This study was attempted to elucidate the mechanism of the regulation of the turnover number on the catalytic sites by the regulatory sites of spinach ribulose-1,5-bisphosphate carboxylase/oxygenase [Rbu(1,5)P2CO]. To this end, we analyzed the effects of the binding of 6-phosphogluconate (6-P-Gln) to the regulatory sites of the enzyme on the progress in the subsequent catalysis assayed with 0.5 mM ribulose 1,5-bisphosphate [Rbu(1,5)P2]. This concentration of Rbu(1,5)P2 hardly binds to the regulatory sites but competes with 6-P-Gln for the catalytic sites. An equilibrium binding assay showed that Rbu(1,5)P2CO bound 8 mol 6-P-Gln/mol enzyme in addition to the catalytic sites. The binding to the eight regulatory sites was positively cooperative. The biphasic reaction course, inherent in the carboxylase reaction of plant Rbu(1,5)P2CO and composed of a burst for an initial few minutes and a subsequent linear phase, became faint with increasing binding of 6-P-Gln to the sites. The change of the reaction progress from the biphasic to linear course was ascribed to the suppression of the functioning form of the enzyme from the high-activity to low-activity form and to the increased turnover number on the catalytic sites though the binding of 6-P-Gln to the regulatory sites.