Energetic contribution of tRNA hybrid state formation to translocation catalysis on the ribosome

Upon transpeptidylation, the 3' end of aminoacyl-tRNA (aa-tRNA) in the ribo somal A site enters the A/P hybrid state. We report that transpeptidylation of Phe-tRNA to fMetPhe-tRNA on Escherichia coli ribosomes substantially lo wers the kinetic stability of the ribosome-tRNA complex and decreases the a ffinity by 18.9 kJ mol(-1). At the same time, the free energy of activation of elongation factor G dependent translocation decreases by 12.5 kJ mol(-1 ), indicating that part of the free energy of transpeptidylation is used to drive translocation kinetically. Thus, the formation of the A/P hybrid sta te constitutes an important element of the translocation mechanism.