Adenosine 5′-O-(3-thio)triphosphate (ATPγS) is a substrate for the nucleotide hydrolysis and RNA unwinding activities of eukaryotic translation initiation factor eIF4A

Dependence of the unwinding activity (k_obs,unw) on the concentration of eIF4A. Unwinding reactions contained 0.75 μM eIF4B, varying concentrations of eIF4A and 500 μM ATPγS•Mg (•) or ATP•Mg (○). The solid curves through the data represent a fit to a line (•), yielding a slope of 4.1 × 10⁻² μM⁻¹min⁻¹, or a fit to the Michaelis-Menten model (○), yielding a K_1/2 for eIF4A of 15 μM and a maximal rate constant of 12 min⁻¹ (k_cat/K_1/2^eIF4A = 0.8 μM⁻¹min⁻¹).