Electron donation to ferredoxin in heterocysts of the N₂ fixing alga Anabaena cylindrica

Abstract

Glucose-6-phosphate dehydrogenase activity is soluble in Anabaena cylindrica and is 6-7 times higher in heterocysts than in vegetative cells of N₂ or NH₄ ⁺-grown cultures. Vegetative cell activity is inhibited by light in vitro, but in heterocyst extracts activity is equally high in the light and dark. This inhibition occurs at equal rates in the presence and absence of thylakoid fragments. NADPH inhibits glucose-6-phosphate dehydrogenase activity competitively with NADP⁺ and non-competitively with glucose-6-phosphate. ATP inhibition also occurs and is competitive with NADP⁺ and non-competitive with glucose-6-phosphate. Dithiothreitol inhibits activity but ribulose 1,5-diphosphate, ADP, phosphoenol pyruvate, glutamine and glutamate do not affect activity. Ferredoxin-NADP⁺ oxidoreductase activity was measured as cytochrome c reduction and as 2,6-dichlorophenol indophenol reduction. Ferredoxin-NADP⁺ oxidoreductase activity remains in solution on centrifugation of extracts at 100000 g × 60 min and in such preparations activity is equally high in vegetative cell and in heterocyst material in the light and dark. In 1000 g × 30 min and in 35000 g × 30 min supernatant fractions cytochrome c reduction is immediately inhibited by light in preparations of vegetative cells, but activity in heterocyst preparations is not inhibited by light. Inhibition by light can be reversed by placing the preparations in the dark. The addition of fractions containing vegetative cell thylakoids inhibits cytochrome c reduction by the 100000 g × 60 min supernatant enzyme from vegetative cells and from heterocysts but the addition of fractions containing heterocyst thylakoids has no such effects. Of various compounds tested, including potential inhibitors of ferredoxin-NADP⁺ oxidoreductase, only NADP⁺ inhibits activity. This inhibition is competitive with NADPH. Antibody to Scenedesmus ferredoxin inhibits cytochrome c reduction. The presence of glucose-6-phosphate dehydrogenase and ferredoxin-NADP⁺ oxidoreductase in heterocysts provides a route of electron transfer from glucose to ferredoxin which is light-independent and which may be regulated by the intracellular levels of oxidized and reduced pyridine nucleotides.