Plant glutaredoxins: pivotal players in redox biology and iron-sulphur centre assembly.

Glutaredoxins are small oxidoreductases structurally related to thioredoxins. They have two major proposed biochemical roles: the reduction of disulphide bonds and the binding of iron-sulphur clusters, both of which involve glutathione. The thiol-disulphide reductase activity regulates the activity of target proteins, either metabolic enzymes or transcription factors, and also helps to regenerate thiol-dependent antioxidant enzymes, namely thiol-peroxidases and methionine sulphoxide reductases, which are key players for the plant response to environmental constraints. In photosynthetic organisms, glutaredoxins are distributed into six classes. Glutaredoxins from class II can exist either as apoforms, which display deglutathionylation activity, or as holoforms, which bind labile [2Fe-2S] clusters and seem to be required for iron-sulphur cluster assembly. This latter role is supported by the ability of the hologlutaredoxins to rapidly and efficiently transfer their clusters to apo-proteins in vitro. It has been proposed that they can act either as scaffold proteins for the de novo synthesis of iron-sulphur clusters or as carrier proteins for the transfer and delivery of preassembled iron-sulphur clusters.