Thermostable properties of the periplasmic selenate reductase from Thauera selenatis

Selenate reductase (SER) from Thauera selenatis is a member of a distinct class of the TAT-translocated type II molybdoenzymes and is closely related to a group of thermostable nitrate reductases (pNAR) found in hyperthermophilic archaea. In the present study the thermostable and thermo-active properties of SER, isolated with either molybdenum (Mo) or tungsten (W) at the active site, are reported. Results show that the purified Mo–SER complex is stable and active upon heat-shock incubation for 10 min at temperatures up to 60 °C. At temperatures greater than 65 °C all three subunits (SerABC) are readily denatured. The optimum temperature for maximum activity recorded was also determined to be 65 °C. T. selenatis can grow readily on a tungstate rich medium up to concentrations of 1 mM. SER isolated from periplasmic fractions from cells grown on 1 mM tungstate displayed selenate reductase activities with a 20-fold reduction in V_max (0.01 μmol [S]/min/mg) and a 23-fold increase in substrate binding affinity (K_m 0.7 μM). The thermo-stability and pH dependence of W–SER was shown to be similar to that observed for Mo–SER. By contrast, the optimum reaction temperature for W–SER exceeded the maximum temperature tested (>80 °C). The combined data from the kinetic analysis and thermal activity profiles provide evidence that W can substitute for Mo at the active site of SER and retain detectable selenate reductase activity. It is argued that despite the similarity in their catalytic and electron conducting subunits, the presence of a membrane anchor in the archaeal pNAR system appears pivotal to the enhanced hyperthermostability. The fact that Mo–SER is thermostable up to 65 °C however, could be advantageous when designing selenate contamination remediation strategies.