Partial characterization of cyclic electron transport in intact chloroplasts
Turnover of the cyclic electron transfer chain around photosystem I in intact chloroplasts was induced by addition of sodium dithionite after poisoning with 3-(3,4-dichlorophenyl)-1,1-dimethylurea. A substantial permeability barrier to dithionite allowed redox poising to a level sufficiently negative to active, but not overreduce, the cycle. Spectral changes could thus be studied without interference from photosystem II reactions. Illumination by repetitive single-turnover flashes showed the participation in the cycle of cytochromes f and b/sub 563/ with an apparent 1:1 stoichiometry. The rise of the flash-induced electrochromic bandshift showed a fast phase with rise time < 10 ..mu..s and a slow phase with rise time variable in the millisecond range. The slow phase had an amplitude equal to that of the fast phase and occurred only when electron transfer between cytochromes b/sub 563/ and f was uninhibited. A kinetic correlation was observed between the rise of the slow phase and the rereduction of cytochrome f, whereas cytochrome b/sub 563/ reoxidation was slower than both. Redox titrations of the appearance of the slow rise in the P518 response showed that it was only observed on repetitive flashes when a component of midpoint potential approx. = -55mV(pH 8.1), n = 2, was reduced before the flash. A comparison is drawn between this protonmotive electron transfer cycle and that of the purple nonsulfur bacterium Rhodopseudomonas capsulata; possible arrangements of electron carriers in the photosystem I cycle are discussed, and a modified Q cycle is proposed to account for the properties observed.
- Research Organization:
- Brookhaven National Lab., Upton, NY
- OSTI ID:
- 5842981
- Journal Information:
- Arch. Biochem. Biophys.; (United States), Vol. 204:2
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
CHLOROPLASTS
PHYSICAL PROPERTIES
ELECTRON TRANSFER
BIOCHEMICAL REACTION KINETICS
ABSORPTION SPECTRA
BIOLOGICAL PATHWAYS
CELL MEMBRANES
COMPARATIVE EVALUATIONS
CYTOCHROMES
PHOTOSYNTHESIS
REDOX REACTIONS
CELL CONSTITUENTS
CHEMICAL REACTIONS
KINETICS
MEMBRANES
PHOTOCHEMICAL REACTIONS
PIGMENTS
REACTION KINETICS
SPECTRA
SYNTHESIS
550200* - Biochemistry