The low molecular-mass organic compound 4-nitrophenol is involved in many chemical processes and is commonly present in soils and in surface and ground waters, thereby causing severe environmental impact and health risk. Several methods have been proposed for its transformation (bio and chemical degradation). However, these strategies not only produce equally or more toxic aromatic species but also require harsh operating conditions and/or time-consuming treatments. In this context, we report a comprehensive and systematic study of the electrochemical reduction of 4-nitrophenol as a viable alternative. We have explored the electrochemical reduction of this pollutant over different metallic and carbonaceous substrata. Specifically, we have focused on the use of gold and silver working electrodes since they combine a high electrocatalytic activity for 4-nitrophenol reduction and a low electrocatalytic capacity for hydrogen evolution. The influence of the pH, temperature, and applied potential have also been considered as crucial parameters in the overall optimization of the process. While acidic media and high temperatures favor the clean reduction of 4-nitrophenol to 4-aminophenol, the simultaneous hydrogen evolution is pernicious for this purpose. Herein, a simple and effective electrochemical method for the transformation of 4-nitrophenol into 4-aminophenol is proposed with virtually no undesired by-products.

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Simple Environmentally-Friendly Reduction of 4-Nitrophenol

Kinetic modelling of the uranium biosorption by Deinococcus radiodurans biofilm.

Novel strategy for biodegradation of 4-nitrophenol by the immobilized cells of Pseudomonas sp. YPS3 with Acacia gum.

A two-component monooxygenase for continuous denitration and dechlorination of chlorinated 4-nitrophenol in Ensifer sp. strain 22-1.

Flavin-dependent dehalogenases use flavin as a cofactor to catalyze carbon-halogen (C-X) bond cleavage from halogenated compounds which are mainly distributed as persistent environmental pollutants via anthropogenic activities. The accumulation of these compounds results in adaptation of bacteria to evolve metabolic pathways to metabolize the agents for four decades. Flavin-dependent enzymes have been evolved to catalyze dehalogenation in addition to its basal function. Apart from bacterial biodegradation, flavin-dependent dehalogenases also naturally appear in cellular metabolisms of higher organisms such as in human thyroid hormone. Although the removal of halogen is required in various applications, the usage of dehalogenases remains limited. In-depth understanding of their enzymatic mechanisms is useful for development of dehalogenases applications. Three main types of flavin-dependent dehalogenases are classified based on their reaction mechanisms reported to date: (1) flavin-dependent O2-utilizing dehalogenases; (2) flavin-dependent reductive dehalogenases; and (3) non-redox flavin-dependent dehalogenases. In this chapter, the catalytic properties, substrate scope, protein structures, enzymatic mechanisms, enzyme engineering, and also development of enzymes for novel applications are discussed.

Flavin-dependent dehalogenases.

Microbial degradation kinetics and molecular mechanism of 2,6-dichloro-4-nitrophenol by a Cupriavidus strain

2-Chloro-4-nitrophenol (2C4NP) is the most common chlorinated nitrophenol pollutant, and its environmental fate is of great concern. Cupriavidus sp. CNP-8, a Gram-negative bacterium, has been reported to degrade 2C4NP via the 1,2,4-benzenetriol (BT) pathway, significantly different from the (chloro)hydroquinone pathways reported in all other Gram-negative 2C4NP-utilizers. Herein, the BT pathway of the catabolism of 2C4NP in this strain was characterized at the molecular, biochemical, and genetic levels. The hnp gene cluster was suspected to be involved in the catabolism of 2C4NP because the hnp genes are significantly upregulated in the 2C4NP-induced strain CNP-8 compared to the uninduced strain. HnpAB, a two-component FAD-dependent monooxygenase, catalyzes the conversion of 2C4NP to BT via chloro-1,4-benzoquinone, with a Km of 2.7 ± 1.1 μΜ and a kcat/Km of 0.17 ± 0.03 μΜ-1 min-1. hnpA is necessary for strain CNP-8 to utilize 2C4NP in vivo. HnpC, a BT 1,2-dioxygenase, was proved to catalyze BT ring-cleavage with formation of maleylacetate by HPLC-MS analysis. Phylogenetic analysis indicated that HnpA likely has different evolutionary origin compared to other functionally identified 2C4NP monooxygenases. To our knowledge, this is the first report revealing the catabolic mechanism of 2C4NP via the BT pathway in a Gram-negative bacterium, increasing our knowledge of the catabolic diversity for microbial 2C4NP degradation at the molecular and biochemical level.

http://ir.yic.ac.cn/bitstream/133337/24871/1/Molecular%20and%20biochemical%20characterization%20of%202-chloro-4-nitrophenol%20degradation%20via%20the%201%2c2%2c4-benzenetriol%20pathway%20in%20a%20Gram-negative%20bacterium.pdf

Lingxue Xu

Papers

Microbial degradation kinetics and molecular mechanism of 2,6-dichloro-4-nitrophenol by a Cupriavidus strain

Molecular and biochemical characterization of 2-chloro-4-nitrophenol degradation via the 1,2,4-benzenetriol pathway in a Gram-negative bacterium