Aldolase-An Important Enzyme in Controlling the Ribulose 1,5-bisphosphate Regeneration Rate in Photosynthesis

概要

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This study was done to explore an enzymatic mechanism for the photosynthetic carbon reduction cycle whereby the rate of synthesis of ribulose 1,5-bisphosphate (RuBP) could be changed while the levels of intermediates other than 3-phosphoglycerate and RuBP were kept constant. Chloroplast aldolase was purified to homogeneity from spinach leaves. When the enzyme was assayed in the direction of fructose 1,6-bisphosphate synthesis in the presence of the concentrations of the substrates reported in vivo, the activity was severely inhibited by physiological concentrations of RuBP. The aldolase reaction proceeded with a sequential mechanism. The K_m for dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate were 0.45 mM and 40 μM, respectively. The activity was competitively inhibited by RuBP with respect to dihydroxyacetone phosphate. The K_i was 0.78 mM. The maximum activity of aldolase in spinach leaves was calculated as 1,360 μmol (mg Chl)^<-1> h^<-1>. An equation to express the reaction for the synthesis of fructose 1,6-bisphosphate by aldolase was constructed to predict the metabolic rate of this reaction in vivo. The calculation clearly showed that aldolase is an important enzyme in controlling the rate of RuBP regeneration.
日本植物生理学会の論文