Purification and characterization of the selenate reductase from Thauera selenatis

Thauera selenatis is one of two isolated bacterial species that can obtain energy by respiring anaerobically with selenate as the terminal electron acceptor. The reduction of selenate to selenite is catalyzed by a selenate reductase, previously shown to be located in the periplasmic space of the cell. This study describes the purification of the enzyme from T. selenatis grown anaerobically with selenate. The enzyme is a trimeric alpha beta gamma complex with an apparent M-r of 180,000. The alpha, beta, and gamma subunits are 96 kDa, 40 kDa, and 23 kDa, respectively, in size. The selenate reductase contains molybdenum, iron, and acid-labile sulfur as prosthetic group constituents. UV-visible absorption spectroscopy also revealed the presence of one cytochrome b per alpha beta gamma complex, The K-m for selenate was determined to be 16 mu M, and the V-max was 40 mu mol/min/mg of protein. The enzyme is specific for the reduction of selenate; nitrate, nitrite, chlorate, and sulfate were not reduced at detectable rates, These studies constitute the first description of a selenate reductase, which represents a new class of enzymes. The significance of this enzyme in relation to cell growth and energy generation is discussed.