Thioredoxins: structure and function in plant cells

Abstract

SUMMARY Thioredoxins are ubiquitous small‐molecular‐weight proteins (typically 100–120 amino‐acid residues) containing an extremely reactive disulphide bridge with a highly conserved sequence ‐Cys‐Gly(Ala/Pro)‐Pro‐Cys‐. In bacteria and animal cells, thioredoxins participate in multiple reactions which require reduction of disulphide bonds on selected target proteins/ enzymes. There is now ample biochemical evidence that thioredoxins exert very specific functions in plants, the best documented being the redox regulation of chloroplast enzymes. Another area in which thioredoxins are believed to play a prominent role is in reserve protein mobilization during the process of germination. It has been discovered that thioredoxins constitute a large multigene family in plants with different‐subcellular localizations, a unique feature in living cells so far. Evolutionary studies based on these molecules will be discussed, as well as the available biochemical and genetic evidence related to their functions in plant cells. Eukaryotic photosynthetic plant cells are also unique in that they possess two different reducing systems, one extrachloroplastic dependent on NADPH as an electron donor, and the other one chloroplastic, dependent on photoreduced ferredoxin. This review will examine in detail the latest progresses in the area of thioredoxin structural biology in plants, this protein being an excellent model for this purpose. The structural features of the reducing enzymes ferredoxin thioredoxin reductase and NADPH thioredoxin reductase will also be described. The properties of the target enzymes known so far in plants will be detailed with special emphasis on the structural features which make them redox regulatory. Based on sequence analysis, evidence will be presented that redox regulation of enzymes of the biosynthetic pathways first appeared in cyanobacteria possibly as a way to cope with the oxidants produced by oxygenic photosynthesis. It became more elaborate in the chloroplasts of higher plants where a co‐ordinated functioning of the chloroplastic and extra chloroplastic metabolisms is required.

Oxidation-Reduction Properties of Chloroplast Thioredoxins, Ferredoxin:Thioredoxin Reductase, and Thioredoxin f-Regulated Enzymes

“…FTR contains a unique [4Fe-4S] cluster that serves to stabilize the one-electron-reduced intermediate formed after the first electron donation by ferredoxin, during the two-electron reduction of the activesite disulfide of the oxidized enzyme to the two cysteine thiols present in reduced FTR (4,5). FTR reduces thioredoxin in a reaction in which the two cysteines at the active site of the reduced enzyme become oxidized to a cystine disulfide, while the active-site disulfide of the oxidized thioredoxin becomes reduced to two cysteine thiols (1)(2)(3)(4)(5). FTR reduces both of the thioredoxins found in chloroplasts, thioredoxin f and thioredoxin m (monomeric proteins with molecular masses of ∼12 kDa that contain a conserved -WCGPCactive site), with equal efficiency.…”

“…The initial step in the thioredoxin regulatory pathway, which has been extensively characterized in spinach and pea chloroplasts, involves the reduction of ferredoxin:thioredoxin reductase (hereafter abbreviated FTR 1 ) by the reduced ferredoxin generated during light-driven noncyclic electron flow (1)(2)(3). Spinach leaf FTR, the best characterized of these enzymes, is a 25.6 kDa heterodimeric protein located in the chloroplast stroma (1)(2)(3).…”

“…The ferredoxin/thioredoxin system of oxygenic photosynthetic organisms plays an important role in the regulation of the carbon metabolism of these organisms (1)(2)(3). The initial step in the thioredoxin regulatory pathway, which has been extensively characterized in spinach and pea chloroplasts, involves the reduction of ferredoxin:thioredoxin reductase (hereafter abbreviated FTR 1 ) by the reduced ferredoxin generated during light-driven noncyclic electron flow (1)(2)(3).…”

“…FTR reduces both of the thioredoxins found in chloroplasts, thioredoxin f and thioredoxin m (monomeric proteins with molecular masses of ∼12 kDa that contain a conserved -WCGPCactive site), with equal efficiency. However, the two chloroplast thioredoxins display differential but overlapping reactivities among the array of identified target proteins (1)(2)(3). Although regulatory reduction by thioredoxin m appears to be restricted to glucose-6-phosphate dehydrogenase and the NADP + -linked malate dehydrogenase (hereafter abbreviated MDH), MDH is even more efficiently activated by thioredoxin f (6)(7)(8), which also regulates fructose-1,6-bisphosphatase (hereafter abbreviated FBPase), phosphor-ibulokinase (hereafter abbreviated PRK), and sedoheptulose-1,7-bisphosphatase (1)(2)(3).…”

“…The initial step in the thioredoxin regulatory pathway, which has been extensively characterized in spinach and pea chloroplasts, involves the reduction of ferredoxin:thioredoxin reductase (hereafter abbreviated FTR 1 ) by the reduced ferredoxin generated during light-driven noncyclic electron flow (1)(2)(3). Spinach leaf FTR, the best characterized of these enzymes, is a 25.6 kDa heterodimeric protein located in the chloroplast stroma (1)(2)(3). FTR contains a unique [4Fe-4S] cluster that serves to stabilize the one-electron-reduced intermediate formed after the first electron donation by ferredoxin, during the two-electron reduction of the activesite disulfide of the oxidized enzyme to the two cysteine thiols present in reduced FTR (4,5).…”