Kinetic studies on ribonuclease F1 using diribonucleoside phosphates as substrates

Transphosphorylation of diribonucleoside phosphates (GpA, GpC, GpU, ApA, CpC and UpU) to corresponding nucleoside 2','3-cyclic phosphate + nucleoside by ribonuclease F1 was followed by high performance liquid chromatography and the kinetic constants. Km and kcat, were determined. The Km values were similar for all the standard substrates. GpN's but the kcat values followed the order GpC > GpA > GpU. This suggests that binding affinity of a substrate to the enzyme is determined primarily by the guanine binding recognition subsite and that the enzyme contains a second subsite preferring C > A > U which contributes to catalysis by interacting with the leaving nucleoside. All the non-standard substrates, NpN's, showed similar kcat / Km Values which were approximately six orders of magnitude smaller than those for GpN's, although individual Km and kcat values varied considerably: small kcat for CpC and UpU, and large Km for ApA. This means that ribonucleas F1 degrades GpN's 10 6 times more effectively than NpN's. The kinetic constants for hydrolysis of guanosine 2',3'-cyclic phosphate by ribonuclease FI were also determined. It showed almost the same Km but three orders of magnitude smaller kcat compared to GpN's.