Bioenergetics of Nitrogenase in Blue-Green Algae. II. Electron Transport to Nitrogenase in Heterocysts of Anabaena Variabilis

Abstract

Conflicting evidence exists on the nature of electron sources for nitrogen fixation and regulation of corresponding electron-transport pathways in heterocysts of cyanobacteria (cf. Bothe 1982). Reduced ferredoxin can in principle be generated by photosystem I, if a donor is available (cf. Houchins, Hind 1982), or in the dark either by pyruvate:ferredoxin oxidoreductase, or by NADP:ferredoxin oxidoreductase (FNR). The latter enzyme, however, requires high NADPH/NADP ratios for ferredoxin reduction, whereas the enzymes suggested to be involved in NADPH generation, i.e. glucose-6-phosphate and isocitrate dehydrogenases are known to be negatively regulated by the reductant charge and by ATP.