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Title: SUBSATURATING RIBULOSE 1,5-BISPHOSPHATE PROMOTES INACTIVATION OF RIBULOSE 1,5-BISPHATE CARBOXYLASE/OXYGENASE

Author
item PORTIS JR, ARCHIE
item LILLEY, ROSS - U OF WOLLONGONG AUSTRALIA
item ANDREWS, T. - AUSTRALIAN NATIONAL UNIV

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/1995
Publication Date: N/A
Citation: N/A

Interpretive Summary: Rubisco is the first enzyme in photosynthesis, the process by which plants use light energy from the sun to make carbohydrates from carbon dioxide and water. The activity of this enzyme varies with light intensity and a mechanism to explain this process has been proposed. A critical aspect of the proposed mechanism is that a spontaneous inactivation of the enzyme occurs when one of its substrates becomes limiting. In this study we obtained the first direct evidence of this process using the isolated enzyme. Our study thus provides strong support for this mechanism in determining the activity of the enzyme. These studies will benefit scientists attempting to modify the properties of rubisco in ways beneficial for increased photosynthesis by crop plants.

Technical Abstract: We developed a continuous-addition method for maintaining subsaturating concentrations of rubisco 1,5-bisphosphate (RuBP) for several minutes while simultaneously monitoring its consumption by ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco. This method enabled us to observe the effects of subsaturating RuBP and CO2 concentrations on the activity of the enzyme declined faster when RuBP was maintained at concentrations near its Km value than when RuBP was saturating. At saturating RuBP, activity declined faster at limiting than at saturating CO2, in accordance with previous observations. The most rapid decline in activity occurred when both CO2 and RuBP concentrations were subsaturating. The activity loss was accompanied by decarbamylation of the enzyme, even though the enzyme was maintained at the same CO2 concentration before and after exposure to RuBP. Rubisco activase ameliorated the decline in activity at subsaturating CO2 and RuBP concentrations. The result are consistent with a proposed mechanism for regulating the carbamylation of Rubisco which postulates that Rubisco activase counteracts Rubisco's unfavourable carbamylation equilibrium in the presence of RuBP by accelerating, in an ATP-dependent manner, the release of RuBP from its complex with uncarbamylated sites.