Plant ribulose bisphosphate carboxylase (RuBP carboxylase, EC 126.96.36.199) displays several physiological and nutritional roles. Indeed, the fundamental importance of this enzyme derives not only from its role on photosynthesis, where it catalyses the initial reaction of the Calvin cycle, but also because it exhibits two important, inevitable chemical constraints that are inherent properties of the enzyme molecule: (i) the ability to catalyse the initial reaction of photorespiration, generally regarded as the main metabolic constriction on plant productivity. (ii) The extremely low rate of catalysis; to grow at convenient rates, plants need to synthesize huge amounts of the enzyme, making it the most abundant protein in nature. Nevertheless, when the strategy is survival rather then growth, plants may use the RuBP carboxylase pool as a foliar storage of carbon, nitrogen and sulphur.
The concentration of RuBP carboxylase, as of proteins in general, is determined by the controlled balance between synthesis and degradation. A considerable amount of information has accumulated in what concerns its mechanism of synthesis. However, fundamental gaps still exists in the knowledge about the catabolism of this enzyme.
The molecular mechanism responsible for RuBP carboxylase catabolism remains largely unknown. Recent reports suggest that the enzyme is first oxidised into a catalytically inactive form and polymerized into very large molecular mass aggregates. Interaction with a membrane and subsequent proteolysis complete the turnover cycle of the enzyme.
The conditions that trigger the degradation of RuBP carboxylase are now relatively well known. Under a normal metabolic conditions, RuBP carboxylase is apparently subjected to continuous turnover at rates that differ widely with the species considered. The metabolic fates of RuBP carboxylase may be tentatively grouped into four categories depending on the plant species under study and the physiological conditions considered: (i) stress situations that do not induce degradation or structural changes in the enzyme molecule; (ii) stress situations that cause non –disulphide oxidation followed by polymerization of the enzyme; (iii) stress situations that produce enzyme degradation in a process leading to plant death; (iv) stress situations that originate “reversible” enzyme proteolysis in a process that does not lead to plant death.
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