title: Thioredoxin/fructose-1,6-bisphosphatase affinity in the enzyme activation by the ferredoxin-thioredoxin system.
abstract: In this work we analyze the affinity relationship between photosynthetic fructose-1,6-bisphosphatase and ferredoxin and thioredoxin from spinach leaves, two components of the proposed light-activation system of this enzyme, using affinity techniques on ferredoxin- and thioredoxin-Sepharose columns. Oxidized and reduced ferredoxin did not show enzyme affinity, whereas thioredoxin, both the oxidized and the dithiothreitol-reduced form, exhibited a strong bisphosphatase affinity at pH 7.5; this thioredoxin/enzyme affinity appears diminished at pH 8.2. When the affinity experiments were performed in the presence of 5 mM Mg2+, only 30% and 12% of the bisphosphatase remained bound to the thioredoxin-Sepharose at pH 7.5 and 8.0, respectively; these percentages were reduced to 6% when the Mg2+ concentration increased to 10 mM. These results suggest that a rise of stromal pH and Mg2+ concentration can account for a loosening of the thioredoxin/bisphosphatase linkage, which could be of physiological significance in the dark-light transition. Studies on the nature of the chemical groups responsible for the affinity have shown that the thioredoxin/bisphosphatase linkage is concerned with the existence of hydrophobic clusters. We have found no difference in the behaviour of the chloroplastic thioredoxins f and m, and the cytoplasmic ones cf and cm. These results support the existence of an in vivo thioredoxin/fructose-1,6-bisphosphatase interaction, in accordance with the light-activation mechanism by the ferredoxin-thioredoxin system.
mesh_terms: D000596:Amino Acids / Q000032:analysis; D001426:Bacterial Proteins / Q000378:metabolism*; D004789:Enzyme Activation / Q000528:radiation effects; D004926:Escherichia coli / Q000032:analysis; D005288:Ferredoxins / Q000378:metabolism*; D006597:Fructose-Bisphosphatase / Q000032:analysis / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008027:Light; D008274:Magnesium / Q000494:pharmacology; D010084:Oxidation-Reduction; D010944:Plants / Q000201:enzymology*; D013879:Thioredoxins / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D001426:Bacterial Proteins; D005288:Ferredoxins; D013879:Thioredoxins; D006597:Fructose-Bisphosphatase; D008274:Magnesium