Regulation of coupling factor in field-grown sunflower: A Redox model relating coupling factor activity to the activities of other thioredoxin-dependent chloroplast enzymes |
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Authors: | David M Kramer Robert R Wise James R Frederick David M Alm John D Hesketh Donald R Ort Antony R Crofts |
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Institution: | (1) Biophysics Division, 156 Davenport Hall, 607 S Mathews Ave., 61801 Urbana, IL, USA;(2) Department of Plant Biology, Agricultural Research Service, Photosynthesis Research Unit, University of Illinois, Urbana, Illinois;(3) Department of Agronomy, Agricultural Research Service, Photosynthesis Research Unit, University of Illinois, Urbana, Illinois;(4) USDA, Agricultural Research Service, Photosynthesis Research Unit, University of Illinois, Urbana, Illinois |
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Abstract: | Simultaneous, non-invasive measurements were made of the rate of photosynthetic CO2 fixation and the state of activation of the chloroplast CF1CF0-ATP synthase (CF) in field-grown sunflower (Helianthus annuus L.) during the dark-to-light transition at sunrise. CO2 fixation showed a linear response with light intensity from zero to about 500–700 E m-2 s-1. However, at light intensities of only 5–22 E m-2 s-1, the energetic threshold for activation of the CF was found to be significantly lowered (as compared to the pre-dawn state), presumably through reduction of the regulatory sulfhdryl groups of the -subunit of the CF. When these studies were extended to chamber-grown plants, it was found that as little as 5 seconds of illumination at 4 E m-2 s-1 caused apparently full CF reduction. It is clear, therefore, that the catalytic activation of CF is not rate limiting to the induction of carbon assimilation under field conditions during a natural dark-to-light transition at sunrise. A model, based on the redox properties of the regulatory sulfhydryls, was developed to examine the significance of sulfhydryl midpoint potential in explaining the differences in light sensitivity and oxidation and reduction kinetics, between the CF and other thioredoxin-modulated chloroplast enzymes. Computer simulations of the light-induced regulation of three representative thioredoxin-modulated enzymes are presented.Abbreviations CF
chloroplast CF1-CF0 ATP synthease or coupling factor
- Ea
active form of CF
- Ea
0
active, oxidized form of CF
- Ea
r
active, reduced form of CF
- Ei
inactive form of CF
- Ei
0
inactive, oxidized form of CF
- Ei
r
inactive, reduced form of CF
- FBPase
fructose-1,6-bisphosphatase
- FTR
ferredoxin-thioredoxin oxidoreductase
- G6PDH
glucose-6-phosphate dehydrogenase
- MDH
NADP-malate dehydrogenase
- pmf
protonmotive force
- pmfT
threshold pmf required to activate CF
- pmfT
0
threshold pmf required to active the oxidized form of CF
- pmfT
r
threshold pmf required to activate the reduced form of CF
- TR
thioredoxin |
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Keywords: | chloroplast coupling factor induction thioredoxin regulation ATP synthase photosynthesis |
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