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Metabolism and Enzymology

Slow Inactivation of Ribulosebisphosphate Carboxylase during Catalysis Is Not Due to Decarbamylation of the Catalytic Site

Research School of Biological Sciences, Australian National University, P.O. Box 475, Canberra ACT 2601, Australia

An investigation was made of the proposal that the slow inactivation of ribulosebisphosphate carboxylase (Rubisco) activity, which occurs during in vitro assays, is due to decarbamylation of the enzyme. The level of carbamylation was compared with catalytic activity during assay conditions in which activity was both increasing and decreasing. Carbamylation level was measured using the reaction-intermediate analogue 2' -carboxy-D-arabinitol-1, 5-bisphosphate (carboxyarabinitol-P₂). A dual isotope procedure was used in which [³H]carboxyarabinitol-P₂ measured total active sites and ¹⁴CO₂ reported the level of carbamylation. The efficacy of the procedure was verified both in the presence and in the absence of the substrate D-ribulose-1, 5-bisphosphate (ribulose-P₂). These measurements showed that changes in activity during assays were not correlated with carbamylation status. Inactivation during assays initiated with both fully and partially carbamylated enzyme was not associated with any change in carbamylation level. This implies that the loss of activity during assays is not due to ribulose-P₂ binding and sequestering the E form of the enzyme. Ribulose-P₂ did not appear to alter the equilibrium between carbamylated and uncarbamylated enzyme, but it did slow the rate at which enzyme was both decarbamylated and carbamylated. The most likely explanation for the loss of activity during assays appears to be the sequestration of carbamylated, Mg²⁺-bound active sites by an inhibitor.

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