Cleavage and synthesis function of high and low redox potential laccases towards 4-morpholinoaniline and aminated as well as chlorinated phenols

Abstract

Laccases are able to mediate both cleavage and synthesis processes. The basis for this dual reaction capability lies in the property of the enzyme laccase to oxidize phenolic, and to some extent non-phenolic substances, to reactive radicals which can undergo on the one hand separations of small substitutents or large molecule parts from the parent compound and on the other hand coupling reactions with other radicals or molecules which are not themselves oxidizable by laccase. The cleavage of the non-phenolic compound 4-morpholinoaniline as well as the deamination of 4-aminophenol and the dechlorination of 4-chlorophenol resulted in the formation of 1,4-hydroquinone which is immediately oxidized by laccase to 1,4-benzoquinone. The formation of the 1,4-hydroquinone/1,4-benzoquinone is the rate limiting step for the synthesis of the heteromolecular dimers and trimers composed of 1,4-benzoquinone and one or two molecules of morpholine. In addition to the synthesis of new compounds from the cleavage products, 4-morpholinoaniline polymerized probably via azo groups and C-N bonds to a homomolecular dimer and trimer. Similarities and differences in cleavage and synthesis reactions catalyzed by the low redox potential laccase of Myceliophthora thermophila (0.46 V) and the high redox potential laccase of Pycnoporus cinnabarinus (0.79 V) were determined. In addition, the dependency of the cleavage and synthesis efficiencies on the (a) structure and redox potential of the laccase, (b) structure and redox potential of the substrate, (c) pH value of the buffer used, (d) incubation temperature, (e) solvent concentration, and (f) laccase activity is discussed in general.

Enzymatic Processes to Unlock the Lignin Value

“…One of the challenges in laccase application for lignin depolymerization as noted by many experts is the fact that the strong polymerization activity of laccase counters the decomposition of lignin by re-polymerizing the degradation products (Roth and Spiess, 2015 ). It was postulated (Hahn et al, 2014 ) that the balance between polymerization and cleavage reactions depend on the structure and redox potential of the laccase, structure and redox potential of the substrate, pH value of the buffer used, incubation temperature, and solvent concentration. Thus, thorough process development is required for directing the reaction into the desired path (Santhanam et al, 2011 ; Singh et al, 2015 ).…”

The taste enhancer divanillin: a review on sources and enzymatic generation

“…This showed that the random formation of phenolic radicals may be channelled into a concerted dimerization. Different reaction conditions may favour polymerization or substitution and cleavage reactions depending on the type of laccase and mediator. No H 2 O 2 or other co‐substrates (except oxygen) are required for the action of laccases, while peroxidases, as used previously, depend on H 2 O 2 by definition.…”

Identification of benzoquinones in pretreated lignocellulosic feedstocks and inhibitory effects on yeast

“…It is well known that laccase-catalyzed oxidation of certain phenolic compounds can result in formation of quinoid products (Lundquist and Kristersson 1985 ). BQ has been reported as a product of oxidation by laccase of 1,4-hydroquinone (Hahn et al 2014 ). Therefore it is noteworthy in this context that while laccase typically has a general positive effect on the fermentability of lignocellulosic hydrolysates, which can be attributed to phenol polymerization, there is at least a theoretical risk with respect to hydroquinones that oxidation reactions to some extent can result in the formation of toxic benzoquinones.…”