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Regulation of Ribulose 1,5-Diphosphate Carboxylase in the Photosynthetic Assimilation of Carbon Dioxide

From the ¹ From the Department of Cell Physiology, University of California, Berkeley, California 94720

The formation of 3-phosphoglycerate from ribulose 1, 5-diphosphate, CO₂, and H₂O by homogeneous preparations of spinach chloroplast ribulose-1,5-P₂ carboxylase was investigated at 1 mm bicarbonate—the lowest concentration that sustained a maximal rate of complete photosynthesis by isolated chloroplasts. Under these conditions, the carboxylase was activated by fructose-6-P and deactivated by fructose-1,6-P₂, two intermediates of the reductive pentose phosphate cycle. Activation by fructose-6-P was most pronounced at a limiting level of Mg²⁺ or at neutral or acidic pH.

Fructose-6-P induced a shift from sigmoidal to Michaelian kinetics and decreased (up to a factor of 6) the K_m for bi-bicarbonate. Fructose-6-P also decreased the K_m for Mg²⁺ and increased the K_m for ribulose-1,5-P₂. On deactivation by fructose-1,6-P₂, the carboxylase showed, in general, its original kinetic characteristics.

Certain other intermediates of the reductive pentose phosphate cycle (ribulose-5-P, xylulose-5-P, erythrose-4-P, ribose-5-P) also activated the carboxylase, but less effectively than did fructose-6-P. 6-Phosphogluconate, a compound not formed by chloroplasts, was a strong activator of the enzyme; its mode of action resembled that of fructose-6-P.

Fructose-1,6-P₂ was the only intermediate of the reductive pentose phosphate cycle that deactivated the carboxylase. Two intermediates in starch synthesis, glucose-1,6-P₂ and glucose-6-P, deactivated the carboxylase as well as did fructose-1,6-P₂.

A regulatory mechanism based on these and other findings is proposed for the conversion of CO₂ to starch in photosynthesis.

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