title: Differential effects of chilling-induced photooxidation on the redox regulation of photosynthetic enzymes.
abstract: Photosynthesis in plant species that are evolutionarily adapted for growth in warm climates is highly sensitive to illumination under cool conditions. Although it is well documented that illumination of these sensitive species under cool conditions results in the photosynthetic production of reactive oxygen molecules, the underlying mechanism for the inhibition of photosynthesis remains uncertain. Determinations of chloroplast fructose-1,6-bisphosphatase and sedoheptulose-1,7-bisphosphatase activity showed that the light-dependent, reductive activation of these key carbon reduction cycle enzymes was substantially inhibited in tomato (Lycopersicon esculentum) following illumination at 4 degrees C. However, other chloroplast enzymes also dependent on thioredoxin-mediated reductive activation were largely unaffected. We performed equilibrium redox titrations to investigate the thermodynamics of the thiol/disulfide exchange between thioredoxin f and the regulatory sulfhydryl groups of fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase, phosphoribulokinase, NADP-glyceraldehyde phosphate dehydrogenase, and the chloroplast ATPsynthase. We determined that the redox midpoint potentials for the regulatory sulfhydryl groups of the various enzymes spanned a broad range ( approximately 50 mV at pH 7. 9). The electron-sharing equilibria among thioredoxin f and its target enzymes largely explained the differential effects of photooxidation induced at low temperature on thioredoxin-mediated activation of chloroplast enzymes in tomato. These results not only provide a plausible mechanism for the low-temperature-induced inhibition of photosynthesis in this important group of plants, but also provide a quantitative basis to evaluate the influence of thioredoxin/target enzyme electron-sharing equilibria on the differential activation and deactivation kinetics of thioredoxin-regulated chloroplast enzymes.
authors.affiliation: Photosynthesis Research Unit, USDA/ARS, and Department of Plant Biology, University of Illinois, Urbana, Illinois 61801-3838, USA.
authors.forename: R S
authors.initials: RS
authors.lastname: Hutchison
authors.forename: Q
authors.initials: Q
authors.lastname: Groom
authors.forename: D R
authors.initials: DR
authors.lastname: Ort
mesh_terms: D025181:ATP Synthetase Complexes; D054479:Chloroplast Thioredoxins; D002736:Chloroplasts / Q000201:enzymology* / Q000378:metabolism; D003080:Cold Temperature; D004220:Disulfides / Q000378:metabolism; D004789:Enzyme Activation / Q000528:radiation effects; D006597:Fructose-Bisphosphatase / Q000378:metabolism; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases / Q000378:metabolism; D008027:Light; D018551:Solanum lycopersicum / Q000201:enzymology*; D009097:Multienzyme Complexes / Q000378:metabolism; D010084:Oxidation-Reduction; D010446:Peptide Fragments / Q000378:metabolism; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism; D017856:Phosphotransferases (Phosphate Group Acceptor) / Q000378:metabolism; D010788:Photosynthesis; D018724:Spinacia oleracea / Q000201:enzymology; D013438:Sulfhydryl Compounds / Q000378:metabolism; D013816:Thermodynamics; D013879:Thioredoxins / Q000378:metabolism
chemical_list: D054479:Chloroplast Thioredoxins; D004220:Disulfides; D009097:Multienzyme Complexes; D010446:Peptide Fragments; D013438:Sulfhydryl Compounds; C068142:glyceraldehyde 3-phosphate dehydrogenase (304-313); D013879:Thioredoxins; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases; D017853:Phosphotransferases (Alcohol Group Acceptor); C022390:phosphoribulokinase; D025181:ATP Synthetase Complexes; D017856:Phosphotransferases (Phosphate Group Acceptor); D006597:Fructose-Bisphosphatase; D010744:Phosphoric Monoester Hydrolases; C022627:sedoheptulose-bisphosphatase