Hexavalent chromate reduction by alkaliphilic Amphibacillus sp. KSUCr3 is mediated by copper-dependent membrane-associated Cr(VI) reductase

Abstract

The present study was aimed to localize and characterize hexavalent chromate [Cr(VI)] reductase activity of the extreme alkaliphilic Amphibacillus sp. KSUCr3 (optimal growth pH 10.5). The resting cells were able to reduce about 62 % of the toxic heavy metal Cr(VI) at initial concentration of 200 μM within 30 min. Cell permeabilization resulted in decrease of Cr(VI) reduction in comparison to untreated cells. Enzymatic assays of different sub-cellular fractions of Amphibacillus sp. KSUCr3 demonstrated that the Cr(VI) reductase was mainly associated with the membranous fraction and expressed constitutively. In vitro studies of the crude enzyme indicated that copper ion was essential for Cr(VI) reductase activity. In addition, Ca²⁺ and Mn²⁺ slightly stimulated the chromate reductase activity. Glucose was the best external electron donor, showing enhancement of the enzyme activity by about 3.5-fold. The K (m) and V (max) determined for chromate reductase activity in the membranous fraction were 23.8 μM Cr(VI) and 72 μmol/min/mg of protein, respectively. Cr(VI) reductase activity was maximum at 40 °C and pH 7.0 and it was significantly inhibited in the presence of disulfide reducers (2-mercaptoethanol), ion chelating agent (EDTA), and respiratory inhibitors (CN and Azide). Complete reduction of 100 and 200 μM of Cr(VI) by membrane associated enzyme were observed within 40 and 180 min, respectively. However, it should be noted that biochemical characterization has been done with crude enzyme only, and that final conclusion can only be drawn with the purified enzyme.