Chromate Reductase 5 (CRB5) Catalyzes the Reduction of CrVII to CrIII

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Chromate is a toxic metal, which can cause skin allergies and irritation. Chromate is also an important oxidizing agent in certain industrial processes. It is used to form a protective coating for copper, nickel and cobalt alloys.

CRB5 has a number of strategies to deal with chromate. One is the production of a soluble enzyme that catalyzes the reduction of Cr(VI) to Cr(III). This process detoxifies the cytoplasm of the cell by reducing chromium from solution and binding it to electronegative surface functional groups on the cell envelope and capsule exopolymers. The reduced chromium is then released from the cytoplasm and precipitates to form chromate deposits (4,5).

The enzyme activity of CRB5 was studied under aerobic and anaerobic conditions using whole-cell suspensions. These were added to Balch tubes containing VB medium and glucose containing various concentrations of chromate, copper and arsenic. Aerobic reduction tests were conducted in crimp-sealed or screw-cap Balch tubes leaving a headspace filled with air and were sampled using sterile needles and syringes.

Anaerobic reduction of cells under resting conditions was enhanced when lactate was present as an electron donor and ceased after 70 h. This may be a detoxification response or it could indicate that lactate is a terminal electron acceptor in anaerobic respiration and that it enhances the reductive activity of the enzyme.

Pseudomonas ambigua G-1, Pseudomonas putida PRS2000 and Pseudomonas fluorescens LB300 have soluble reductases that reduce chromate under both aerobic and anaerobic conditions. The soluble reductases can be activated by acetic acid or ethanol and the reduction rates increased with a higher initial chromium concentration. This is a different mechanism from membrane-bound reduction activity of Pseudomonas ambivalence G-1 and Pseudomonas putida MK1 (19, 30). The presence of both soluble and membrane-associated reduction systems in chromate-reducing bacteria suggests that the processes are distinct.