SDS-PAGE analysis of the lignin-bound enzymes indicated that very strong interactions form between the residue and the enzymes at 45°C, because the enzymes were not released from the residue in the electrophoresis. These results suggest that heat-induced denaturation may take place on the surface of softwood lignin at the hydrolysis temperature.     

Enhancing the enzymatic hydrolysis of lignocellulosic biomass by increasing the carboxylic acid content of the associated lignin

To assess the effects that the physical and chemical properties of lignin might have on the enzymatic hydrolysis of pretreated lignocellulosic substrates, protease treated lignin (PTL) and cellulolytic enzyme lignin (CEL) fractions, isolated from steam and organosolv pretreated corn stover, poplar, and lodgepole pine, were prepared and characterized. The adsorption of cellulases to the isolated lignin preparations corresponded to a Langmuir adsorption isotherm. It was apparent that, rather than the physical properties of the isolated lignin, the carboxylic acid functionality of the isolated lignin, as determined by FTIR and NMR spectroscopy, had much more of an influence when lignin was added to typical hydrolysis of pure cellulose (Avicel). An increase in the carboxylic content of the lignin preparation resulted in an increased hydrolysis yield. These results suggested that the carboxylic acids within the lignin partially alleviate non-productive binding of cellulases to lignin. To try to confirm this possible mechanism, dehydrogenative polymers (DHP) of monolignols were synthesized from coniferyl alcohol (CA) and ferulic acid (FA), and these model compounds were added to a typical enzymatic hydrolysis of Avicel. The DHP from FA, which was enriched in carboxylic acid groups compared with the DHP from CA, adsorbed a lower mount of cellulases and did not decrease hydrolysis yields when compared to the DHP from CA, which decreased the hydrolysis of Avicel by 8.4%. Thus, increasing the carboxylic acid content of the lignin seemed to significantly decrease the non-productive binding of cellulases and consequently increased the enzymatic hydrolysis of the cellulose.     

Pretreatment and saccharification of rice hulls for the production of fermentable sugars

Hydrolytic conditions of rice hulls by acid and alkaline treatments before enzymatic saccharification were optimized in this study. Based on the results of single-factor experiments and an orthogonal array experiment, reaction time was found to be the most important factor for the acidic hydrolysis of rice hulls. Maximum yield of sugars from 1 g of rice hulls by acidic treatment under optimized conditions was 213.6 mg. The yield of lignin removal from acidic pretreated rice hulls by alkaline treatment increased with increase in reaction temperature and time. The amount of sugars obtained from 1 g of pretreated rice hulls by enzymatic saccharification was 307.7 mg, and the conversion rate of sugars from crude fibers in pretreated rice hulls was about 72%. Instrumental analyses with FTIR and SEM indicated that lignin in rice hulls was partially removed by alkaline treatment, and the structure of rice hulls became deformed and more fibers were exposed to cellulases after acidic treatment.     

Laccase-Mediator Pretreatment of Wheat Straw Degrades Lignin and Improves Saccharification

Agricultural by-products such as wheat straw are attractive feedstocks for the production of second-generation bioethanol due to their high abundance. However, the presence of lignin in these lignocellulosic materials hinders the enzymatic hydrolysis of cellulose. The purposes of this work are to study the ability of a laccase-mediator system to remove lignin improving saccharification, as a pretreatment of wheat straw, and to analyze the chemical modifications produced in the remaining lignin moiety. Up to 48 % lignin removal from ground wheat straw was attained by pretreatment with Pycnoporus cinnabarinus laccase and 1-hydroxybenzotriazole (HBT) as mediator, followed by alkaline peroxide extraction. The lignin removal directly correlated with increases (～60 %) in glucose yields after enzymatic saccharification. The pretreatment using laccase alone (without mediator) removed up to 18 % of lignin from wheat straw. Substantial lignin removal (37 %) was also produced when the enzyme-mediator pretreatment was not combined with the alkaline peroxide extraction. Two-dimensional nuclear magnetic resonance (2D NMR) analysis of the whole pretreated wheat straw material swollen in dimethylsulfoxide-d ₆ revealed modifications of the lignin polymer, including the lower number of aliphatic side chains involved in main β-O-4′ and β-5′ inter-unit linkages per aromatic lignin unit. Simultaneously, the removal of p-hydroxyphenyl, guaiacyl, and syringyl lignin units and of p-coumaric and ferulic acids, as well as a moderate decrease of tricin units, was observed without a substantial change in the wood polysaccharide signals. Especially noteworthy was the formation of Cα-oxidized lignin units during the enzymatic treatment.     

Pulp properties and their influence on enzymatic degradability

Endoglucanase treatment of pulp for the adjustment of viscosity and the increase in pulp reactivity is a promising step in the concept for the beneficial production of dissolving pulps from paper grade pulps. To promote the commercial applicability of these enzymes, the influence of pulp properties such as carbohydrate composition, pulp type and cellulose morphology on the enzymatic degradability of a pulp was examined. High contents of hemicelluloses and lignin were shown to impair the accessibility of the cellulose to the enzymes. Due to the elevated swelling capacity of cellulose II, conversion of the cellulose morphology from I to II upon alkaline treatments showed a large increasing effect on the cellulose accessibility, and enzymatic degradability. Reactivity measurements of softwood sulfite pulps after enzymatic degradation and acid-catalyzed hydrolysis, respectively, revealed elevated reactivity for the pulp after acid treatment. This is in contrast to effects of enzyme treatments reported for CCE treated kraft pulps.     

The effect of lignin model compound structure on the rate of oxidation catalyzed by two different fungal laccases

Laccases (EC 1.10.3.2) are multicopper oxidases able to oxidize various substrates, such as phenolic subunits of lignin. The substrate range can be widened to non-phenolic units by the use of mediators. Since discovery of the laccase-mediator system, direct reactions of lignin and laccase without mediated electron-transfer have gained much less attention. The objective of this study was to investigate lignin as a substrate for fungal laccases by using lignin model compounds. These model compounds contained guaiacylic and syringylic moieties and also compounds of guaiacylic origin at a higher oxidation level. Some of these compounds are commercially available, but most of them were synthesized. The oxidation reaction rates of the lignin model compounds were studied by monitoring consumption of the co-substrate oxygen, in reactions catalyzed by laccases from two different fungi; Melanocarpus albomyces and Trametes hirsuta, possessing different molecular and catalytic properties. These reaction rate studies were compared to physicochemical properties of the lignin model compounds: relative redox potentials determined using cyclic voltammetry and pK_a-values. Docking of syringylic and biphenylic compounds to the active sites of both laccases was performed and the resulting model complex structures were used to further interpret the reaction rate results. Reaction rates of laccases are mainly affected by the ability of a lignin model compound to be oxidized and the pK_a-value of the substrate seems to be less important. As a consequence, syringylic compounds are oxidized with the highest rates and compounds at a higher oxidation level and redox-potential, such as vanillin, are oxidized at a much lower rate. Both guaiacylic and syringylic type compounds fit well in the active sites of both laccases. Only for a biphenylic compound, steric clashes were observed, and they are likely to have an effect on the reaction rate. When the oxidation rates on the selected model compounds with the two different laccases were compared, the redox-potential difference between laccases T1 copper and the lignin model compound (ΔE) was not the only property that determined the oxidation rate. In the case of lignin model substrates, also the selectivity of a specific laccase, reflected in the k_cat/K_m value, plays an important role.