INTERACTION OF MG2+ AND ATP4- WITH ATP - CREATINE PHOSPHOTRANSFERASE

1. 1. The forward reaction catalysed by creatine kinase (ATP:creatine phosphotransferase, EC 2.7.3.2) has been studied under conditions where Mg2+, ATP4t- and MgATP2- were each present in significant concentration. The results indicate the presence of significant concentrations of enzyme-Mg and enzyme-ATP complexes, as well as of the enzyme-MgATP complex, under these conditions. 2. 2. The inhibition of the forward reaction by concentrations of Mg2+ or ATP4- in excess of the MgATP2- concentration has been shown to be less effective than would be expected on the basis of the results obtained when Mg2+, ATP4- and MgATP2- are all present. Excess Mg2+ cause noncompetitive inhibition with respect to both creatine and MgATP2-, and it is concluded that detectable inhibition is caused only by the interaction of Mg2+ at a site on the enzyme distinct from the active site. This conclusion is in agreement with previous results obtained in the reverse reaction. 3. 3. From comparison of these two types of kinetic data and previous thermodynamic data, it appears probable that Mg2+ does not interact directly with creatine kinase at the active site, and that the enzyme-Mg complex is formed only by the dissociation of ATP4- from the enzyme-MgATP complex. 4. 4. ATP4- is a much weaker inhibitor with respect to MgATP2- than ADP3- was previously found to be with respect to MgADP- in the reverse reaction. This indicates that, while ADP3- may react with free creatine kinase at the active site, a similar reaction involving ATP4- is unlikely. © 1969.