Selenate reduction by Enterobacter cloacae SLD1a-1 is catalysed by a molybdenum-dependent membrane-bound enzyme that is distinct from the membrane-bound nitrate reductase

Enterobacter cloacae SLD1a-1 is capable of reducing selenium oxyanions to elemental selenium under both aerobic and anaerobic conditions. In this study the enzyme that catalyses the initial reduction of selenate (SeO42−) to selenite (SeO32−) has been localised to isolated cytoplasmic membrane fractions. Experiments with intact cells have shown that the putative selenate reductase can accept electrons more readily from membrane-impermeable methyl viologen than membrane-permeable benzyl viologen, suggesting that the location of the catalytic site is towards the periplasmic side of the cytoplasmic membrane. Enzyme activity was enhanced by growing cells in the presence of 1 mM sodium molybdate and significantly reduced in cells grown in the presence of 1 mM sodium tungstate. Non-denaturing polyacrylamide gel electrophoresis (PAGE) gels stained for selenate and nitrate reductase activity have revealed that two distinct membrane-bound enzymes catalyse the reduction of selenate and nitrate. The role of this membrane-bound molybdenum-dependent reductase in relation to selenate detoxification and energy conservation is discussed.