This is the first report ever on the nuclear spin selective 43 Ca 40 Ca isotope effect expressed in the Calcium affected enzymatic ATP synthesis. Once the creatine kinase (CK) catalytic sites were loaded with 43 Ca 2+ , the resulting enzyme activity goes up (2.0±0.3 – fold) compared to the CKdirected ATP synthesis rate occurred in the presence of nonmagnetic, spinless, 40 Ca nuclei. The effect manifests itself predominantly within a high concentration range (Calcium chloride,40 mM) indicating that the ATP synthesis is a spin-selective ion-radical process. Primary reaction of this path is an electron transfer between the reaction partners, С a(H2O)n 2+ ( n ≤ 4) and Ca(ADP) 3- , generating an ion-radical pair where the singlet -triplet spin conversion takes place as a result of the nuclear spin selectivity and a consequent magnetic isotope effect. This, in turn, makes an essential impact on the final reaction’s ATP yield. To the contrast, there is no any isotope effect found within a low Ca 2+ concentration range showing that the ATP synthesis proceeds in a most common nucleophylic way. The submolecular mechanisms of phenomena described and their possible applied significance are under discussion.