Synthesizes selenophosphate from selenide and ATP. ATP + H2O + hydrogenselenide = AMP + 2 H(+) + phosphate + selenophosphate Binds 1 Mg(2+) ion per monomer. Activated by phosphate ions and by potassium ions. Homodimer (PubMed:20471958, PubMed:19477186). Heterodimer with isoform 3 (PubMed:20471958). Homodimer (PubMed:20471958). Heterodimer with isoform 4 (PubMed:20471958). Homodimer (PubMed:20471958). Heterodimer with isoform 1 (PubMed:20471958). Homodimer (PubMed:20471958). Heterodimer with isoform 2 (PubMed:20471958). Gradually expressed during the cell cycle until G2/M phase and then decreases. Gradually expressed during the cell cycle until G2/M phase and then decreases. Gradually expressed during the cell cycle until S phase and then decreases. 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 proven, it is possible Cys-31 is the active site.