Structure and function of plant-type ferredoxins.

The plant-type ferredoxins (Fds) are the [2Fe-2S] proteins that function primarily in photosynthesis; they transfer electrons from photoreduced Photosystem I to ferredoxin NADP(+) reductase in which NADPH is produced for CO(2) assimilation. In addition, Fds partition electrons to various ferredoxin-dependent enzymes not only for assimilations of inorganic nitrogen and sulfur and N(2) fixation but also for regulation of CO(2) assimilation cycle. Although Fds are small iron-sulfur proteins with molecular weight of 11 KDa, they are expected to interact with surprisingly many enzymes. Several Fd isoforms were found in non-photosynthetic cells as well as Fds in photosynthetic cells, leading to the recognition that they have differentiated physiological roles. In a quarter of century, X-ray crystallography and NMR spectroscopy provided wealth of structural data, which shed light on the structure-function relationship of the plant-type Fds and gave structural basis for the biochemical and spectroscopic properties so far accumulated. Thus the structural studies of Fds have come to a new era in which different roles of Fds and interactions with various enzymes are clarified on the basis of the tertiary and quaternary structures, although they are premature at present. This article reviews briefly the structures of the plant-type Fds together with their functions, properties, and interactions with Fd related enzymes. Lastly the folding motif of Fd, that has grown to be a large family by including many functionally unrelated proteins, is noted.