The enzymes of the citric acid and glyoxylate cycles as well as RuBP4 carboxylase were measured in cell-free extracts from Rhodopseudomonas palustris after growth under chemoheterotrophic, photoheterotrophic and photolithotrophic conditions. Although the citric acid cycle was found to be complete under all growth conditions, significant differences in certain enzyme activities occurred as a function of the different energy sources applied. The glyoxylate cycle also was complete under all growth conditions with highest isocitrate lyase activity seen after photoheterotrophic growth on acetate. Photo- and chemoheterotrophic growth on malate reduced the isocitrate lyase. The activity was not repressed further by photolithotrophic growth on thiosulfate. RuBP carboxylase activity, present under photolithotrophic conditions, was repressed by chemoheterotrophic growth but was not decreased by the presence of organic substrates during photoheterotrophic growth.
The enzymes of the glyoxylate cycle, isocitrate lyase (EC.4.1.3.1) and malate synthase (EC.4.1.3.2), were measured in cell‐free extracts from the cyanobacterium Anacystis nidulans Drouet during photoautotrophic growth in medium aerated with ordinary air (0.03% CO2). Isocitrate lyase had an average specific activity of 112 nmoles·min−1·mg protein−1 whereas malate synthase had an average specific activity of 12.5 nmoles·min−1·mg protein−1. Unpurified isocitrate lyase showed classical Michaelis kinetics with a Km of 8 mM. Isocitrate lyase activity was strongly inhibited by numerous cellular metabolites at 10 mM concentration. The previously reported low specific activity for isocitrate lyase may be due to metabolite inhibition caused by growth in high CO2 concentrations. The activities reported for isocitrate lyase and malate synthase suggest the operation of the glyoxylate cycle in Anacystis nidulans under CO2‐limiting growth conditions.
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