ATP synthesis by creatine kinase with calcium ions is accompanied by Ca-43/Ca-40 isotope effect: the enzyme with Ca-43(2+) was found to be 2.0 +/- 0.3 times more active than enzymes, in which Ca2+ ions have nonmagnetic nuclei Ca-40. The effect demonstrates that primary reaction in ATP synthesis is electron transfer between reaction partners, Ca(H2O)(n)(2+) (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(Ca-43) and a(P-31)) confirm that namely terminal oxygen atom of the ADP ligand in the complex Ca2+(ADP)(3) donates electron to the Ca(H2O)(n)(2+) ion. (C) 2011 Elsevier B.V. All rights reserved.