The Structure and Function of the Ferredoxin/Thioredoxin System in Photosynthesis

The demonstration that thioredoxin could function in regulation grew out of CO2 assimilation experiments initiated more than thirty years ago with isolated chloroplasts. This work ultimately led to the description of the ferredoxin/thioredoxin system, whereby the activity of enzymes, key to oxygenic photosynthetic processes, is linked to light. Electrons provided by excited chlorophyll are transferred to ferredoxin and then sequentially to the enzyme ferredoxin: thioredoxin reductase (FTR) and a thioredoxin (f or m in chloroplasts). FTR converts an electron signal to a sulfhydryl (SH) signal that can be recognized by specific enzymes with a complementary disulfide (S-S) site. Through a reversible reduction of regulatory disulfide elements, thioredoxin brings about a structural change that alters the catalytic properties of the target enzymes. By selectively activating and deactivating regulatory enzymes of opposing pathways, the ferredoxin/thioredoxin system makes it possible for the oxygenic photosynthetic system (chloroplast or prokaryote) to house biosynthetic and catabolic processes in a single compartment and separate their activities diurnally—i.e., by the presence or absence of light. In this way autotrophic (photosynthetic) and heterotrophic lifestyles can be accommodated and function in a single organism. We summarize below the recent developments on the structure, mechanism of action and function of the individual thiol components of the ferredoxin/thioredoxin system. As a result of impressive progress made during this decade, we now have a better understanding of the events underlying the regulatory role of thioredoxin in oxygenic photosynthesis and associated processes linked to light. This work will also assist in the understanding of the growing array of heterotrophic plant and animal processes found to be regulated by thioredoxin reduced enzymatically with NADPH.