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Metabolism and Enzymology

Fallover of Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase Activity ¹

Decarbamylation of Catalytic Sites Depends on pH

Department of Biochemistry, University of Arizona, Tucson, Arizona 85721, Department of Plant Science, University of Arizona, Tucson, Arizona 85721

Loss of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity during CO₂ fixation, called fallover, occurred with or without loss of activator CO₂ from catalytic sites depending on pH. At pH 7.5, but not at pH 8.5, the fraction of Rubisco sites that were carbamylated decreased during fallover. Inhibitors which formed during fallover were identified following NaBH₄ reduction and separation of the products by high performance anion-exchange chromatography and pulsed amperometric detection. They were xylulose 1,5-bisphosphate (XuBP) and 3-ketoarabinitol 1,5-bisphosphate. During fallover at pH 8.5, 3-ketoarabinitol-P₂ was the only inhibitor binding to Rubisco and this binding was at carbamylated sites, although both inhibitors were made. At pH 7.5, both inhibitors were bound to catalytic sites of Rubisco with XuBP bound tightly to decarbamylated sites, whereas 3-ketoarabinitol-P₂ bound to carbamylated sites. The pH during fallover also influenced the ratio of 3-ketoarabinitol-P₂ to XuBP formed. When fallover occurred at pH 7.5, both the formation of XuBP and its binding affinity to decarbamylated Rubisco sites were increased compared with those at pH 8.5. 3-Ketoribitol-P₂ was not found at either pH.

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