Review Bacterial chromate reductase, a potential enzyme for bioremediation of hexavalent chromium: A review Hrudayanath Thatoi a, * , Sasmita Das a , Jigni Mishra a , Bhagwat Prasad Rath a , Nigamananda Das b a Department of Biotechnology, College of Engineering and Technology, Biju Patnaik University of Technology, Techno-Campus, Ghatikia, Bhubaneswar 751003, Odisha, India b Department of Chemistry, North Orissa University, Takatpur, Baripada 757003, Odisha, India article info Article history: Received 28 March 2014 Received in revised form 3 July 2014 Accepted 10 July 2014 Available online 8 September 2014 Keywords: Bioremediation Hexavalent chromium Chromium resistance Chromate reductase abstract Hexavalent chromium is mobile, highly toxic and considered as a priority environmental pollutant. Chromate reductases, found in chromium resistant bacteria are known to catalyse the reduction of Cr(VI) to Cr(III) and have recently received particular attention for their potential use in bioremediation process. Different chromate reductases such as ChrR, YieF, NemA and LpDH, have been identified from bacterial sources which are located either in soluble fractions (cytoplasm) or bound to the membrane of the bacterial cell. The reducing conditions under which these enzymes are functional can either be aerobic or anaerobic or sometimes both. Enzymatic reduction of Cr(VI) to Cr(III) involves transfer of electrons from electron donors like NAD(P)H to Cr(VI) and simultaneous generation of reactive oxygen species (ROS). Based on the steps involved in electron transfer to Cr(VI) and the subsequent amount of ROS generated, two reaction mechanisms, namely, Class I “tight” and Class II “semi tight” have been proposed. The present review discusses on the types of chromate reductases found in different bacteria, their mode of action and potential applications in bioremediation of hexavalent chromium both under free and immobilize conditions. Besides, techniques used in characterization of the Cr (VI) reduced products were also discussed. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Environmental pollution due to indiscriminate discharge of hazardous and harmful wastes containing toxic heavy metals at elevated concentrations from industries and mining sites has been a growing concern all over the world and therefore, underline the importance of applying effective treatment methods to reduce the concentration of heavy metals down to acceptable limit. Among various approaches, bioremediation using biological agents such as bacteria, fungi, and their enzyme is one of the attractive and effective methods for cleaning the environment from toxic pollut- ants (Ruggaber and Talley, 2006). The microorganisms play an important role in bioremediation processes which is, however, limited by several factors. For instance, the microorganisms that are actively involved in the bioremediation of a specific pollutant might be inhibited by other pollutants present in the same environment. Further, the rate of degradation of pollutants by microorganisms is often very slow which limits the feasibility of using them in practice for bioremediation processes (Whiteley and Lee, 2006). In this context, the use of sole enzymes isolated from bacterial species is more advantageous than using whole microorganisms as revealed from several studies undertaken during last few years (Sutherland et al., 2004; Pieper et al., 2004). Moreover, the enzymatic bio- transformations do not generate toxic side products as often found in the case of chemical and some microbiological processes and therefore, possess less risk of biological contamination on ecosystem. Their action is specific to the substrate in comparison to microorganisms and they are also more mobile than microorgan- isms because of their smaller size (Gianfreda and Bollag, 2002). Although the enzymatic treatment processes have tremendous scope for bioremediation, its practical application often faces with several challenges in terms low activity, productivity and stability of the enzyme in addition to sustainability of their application. Efforts are on in search of potential microbes capable of producing enzymes that can transform the toxic metal ions to their less/non- toxic forms under wide range of environmental conditions (e.g. pH, * Corresponding author. E-mail address: hn_thatoi@rediffmail.com (H. Thatoi). Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman http://dx.doi.org/10.1016/j.jenvman.2014.07.014 0301-4797/© 2014 Elsevier Ltd. All rights reserved. Journal of Environmental Management 146 (2014) 383e399