In enzymology, an opheline kinase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are ATP and guanidinoethyl methyl phosphate, whereas its two products are ADP and N'-phosphoguanidinoethyl methylphosphate.
This enzyme belongs to the family of transferases, specifically those transferring phosphorus-containing groups (phosphotransferases) with a nitrogenous group as acceptor. The systematic name of this enzyme class is ATP:guanidinoethyl-methyl-phosphate phosphotransferase.