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pH Dependence of the Km(CO₂) of Ribulose 1,5-Diphosphate Carboxylase ¹

William L. Ogren^b

Richard H. Hageman^c

^a Department of Botany, University of Florida, Gainesville, Florida 32611, ^b United States Regional Soybean Laboratory, Agricultural Research Service, United States Department of Agriculture, Urbana, Illinois 61801, Department of Agronomy, University of Illinois, Urbana, Illinois 61801

The Km(CO₂) values of ribulose 1,5-diphosphate carboxylase in freshly ruptured spinach (Spinacia oleracea L.) chloroplasts and in the purified form isolated from spinach leaves were found to be pH dependent. Raising the pH of the assay solution produced a substantial decrease in the Km(CO₂) of both enzyme systems. In freshly ruptured chloroplasts at pH 7.2 the Km(CO₂) was 25 µM, at pH 8 it decreased to 19 µM, and at pH 8.8 a further decrease to 7 µM was found. With the purified enzyme at pH 7.2 the Km(CO₂) was 147 µM, while the corresponding Km values for pH 8 and 8.8 were 34 and 15 µM CO₂, respectively. The latter figure approximates the physiological Km(CO₂) of 10 µM estimated for photosynthesizing leaves and intact chloroplasts. The maximum velocity for both enzyme systems at optimum substrate levels was at pH 8, but the highest calculated rate of CO₂ uptake at atmospheric CO₂ levels occurred at pH 8.8. These results support the proposal that the light-induced efflux of protons out of the chloroplast stroma may be a major factor involved with the reported in vivo light activation of ribulose 1,5-diphosphate carboxylase.

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