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Properties of Phosphoribulokinase from Thiobacillus neapolitanus

Partially purified preparations of ribulose-5-phosphate kinase (specific activity, 50 to 125 μmoles per min per mg of protein) were employed in a series of kinetic experiments in the presence of several concentrations of H+, Mg2+, adenosine triphosphate (ATP), and phosphoenolpyruvate (PEP). The pH optimum of the enzyme was found to be 7.9; at this pH and above, response of the enzyme to variations in ATP concentration was hyperbolic, exhibiting a Km of 7 × 10−4m ATP. At pH values below the optimum the response to ATP was sigmoidal, as it was throughout the entire pH range in the presence of PEP at a concentration greater than 5 × 10−4m. In the presence of PEP the pH optimum shifted to pH 8.4. In contrast, phosphoribulokinase from spinach exhibited hyperbolic responses throughout its pH range with no inhibition caused by PEP. Thiobacillus neapolitanus phosphoribulokinase was inhibited by PEP in a sigmoidal manner; however, in the presence of suboptimal concentrations of Mg2+ the addition of PEP caused significant stimulation of activity. It is postulated that the enzyme consists of interacting subunits with several sites on the enzyme for binding ATP and with several separate sites binding PEP. It is suggested that PEP functions as a regulator of CO2 fixation when the organism is under conditions of unlimited concentrations of substrate and CO2.