Bioremediation of Highly Toxic Hexavalent Chromium by Bacterial Chromate Reductases Family: A Structural and Functional Overview

Chromium is a heavy metal with a vast range of industrial applications. However, extensive mining of chromite ore and its industrial application pose a serious threat to human health due to emission of toxic hexavalent chromium ions to the environment. Cr(VI) (chromate) is highly soluble and toxic ion which is a widespread environmental contaminant. Presently, several physicochemical methods are there for detoxification of Cr(VI) which are associated with high cost and typically cause secondary pollution. In this context, bacteria-mediated chromate reduction process is both economical and eco-friendly. Different chromate reductases have been identified from bacterial sources. According to sub-cellular localization, bacterial chromate reductase has been classified as cytoplasmic and membrane bound. Cytoplasmic chromate reductases are promising candidate for protein engineering and thus can be used in bioremediation program. The enzyme chromate reductase can catalyze its reduction mechanism in either aerobic or anaerobic condition. Chromate reductase–induced reduction of Cr(VI) to Cr(III) is mediated by the shifting of electrons from a nucleophilic cofactor like NAD(P)H to Cr(VI) and concomitant production of reactive oxygen species (ROS). Depending upon the steps present in the reduction process and the amount of ROS generation, two mechanism of action for the enzyme chromate reductase has been proposed known as Class I “tight” and Class II “semi-tight.” This chapter highlights various classes of bacterial chromate reductases, their three-dimensional structure, functions, and prospective uses in the bioremediation of hexavalent chromium.