Mechanism of photoinhibition in pea thylakoids: effects of irradiance level and pH (original) (raw)
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Zeitschrift für Naturforschung. C, Journal of biosciences
The relationship between thylakoid membrane fluidity and the process of photoinhibition at room and low (4 degrees C) temperature was investigated. Two different membrane perturbing agents--cholesterol and benzylalcohol were applied to manipulate the fluidity of isolated pea thylakoids. The photochemical activity of photosystem I (PSI) and photosystem II (PSII), polarographically determined, were measured at high light intensity for different time of illumination at both temperatures. The exposure of cholesterol- and benzylalcohol-treated thylakoid membranes to high light intensities resulted in inhibition of both studied photochemical activities, being more pronounced for PSII compared to PSI. Time dependencies of inhibition of PSI and PSII electron transport rates for untreated and membranes with altered fluidity were determined at 20 degrees C and 4 degrees C. The effect is more pronounced for PSII activity during low-temperature photoinhibition. The data are discussed in terms o...
Biochemistry, 1992
Thylakoid membranes were isolated from pea seedlings grown under intermittent light (2-min light/118-min dark cycles). These preparations differed from controls (thylakoids from plants grown under 16-h light/8-h dark cycles) in the following respects: 15 times smaller chlorophyll/protein ratio, 10 times greater chlorophyll a/b ratio, absence of light-harvesting chlorophyll a/b binding proteins, and 2-3-fold greater ratio of photosystem II over photosystem I. In addition we found the following: (1) Electrogenic electron transfer around cytochrome b6/f under flashing light was greatly enhanced, probably as a consequence of the greater photosystem II/photosystem I ratio. (2) The rate of proton uptake from the medium at the acceptor side of photosystem II was enhanced, probably by unshielding of the quinone binding domain. (3) The N,N'-dicyclohexylcarbodiimide sensitivity of the proton-pumping activity of photosystem II was absent, which was consistent with the attribution of a N,N'-dicyclohexylcarbodiimide-induced protonic short circuit to chlorophyll a/b binding proteins. (4) The sensitivity of oxygen evolution under continuous light to variations of pH or the concentration of Ca2+ was altered. Chlorophyll a/b binding proteins serve as light-harvesting antennas. We found in addition that they modulated the activity of water oxidation and, in particular, the proteolytic reactions around photosystem II.
Photochem. Photobiol. Sci., 2008
Heat-induced changes in photosystem I (PSI) have been studied in terms of rates of oxygen consumption using various donors (DCPIPH 2 , TMPD red and DAD red ), formation of photo-oxidized P700 and changes in Chl a fluorescence emission at 77 K. Linear heating of thylakoid membranes from 35 • C to 70 • C caused an enhancement in PSI-mediated electron transfer rates (DCPIPH 2 →MV) up to 55 • C. However, no change was observed in PSI rates when other electron donors were used (TMPD red and DAD red ). Similarly, Chl a fluorescence emission spectra at 77 K of heat-treated thylakoid membranes did not show any increase in peak at 735 nm, however, a significant decrease was observed as a function of temperature in the peaks at 685 and 694 nm. In DCMU-treated control thylakoid membranes maximum photo-oxidized P700 was generated at g = 2.0025. In heat-treated thylakoid membranes maximum intensity of photo-oxidized P700 signal was observed at ∼50-55 • C without DCMU treatment. The steady-state signal of the photo-oxidized P700 was studied in the presence of DCPIPH 2 and TMPD red as electron donors in DCMU-treated control and in 50 • C treated thylakoid membranes. We present here the first of such comparative study of PSI activity in terms of the rates of oxygen consumption and re-reduction kinetics of photo-oxidized P700 in the presence of different electron donors. It appears that the formation of the P700 + signal in heat-treated thylakoid membranes is due to an inhibited electron supply from PSII and not due to spillover or antenna migration.
FEBS Letters, 1994
Proton and electron transfer at the reducing side of photosystem II of green plants was studied under flashing light, the former at improved time resolution by using Neutral red. The rates of electron transfer within QAFeQB were determined by pump-probe flashes through electrochromic transients. The extent of proton binding was about 1 H+/e-. The rates of proton transfer were proportional to the concentration of Neutral red (collisional transfer), whereas the rates of electron transfer out of QA- and from QAFeQB- to the cytochrome b6f complex were constant. The half-rise times of electron transfer (tau e) and the apparent times of proton binding (tau h) at 30 microM Neutral red were: QA- --> FeIIIQB (tau c < or = 100 microseconds, tau h = 230 microseconds); QA- --> FeIIQB (tau c = 150 microseconds, tau h = 760 microseconds); and QA- --> FeIIQB (tau c = 150 microseconds, tau h = 760 microseconds); and QA- --> FeIIQB (tau c = 620 microseconds, tau h = 310 microseconds).
Oxygen exchange associated with electron transport and photophosphorylation in spinach thylakoids
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1983
0 2 uptake in spinach thylakoids was composed of ferredoxin-dependent and-independent components. The ferredoxin-independent component was largely 3-(3,4-didflorophanyl)-l,l-dimethylurea (DCMU) insensitive (60%). Light-dependent O 2 uptake was stimulated 7-fold by 70 pM ferredoxin and both uptake and evolution (with 0 2 as the only electron aeeeptor) responded almost linearly to ferredoxin up to 40 #M. NADP + reduction, however, was saturated by less than 20 ttM ferredoxin. The affinity of 0 2 uptake for for 0 2 was highly dependent on ferredoxin concentration, with Kt/2(02) of less than 20/tM at 2 pM ferredoxin but greater than 60 pM 02 with 25/tMferredoxin. 0 2 uptake could be suppressed up to 80% with saturating NADP + and it approximated a competitive inhibitor of 0 2 uptake with a K l of 8-15 pM. Electron transport in these thylakoids supported high rates of photoplu~phor~lation with NADP + (600 pmol ATP/mg Chl per h) or 0 2 (280/tmol/mg Chi per h) as electron aeeeptors, with ATP/2e ratios of 1.15-1.55. Variation in ATP/2e ratios with ferredoxin concentration and effects of antimycin A indicate that cyclic electron flow may also be occurring in this thylakoid system. Results are discussed with regard to photoreduction of 0 2 as a potential source of ATP in viva. Introdnetion Photoreduction of oxygen by the chloroplast electron-transport chain was first recognised over thee decades ago [1] and has been termed the 'Mehler reaction' after its discover. It is clear from an examination of subsequent literature that a number of different reactions involving the photoreduction of oxygen have been studied under this
Biophysical Chemistry, 2004
The time dependence of photobleaching of photosynthetic pigments under high light illumination of isolated spinach thylakoid membranes at 22 and 4 8C was investigated. At 22 8C, the bleaching at 678, 472 and 436 nm was prominent but lowering the temperature up to 4 8C during illumination prevented the pigments from bleaching almost completely. The accelerating effect on pigment photobleaching by the presence of 3-(3,4 dichlorophenyl)-1,1dimethyl-urea)-(DCMU), a well-known inhibitor of the electron transport and known to prevent photosystem I (PSI) and photosystem II (PSII) against photoinhibitory damage, was also suppressed at low temperature. At 22 8C in the presence and absence of DCMU, the decrease of the absorption at 678 and 472 nm was accompanied by a shift to the shorter wavelengths. To check the involvement of reactive oxygen species in the process, pigment photobleaching was followed in anaerobiosis. The effects of the three different environmental factors-light, temperature and DCMU-on the dynamics of photobleaching are discussed in terms of different susceptibility of the main pigment-protein complexes to photoinhibition. ᮊ