title: Association kinetics and binding constants of nucleoside triphosphates with G-actin.
abstract: The dissociation of the complex between 1:N6-ethenoadenosine, 5'-triphosphate (xiATP) and G-actin was initiated by dilution to concentrations between 1 micronM and 5 nM and monitored by the fluorescence change of xiATP. The results were quantitatively explained by a two-step mechanism: a reversible dissociation of the actin-nucleotide complex followed by a fast irreversible inactivation of nucleotide-free G-actin. Under normal conditions (0.8 mM CaCl2, pH 8.2,21 degrees C), the rate-limiting step was the dissociation of the nucleotide-G-actin complex. The half-time of the dissociation of xiATP from G-actin was 290 s as compared to only 13 s for the following denaturation step of nucleotide-free actin. 1 mM EDTA highly accelerated the dissociation step and, regardless of its concentration, the complex dissociated quantitatively within 1 min. Addition of Ca2+ within 20 s after EDTA addition induced a re-association of xiATP with nucleotide-free but still native G-actin. This reversal was kinetically resolved by means of a multimixing stopped-flow apparatus. The association rate constant was 6 X 10(6) M-1s-1. From the association and dissociation rate constant, a value of 2.5 X (10(9) M-1 was calculated for the binding constant of xiATP to G-actin. The binding constant of ATP (1.4 X 10(10) M-1) was derived from the relative binding constant of xiATP and ATP as determined by fluorescence titration of xiATP-G-actin with ATP. These binding constants are 10(3)-10(4) times higher than values reported earlier on the basis of more indirect data.
mesh_terms: D000199:Actins / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000031:analogs & derivatives; D001665:Binding Sites; D004492:Edetic Acid; D004985:Ethenoadenosine Triphosphate; D007700:Kinetics; D008433:Mathematics; D008722:Methods; D011485:Protein Binding; D013050:Spectrometry, Fluorescence
chemical_list: D000199:Actins; D004985:Ethenoadenosine Triphosphate; D000255:Adenosine Triphosphate; D004492:Edetic Acid