INVESTIGATION OF SUBSTRATE-BINDING SITES OF CYCLIC-RIBONUCLEOTIDE PHOSPHOMUTASE-5'-PHOSPHODIESTERASE USING SUBSTRATE-ANALOGS

The substrate-binding sites of a bifunctional enzyme, cyclic-ribonucle otide phosphomutase-5'-phosphodiesterase from Aspergillus niger, FS-44 , were investigated by kinetic studies. A series of adenine nucleotide s' inhibited the mutase and diesterase reactions, but adenosine and ad enine did not at all. Adenosine monophosphate derivatives acted as com petitive inhibitors with respect to both adenosine 3',5'-cyclic monoph osphate in the mutase reaction and adenosine 5'-p-nitrophenyl phosphat e in the diesterase reaction with the same decreasing order of inhibit ion constant. However, the inhibition constants of these compounds in the mutase reaction were 1 order of magnitude lower than those in the diesterase reaction, These results suggest that the active site(s) on the enzyme catalyzing the mutase and diesterase reactions have similar properties, but the substrate-binding sites of both reactions are dis tinct at least.