Synthesizes selenophosphate from selenide and ATP. ATP + H2O + hydrogenselenide = AMP + 2 H(+) + phosphate + selenophosphate Binds 1 Mg(2+) ion per monomer. Homodimer. Belongs to the selenophosphate synthase 1 family. Class II subfamily. The conserved active site Cys (or selenocysteine) residue in position 29 is replaced by a Thr. However, as function in selenoprotein synthesis is probable, it is possible Cys-31 is the active site.