The NprA nitroreductase required for 2,4‐dinitrophenol reduction in Rhodobacter capsulatus is a dihydropteridine reductase

Abstract

The Rhodobacter capsulatus nprA gene codes for a putative nitroreductase. A recombinant His(6)-NprA protein was overproduced in Escherichia coli and purified by affinity chromatography. This protein contained FMN and showed nitroreductase activity with a wide range of nitroaromatic compounds, such as 2-nitrophenol, 2,4-dinitrophenol, 2,6-dinitrophenol, 2,4,6-trinitrophenol (picric acid), 2,4-dinitrobenzoate and 2,4-dinitrotoluene, and with the nitrofuran derivatives nitrofurazone and furazolidone. NADPH was the main electron donor and the ortho nitro group was preferably reduced to the corresponding amino derivative. The apparent K(m) values of NprA for NADPH, 2,4-dinitrophenol, picric acid and furazolidone were 40 microM, 78 microM, 72 microM and 83 microM, respectively, at pH and temperature optima (pH 6.5, 30 degrees C). Escherichia coli cells overproducing the NprA protein were much more sensitive to the prodrug 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB1954) used in cancer therapy than non-transformed cells. NprA showed the highest activity with the quinonoid form of 6,7-dimethyl-7,8-dihydropterine as substrate, so that NprA may be involved in the synthesis of tetrahydrobiopterin in R. capsulatus. Expression of a transcriptional nprA-lacZ gene fusion was induced by phenylalanine or tyrosine, but not by other amino acids like glutamate or alanine. Furthermore, both nitroreductase activity and phenylalanine assimilation were inhibited in vivo by ammonium. A mutant defective in the nprA gene showed better growth rate with Phe or Tyr as nitrogen source than the wild-type strain, although both strains showed similar growth in media with Glu or without added nitrogen. These results suggest that the NprA nitroreductase may act in vivo as a dihydropteridine reductase involved in aromatic amino acids metabolism.

Understanding the Broad Substrate Repertoire of Nitroreductase Based on Its Kinetic Mechanism

“…of possibilities to be considered (5,39). Recent data suggest a role as a dihydropteridine reductase for the NR homologue of Rhodobacter capsulatus (40). Structural explanations for the broad substrate repertoires of NR homologues have been discussed (4,41).…”

Reduction of polynitroaromatic compounds: the bacterial nitroreductases

“…However, the physiological roles of nitroreductases as lipoamide dehydrogenase or dihydropteridine reductase have not yet been demonstrated. We have recently found that the purified NprA nitroreductase from R. capsulatus shows dihydropteridine reductase activity in vitro (Pérez‐Reinado et al , 2008).…”

The microbial nitrogen cycle

“…In addition to the conversion of inorganic nitrogen molecules discussed above, many bacteria are capable to transform recalcitrant organic nitrogen compounds under various environmental conditions. In this issue one such an example, the conversion of 2,4‐dinitrophenol by Rhodobacter capsulatus , is presented (Pérez‐Reinado et al ., 2008). In this study the npr A gene, encoding for a putative nitroreductase, was heterologously expressed in E. coli and shown to catalyse the reduction a number of nitroaromatic compounds using NADPH as electron donor.…”