Studies of catalysis by ribonuclease U2. Steady-state kinetics for transphosphorylation of oligonucleotide and synthetic substrates.

The values of the steady-state kinetic parameters for the RNase, U2 catalyzed transphosphorylation were measured for several di- and trinucleotides such as ApXp (X = A, C, G, or U), ApYpGp, and YpApGp (Y = C or U). The pH dependence of kcat/Km for ApUp indicated a dependence for catalysis upon an unprotonated group with pK = 3.8 and two proton-associated groups with pK = 4.4 and 5.0. As judged from kcat/Km values, ApUpGp and ApCpGp are better substrates than the corresponding parent dimers, ApUp and ApCp, respectively. By contrast, the kcat/Km for UpApGp was about 1/20 relative to that of the parent dimer, ApGp. The results were compared for possible thermodynamic and structural information about the chemical consequences. The inhibition of the RNase U2 catalyzed reaction of ApUp by a series of nucleosides and nucleotides was also studied. Evidence for similarities and dissimilarities in the binding and catalysis by RNase U2 and RNase T1 is presented.