Microbial reduction of selenate and nitrate: common themes and variations

Abstract

A number of biochemically distinct systems have been characterized for the microbial reduction of the oxyanions, selenate (SeO(4)(2-)) and nitrate (NO(3)(-)). Two classes of molybdenum-dependent nitrate reductase catalyse the respiratory-linked reduction of nitrate (NO(3)(-)) to nitrite (NO(2)(-)). The main respiratory nitrate reductase (NAR) is membrane-anchored, with its active site facing the cytoplasmic compartment. The other enzyme (NAP) is water-soluble and located in the periplasm. In recent years, our understanding of each of these enzyme systems has increased significantly. The crystal structures of both NAR and NAP have now been solved and they provide new insight into the structure, function and evolution of these respiratory complexes. In contrast, our understanding of microbial selenate (SeO(4)(2-)) reduction and respiration is at an early stage; however, similarities to the nitrate reductase systems are emerging. This review will consider some of the common themes and variations between the different classes of nitrate and selenate reductases.

Resolution of Distinct Membrane-Bound Enzymes fromEnterobacter cloacaeSLD1a-1 That Are Responsible for Selective Reduction of Nitrate and Selenate Oxyanions

“…Previous studies of whole cells of E. cloacae SLD1a-1 have suggested that the membrane-bound nitrate reductase is not involved in the process of selenate reduction (31). To test this hypothesis, the nitrate reductase was purified and assayed for selenate reductase activity in vitro.…”

“…Similarly, both NarG and NarZ in membrane fractions from Escherichia coli have been reported to show selenate reductase activity, but only when assayed in the presence of excess (160 mM) sodium selenate (1). Clearly, it is evident that membrane-bound nitrate reductases are poor reducers of selenate (1,31) and may not contribute significantly to global selenate reduction, particularly in areas enriched with both selenate and nitrate. Consequently, novel enzyme systems that selectively catalyze the reduction of selenate have been sought (2,26,32), and to date, detailed biochemical studies have been limited mainly to a single species, Thauera selenatis (7,17,19,26).…”

“…Enterobacter cloacae SLD1a-1 (ATCC 700258), a bacterium isolated from Se-contaminated drainage water in the San Joaquin Valley, California, can also reduce Se oxyanions to elemental selenium (9,18,31,32) but cannot utilize selenate as the sole electron acceptor when grown anaerobically on nonfermentable carbon sources (32). However, it is the ability of this organism to readily catalyze the reduction of selenate to selenium under aerobic conditions that has interested those developing bioremediation strategies.…”

Bioaugmentation of UASB reactors with immobilized Sulfurospirillum barnesii for simultaneous selenate and nitrate removal

“…The specific activities for selenate reduction by nitrate reductases are, however, 15 to 518 times lower (Watts et al 2005) and the affinity constants (K M ) 2.3 times higher for selenate compared to nitrate reduction (Sabaty et al 2001). Consequently, selenate is reduced by these enzymatic systems at low nitrate concentrations only .…”

Genetic and Biochemical Evidence for the Involvement of a Molybdenum-Dependent Enzyme in One of the Selenite Reduction Pathways ofRhodobacter sphaeroidesf. sp.denitrificansIL106

“…In addition to this chemical reduction, enzymatic processes have also been described. The reduction of oxyanions has often been associated with denitrification, since bacterial nitrate reductases have been shown to catalyze selenate reduction (24,40). However, T. selenatis and E. cloacae SLD1a-1 possess a specific selenate reductase (4,39).…”