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Interactions between ribulose-1,5-bisphosphate carboxylase and stromal metabolites. I. Modulation of enzyme activity by Benson-Calvin cycle intermediates

1987; Elsevier BV; Volume: 894; Issue: 2 Linguagem: Inglês

10.1016/0005-2728(87)90185-x

1879-2650

Christine H. Foyer, Robert T. Furbank, David A. Walker,

Light effects on plants

Changes in the concentrations of glycerate 3-phosphate, ribulose 1,5-bisphosphate, fructose 1,6-bisphosphate and dihydroxyacetone phosphate were measured in leaf discs under various conditions of light intensity and CO2 concentration. The range of measured metabolite concentrations was then used to study metabolite regulation of ribulose-1,5-bisphosphate carboxylase. The effects of physiological concentrations of glycerate 3-phosphate, fructose 1,6-bisphosphate and dihydroxyacetone phosphate on ribulose-1,5-bisphosphate carboxylase activity of isolated stromal protein at saturating and limiting ribulose 1,5-bisphosphate were measured by incorporation of 14CO2. A system containing ribulose 5-phosphate and an ATP-generating mechanism is described which was used to produce a constant low level of ribulose 1,5-bisphosphate in the reaction mixture. Of the metabolites measured glycerate 3-phosphate was the only one which was found to be present at a sufficiently high concentration relative to ribulose 1,5-bisphosphate in the light to be an effective inhibitor of ribulose-1,5-bisphosphate carboxylase activity in situ. Physiological concentrations of glycerate 3-phosphate were found to inhibit carboxylation severely, particularly at rate-limiting ribulose 1,5-bisphosphate concentrations. The rate of 14CO2 assimilation was decreased 85% (with 0.2 mM ribulose 1,5-bisphosphate) by the presence of 20 mM glycerate 3-phosphate and the Km (ribulose 1,5-bisphosphate) of ribulose-1,5-bisphosphate carboxylase was increased by over 10-fold. The activity of the enzyme was inhibited substantially by a mixture of metabolites chosen to mimic in vivo conditions (20 mM glycerate 3-phosphate, 0.5 mM fructose 1,6-bisphosphate and 1.5 mM dihydroxyacetone phosphate). With this mixture of metabolites the Km (ribulose 1,5-bisphosphate) of ribulose-1,5-bisphosphate carboxylase was increased 10-fold above the control. A pulse of 10 mM glycerate 3-phosphate completely inhibited CO2 uptake when the activity of the Benson-Calvin cycle as a whole was assayed in chloroplast extracts. Using these data we have examined the physiological significance of glycerate 3-phosphate modulation of ribulose-1,5-bisphosphate carboxylase at different metabolite levels.