Exchange of ADP, ATP and 1:N⁶‐Ethenoadenosine 5′‐Triphosphate at G‐Actin

Nucleotide exchange was monitored by the strong increase of fluorescence accompanying binding of 1: N 6 ‐ethanoadenosine 5′‐triphosphate (ɛATP) to G‐actin. ɛATP · G‐actin was titrated by ADP and a relative binding constant K ADP/ɛATP = 0.03 was obtained. With K ɛATP/ATP = 0.19, the physiologically important relative binding constant K ADP/ATP = 0.0057 was calculated. From kinetic measurements of the exchange reaction a rate constant of ADP dissociation k − = 8 × 10 −4 s −1 was derived which was compared with the corresponding values for ATP (8 × 10 −4 S −1 ) and eATP (4 × 10 −3 S −1 ) (all values in 20 mM Tris · HC1 buffer, pH 8.2 containing 0.8 mM CaCl 2 at21°C). The exchange of ADP versus ATP is an important step in the steady‐state equilibrium of actin polymerization. Since polymerization of ATP · G‐actin to ADP · F‐actin is irreversible, depolymerization can take place only by dissociation of ADP · G‐actin followed by nucleotide exchange. It was found from a comparison of k‐ with the rate constants of the polymerization steps that the ratio of ADP · G‐actin to ATP · G‐actin is much higher in the steady state than at equilibrium.