SEQUENTIAL GENERAL BASE ACID CATALYSIS IN THE HYDROLYSIS OF RNA BY IMIDAZOLE

The cleavage of poly-U by imidazole buffer was studied by using a recently described kinetic assay technique. A bell-shaped profile is seen for rate vs. buffer protonation state, but the reaction is kinetically first order in buffer. It is proposed that a sequential base-acid mechanism is involved: the base catalyzes formation of a 5-coordinate phosphorus isomer of the starting material by transferring a proton from nucleophilic hydroxyl to equatorial oxygen, and the acid then catalyzes the forward decomposition of that intermediate to form the products. Some kinetically equivalent alternative mechanisms have been excluded. The proton transfer catalysis mechanism is suggested for the enzyme ribonuclease as well.