In enzymology, a selenide, water dikinase () is an enzyme that catalyzes the chemical reaction
The 3 substrates of this enzyme are ATP, selenide, and H<sub>2</sub>O, whereas its 3 products are AMP, selenophosphate, and phosphate.
This enzyme belongs to the family of transferases, to be specific, those transferring phosphorus-containing groups (phosphotransferases) with paired acceptors (dikinases). The systematic name of this enzyme class is ATP:selenide, water phosphotransferase. This enzyme is also called selenophosphate synthetase. This enzyme participates in selenoamino acid metabolism.
Vertebrates including humans carry two versions of this enzyme, with one (SEPHS2) being a selenoprotein and the other (SEPHS1) replacing it with a threonine, though still with a vestigial SECIS element. Analysis of animal versions of this enzyme show that the original animal version is a selenoprotein, with SEPHS1 arising later through gene duplication.
Among prokaryotes, most bacteria have a version with cystine instad of selenocystine, suggesting that this may be the ancestral state (which would avoid the chicken-and-egg problem). Some have two versions, one with Sec and the other with Cys. Archaea mostly have the Sec version.