28] Ferredoxin: Thioredoxin system

Abstract

This chapter focuses on the ferredoxin: thioroxin system. The ferredoxin: thioroxin system is a well-established regulatory system in oxygenic photosynthesis. It regulates, by light-driven thiol-disulfide exchange, the activity of key photosynthetic enzymes. Electrons provided by the excitation of chlorophyll are transferred via ferredoxin and ferredoxin : thioredoxin reductase (FTR) to two types of enzyme-specific thioredoxins, f and m. The thioredoxins as well as FFR each contain one redoxactive disulfide bridge involved in catalysis, and FTR has in addition a 4Fe-4S cluster. The ferredoxin : thioroxin system has been demonstrated in several organisms, and its components have been isolated and characterized. The best known system is the one from spinach. Both types of spinach chloroplast thioredoxins have been cloned and expressed in Escherichia coli, which considerably simplifies their purification. The FTR has also been cloned, but not overexpressed yet. Recombinant thioredoxin f, strongly expressed in E. coli, is present in the cells mostly in insoluble form deposited in inclusion bodies. This presents the advantage that the inclusion bodies can be easily purified and the insoluble thioredoxin freed from the bulk of contaminating proteins.