Flavodoxin (Fld) is a soluble electron shuttle flavoprotein (it binds a flavin mononucleotide) that is isofunctional with ferredoxin (Fd). In photosynthetic organisms flavodoxin is reduced by the terminal electron acceptors of photosystem I.
Fd and Fld are involved in multiple redox reactions. They can both sustain cyclic as well as linear electron flow. The cyclic electron flow creates a proton gradient across the photosynthetic membrane and allows ATP synthesis independent of photosystem II activity. Linear electron flow, on the other hand, utilizes electrons from the reduced Fd and Fld for NADP+ reduction in a reaction catalyzed by ferredoxin-NADP oxidoreductase (FNR). The source of electrons during linear electron flow is the dissociation of water by photosystem II.
Fd serves also as an electron donor to a number of soluble enzymes involved in nitrogen metabolism, sulfur metabolism and the regulation of carbon metabolism. Fld can substitute Fd under conditions of iron deprivation in most cyanobacteria and some algae and may be efficient in reducing most or all of these soluble enzymes [Setif01].
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