Microbial Chromate Reductases: Novel and Potent Mediators in Chromium Bioremediation-A Review

Heavy metal pollution from the growing industrialization are a significant cause of environmental concern.Chromium (Cr) is commonly used in the production of stainless steel, textile dyeing, electroplating, as nuclear coolants and largely in chrome tanning of hides and skins. About 90% of leather is produced by chrome tanning and the leather industry contributes to an overload of Cr toxicity in tannery effluents. Accumulation of Cr6+ is carcinogenic, genotoxic and teratogenic to organisms. Biological methods are ‘green’ approaches for chromium bioremediation and microorganisms are the desired candidates for pollution abatement. Microbial chromate reduction is mediated by chromate reductases (ChrRs) which may be expressed constitutively or inducibly. ChrRs have been produced by a number of bacteria, fungi and yeasts and may be extracellular or localized in the membrane or cytosol. ChrRs are dependent on electron donors such as reduced Nicotinamide adenine dinucleotide (NADH) or reduced Nicotinamide adenine dinucleotide phosphate (NADPH) or reduced Glutathione (GSH) as cofactors. In chromate reduction by ChrRs, Cr6+ undergo one electron transfer to producean unstable Cr5+ radical that is converted to stable and less toxic Cr3+. Putative ChrR genomic sequences have been studied with 99% sequence similarity in Gram negative bacteria. ChrRs are valuable resources in different environments for chromate reduction. This review is to discuss the expression and characteristics of ChrRs and their mechanisms in reduction of Cr6+ toxicity in order to provide a comprehensive understanding of this novel class of enzymes for promisingapplications in Cr bioremediation.

Vibrio harveyi Nitroreductase Is Also a Chromate Reductase

ABSTRACT The chromate reductase purified from Pseudomonas ambigua was found to be homologous with several nitroreductases. Escherichia coli DH5α and Vibrio harveyi KCTC 2720 nitroreductases were chosen for the present study, and their chromate-reducing activities were determined. A fusion between glutathione S-transferase (GST) and E. coli DH5α NfsA (GST-EcNfsA), a fusion between GST and E. coli DH5α NfsB (GST-EcNfsB), and a fusion between GST and V. harveyi KCTC 2720 NfsA (GST-VhNfsA) were prepared for their overproduction and easy purification. GST-EcNfsA, GST-EcNFsB, and GST-VhNFsA efficiently reduced nitrofurazone and 2,4,6-trinitrotoluene (TNT) as their nitro substrates. The Km values for GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA for chromate reduction were 11.8, 23.5, and 5.4 μM, respectively. The V max values for GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA were 3.8, 3.9, and 10.7 nmol/min/mg of protein, respectively. GST-VhNfsA was the most effective of the three chromate reductases, as determined by each V max/Km value. The optimal temperatures of GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA for chromate reduction were 55, 30, and 30°C, respectively. Thus, it is confirmed that nitroreductase can also act as a chromate reductase. Nitroreductases may be used in chromate remediation. GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA have a molecular mass of 50 kDa and exist as a monomer in solution. Thin-layer chromatography showed that GST-EcNfsA, GST-EcNfsB, and GST-VhNfsA contain FMN as a cofactor. GST-VhNfsA reduced Cr(VI) to Cr(III). Cr(III) was much less toxic to E. coli than Cr(VI).