Competition of pyridoxal 5'-phosphate with ribulose 1,5-bisphosphate and effector sugar phosphates at the reaction centers of the spinach ribulose 1,5-bisphosphate carboxylase/oxygenase.
The stimulation of the carboxylase reaction by effectors of ribulose 1,5-bisphosphate carboxylase/oxygenase displays higher sensitivity towards pyridoxal 5'-pyridoxal 5'-phosphate inhibition than the catalytical process itself. Pyridoxal 5'-phosphate binding to the enzyme is not affected by the modulators 6-phosphogluconate and fructose 1,6-bisphosphate at low concentrations at which these agents stimulate the carboxylation rate. At higher concentrations these sugar phosphates protect the enzyme against pyridoxal 5'-phosphate inhibition in a similar fashion like the substrate ribulose 1,5-bisphosphate. Such protection experiments in combination with spectrophotometrical studies of pyridoxal 5'-phosphate binding demonstrate two binding states of ribulose 1,5-bisphosphate at the reaction centers of the enzyme with different requirements for Mg2+. 6-Phosphogluconate functions as protector only in the presence of Mg2+. Our results imply a competition between pyridoxal 5'-phosphate and substrate or effector sugar phosphates at the reaction centers of the spinach carboxylase. It is proposed that the pyridoxal 5'-phosphate inhibition of the stimulatory activity of these effectors originates from a modification of the regulatory sites of the enzyme caused by pyridoxal 5'-phosphate binding to the catalytical sites.
D001665:Binding Sites; D001667:Binding, Competitive; D002262:Carboxy-Lyases / Q000378:metabolism*; D007700:Kinetics; D010428:Pentosephosphates / Q000494:pharmacology*; D010944:Plants / Q000201:enzymology*; D011485:Protein Binding; D011732:Pyridoxal Phosphate / Q000494:pharmacology*; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism*; D012274:Ribulosephosphates / Q000494:pharmacology*; D013050:Spectrometry, Fluorescence; D013053:Spectrophotometry
D010428:Pentosephosphates; D012274:Ribulosephosphates; C001933:ribulose-1,5 diphosphate; D011732:Pyridoxal Phosphate; D002262:Carboxy-Lyases; D012273:Ribulose-Bisphosphate Carboxylase