ATP synthesis by creatine kinase with calcium ions is accompanied by 43Ca/40Ca isotope effect: the enzyme with 43Ca2+ was found to be 2.0 ± 0.3 times more active than enzymes, in which Ca2+ ions have nonmagnetic nuclei 40Ca. The effect demonstrates that primary reaction in ATP synthesis is electron transfer between reaction partners, Сa(H2O)2+n (n ⩽ 3) and Ca2+(ADP)3−. It generates ion-radical pair, in which spin conversion results in the isotope effect. Magnetic parameters (g-factors and HFC constants a(43Ca) and a(31P)) confirm that namely terminal oxygen atom of the ADP ligand in the complex Ca2+(ADP)3− donates electron to the Ca(H2O)2+n ion.