Redox Chemistry in Laccase-Catalyzed Oxidation of N-Hydroxy Compounds

Abstract

1-Hydroxybenzotriazole, violuric acid, and N-hydroxyacetanilide are three N-OH compounds capable of mediating a range of laccase-catalyzed biotransformations, such as paper pulp delignification and degradation of polycyclic hydrocarbons. The mechanism of their enzymatic oxidation was studied with seven fungal laccases. The oxidation had a bell-shaped pH-activity profile with an optimal pH ranging from 4 to 7. The oxidation rate was found to be dependent on the redox potential difference between the N-OH substrate and laccase. A laccase with a higher redox potential or an N-OH compound with a lower redox potential tended to have a higher oxidation rate. Similar to the enzymatic oxidation of phenols, phenoxazines, phenothiazines, and other redox-active compounds, an "outer-sphere" type of single-electron transfer from the substrate to laccase and proton release are speculated to be involved in the rate-limiting step for N-OH oxidation.Laccases (EC 1.10.3.2) are multi-Cu oxidases that can catalyze the oxidation of a range of reducing substances with the concomitant reduction of O 2 (for recent reviews, see reference 24 and references therein). Because of their capability of catalyzing the oxidation of aromatic compounds, laccases are receiving increasing attention as potential industrial enzymes in various applications, such as pulp delignification, wood fiber modification, dye or stain bleaching, chemical or medicinal synthesis, and contaminated water or soil remediation (15, 37).Laccases contain one type 1 (T1) Cu center, one type 2 (T2) Cu center, and one type 3 (T3) Cu center. The T2 and T3 sites form a trinuclear Cu cluster onto which O 2 is reduced. The T1 Cu oxidizes the reducing substrate and transfers electrons to the T2 and T3 Cu. Laccase is able to oxidize certain phenols with E 0 values higher than its own (0.5 to 0.8 V versus the normal hydrogen electrode [NHE]) (36). However, many inorganic and organic compounds with comparable E 0 values (such as 1,2,3,5-tetramethoxybenzene [18]) are not laccase substrates due to unfavorable kinetics. Under certain conditions, however, these compounds can be indirectly oxidized by laccase through the mediation of small, redox-active laccase substrates. 2,2Ј-Azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) was the first compound found capable of efficiently mediating the laccase oxidation of high-E 0 , nonsubstrate lignin model compounds (such as veratryl alcohol and nonphenolic lignin model dimers) (8). Based on product structure analysis, it has been proposed that laccase-oxidized ABTS can abstract an H atom from the lignin model compounds, leading to indirect laccase catalysis upon the oxidation of the compounds (25). To date, other types of mediators, particularly phenoxazines and N-OH compounds, also have been recognized for their mediation function in laccase catalysis (1, 6,17,29).Mediated laccase catalysis has been applied to a wide range of applications, such as pulp delignification (9, 10, 12, 22, 32), textile dye bleaching (31), polycyclic aromatic h...

Voltammetric monitoring of laccase-catalysed mediated reactions

“…Thorough studies previously reported by Xu et al [1,9,[18][19][20] evidenced a clear relationship between the enzyme activity and the potential difference between its active T1 site and the formal potential of the mediator (DE*). Moreover, the OH À contribution to this activity profile should be taken into consideration.…”

“…Xu et al [19] carried out voltammetric studies to elucidate the redox chemistry of various N-hydroxy compounds used as laccase substrates or different redox couples applied for the estimation of the redox potential at the T1 site of several laccases [21]. Likewise, Bourbonnais et al [8] carried out a thorough voltammetric study of the redox chemistry of two different laccase mediators and their interaction with lignin model compounds, in which a glassy carbon electrode (GCE) simulates the laccase activity towards both electron donors.…”

Efficient bleaching of non-wood high-quality paper pulp using laccase-mediator system

“…On the other hand, the laccase from P. cinnabarinus was selected for flax bleaching because of its higher selectivity removing residual lignin. This is a high redox potential laccase [17,18] that can be produced in high levels for different applications using selected monokaryotic strains of P. cinnabarinus [34]. This laccase has been extensively investigated during last years including gene cloning [35], protein crystallization [36], and expression in different host systems [37].…”

“…These mediators enhance the oxidative capabilities of the enzyme enabling oxidation of non-phenolic lignin [11]. During last decade, much research has focused on the laccase-mediator system for pulp bleaching including understanding the action mechanisms [12][13][14] and description of more suitable mediators [15][16][17][18]. Several patents on laccase-mediator bleaching have been deposited since the first Call's patent [19].…”

Paper pulp delignification using laccase and natural mediators

“…One of them is the cost of the synthetic mediators, which is difficult to lower enough for economic application. Moreover, a concern exists about the possible toxicity of some of the most powerful laccase mediators, such as the -N(OH)-compounds [15], or their reaction products.…”