The Kinetics of Exchange of Adenosine Triphosphate and Calcium with G-Actin

Abstract A kinetic analysis of the exchange of ATP bound to G-actin shows that it is consistent with a unimolecular mechanism at least under the conditions studied. The effects of varying the concentrations of Ca2+, ATP, actin, H+, and salt were determined. The first three of these can be related to the finding that the ATP exchange rate is proportional to 1/ (Ca2+). The effect of chemical modification—viz. blocking of —SH groups and photooxidation—on biological properties, including ATP exchange, has been studied. These specific modifications permit the study of G-actin under conditions usually leading to polymerization. Calcium exchange has been compared with ATP exchange. Finally, the results have been discussed in the context of the active site (sites) of G-actin.