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1.
Removal of 23 and 17 kDa water-soluble polypeptides from PS II membranes causes a marked decrease in oxygen-evolution activity, exposes the oxidizing side of PS II to exogenous reductants (Ghanotakis, D.F., Babcock, G.T. and Yocum, C.F. (1984) Biochim. Biophys. Acta 765, 388–398) and alters a high-affinity binding site for Ca 2+ in the oxygen-evolving complex (Ghanotakis, D.F., Topper, J.N., Babcock, G.T. and Yocum, C.F. (1984) FEBS Lett. 170, 169–173). We have examined further the state of the functional Mn complex in PS II membranes from which the 17 and 23 kDa species have been removed by high-salt treatment. These membranes contain a structurally altered Mn complex which is sensitive to destruction by low concentrations of NH 2OH which cannot, in native PS II membranes, cause extraction of functional Mn. In addition to NH 2OH, a wide range of other small (H 2O 2, NH 2NH 2, Fe 2+) and bulky (benzidine, hydroquinone) electron donors extract Mn (up to 80%) from the polypeptide-depleted PS II preparations. This extraction is due to reduction of the functional Mn complex since light, which would generate higher oxidation states within the Mn complex, prevents Mn release by reductants. Release of Mn by reductants does not extract the 33 kDa water-soluble protein implicated in Mn binding to the oxidizing side of PS II, although the protein can be partially or totally extracted from Mn-depleted preparations by exposure to high ionic strength or to high (0.8 M) concentrations of Tris. We view our results as evidence for a shield around the Mn complex of the oxygen-evolving complex comprised of the 33 kDa polypeptide along with the 23 and 17 kDa proteins and tightly bound Ca 2+. 相似文献
2.
The kinetics of flash-induced electron transport were investigated in oxygen-evolving Photosystem II preparations, depleted of the 23 and 17 kDa polypeptides by washing with 2 M NaCl. After dark-adaptation and addition of the electron acceptor 2,5-dichloro- p-benzoquinone, in such preparations approx. 75% of the reaction centers still exhibited a period 4 oscillation in the absorbance changes of the oxygen-evolving complex at 350 nm. In comparison to the control preparations, three main effects of NaCl-washing could be observed: the half-time of the oxygen-evolving reaction was slowed down to about 5 ms, the misses and double hits parameters of the period 4 oscillation had changed, and the two-electron gating mechanism of the acceptor side could not be detected anymore. EPR-measurements on the oxidized secondary donor Z + confirmed the slower kinetics of the oxygen-releasing reaction. These phenomena could not be restored by readdition of the released polypeptides nor by the addition of CaCl 2, and are ascribed to deleterious action of the highly concentrated NaCl. Otherwise, the functional coupling of Photosystem II and the oxygen-evolving complex was intact in the majority of the reaction centers. Repetitive flash measurements, however, revealed P +Q − recombination and a slow Z + decay in a considerable fraction of the centers. The flash-number dependency of the recombination indicated that this reaction only appeared after prolonged illumination, and disappeared again after the addition of 20 mM CaCl 2. These results are interpreted as a light-induced release of strongly bound Ca 2+ in the salt-washed preparations, resulting in uncoupling of the oxygen-evolving system and the Photosystem II reaction center, which can be reversed by the addition of a relatively high concentration of Ca 2+. 相似文献
3.
Oxygen-evolving Photosystem II (PS II) particles were prepared from the thylakoid membranes of a chlorophyll b-less rice mutant, which totally lacks light-harvesting chlorophyll a/b proteins, after solubilization with β-octylglucoside. The preparation was essentially free of Photosystem I as judged from its low-temperature fluorescence spectrum and polypeptide composition. The PS II particles contained all the major subunit polypeptides of the PS II reaction center core complexes and the three extrinsic proteins related to oxygen evolution. The relative abundances of the 33, 21 and 15 kDa proteins were 100, 64 and 20%, respectively, of the corresponding proteins in the mutant thylakoids. The chlorophyll-to-Q A ratio was 53 and there was only one bound Ca 2+ per Q A. Thus, one of the two bound Ca 2+ present in the oxygen-evolving PS II membrane preparations from wild-type rice (Shen J.-R., Satoh, K. and Katoh, S. (1988) Biochim. Biophys. Acta 933, 358–364) is missing. The mutant PS II particles were highly active in oxygen evolution in the absence of exogenously added Ca 2+, although addition of 5 mM Ca 2+ enhanced the activity by 30%. When the 21 and 15 kDa proteins were supplemented to the particles, the Ca 2+-effect disappeared and the rate of oxygen evolution increased to a level exceeding 1000 μmol O 2 per mg chlorophyll per h. The results indicate that the number of Ca 2+ needed to promote a high rate of oxygen evolution is one per PS II in higher plants. 相似文献
4.
The mechanism by which Cl − activates the oxygen-evolving complex (OEC) of Photosystem II (PS II) in spinach was studied by 35Cl-NMR spectroscopy and steady-state measurements of oxygen evolution. Measurements of the excess 35Cl-NMR linewidth in dark-adapted, Cl −-depleted thylakoid and Photosystem II membranes show an overall hyperbolic decrease which is interrupted by sharp increases in linewidth (linewidth maxima) at approx. 0.3 mM, 0.75 mM, 3.25 mM (2.0 mM in PS II membranes), and 7.0 mM Cl −. The rate of the Hill reaction (H 2O → 2,6-dichlorophenolindophenol) at low light intensities (5% of saturation) as a function of [Cl −] in thylakoids shows three intermediary plateaus in the concentration range between 0.1 and 10 mM Cl − indicating kinetic cooperativity with respect to Cl −. The presence of linewidth maxima in the 35Cl-NMR binding curve indicates that Cl − addition exposes four types of Cl − binding site that were previously inaccessible to exchange with Cl − in the bulk solution. These results are best explained by proposing that Cl − binds to four sequestered (salt-bridged) domains within the oxygen-evolving complex. Binding of Cl − is facilitated by the presence of H + and vice versa. The pH dependence of the excess 35Cl-NMR linewidth at 0.75 mM Cl − shows that Cl − binding has a maximum at pH 6.0 and two smaller maxima at pH 5.4 and 6.5 which may suggest that as many as three groups (perhaps histidine) with p Ka values in the region may control the binding. 相似文献
5.
The thermoluminescence band observed in chloroplasts after flash excitation at ambient temperatures has recently been identified as being due to recombination of the electron on the semiquinone form of the secondary plastoquinone acceptor, Q B, with positive charges on the oxygen-evolving enzyme, S 2 and S 3 (Rutherford, A.W., Crofts, A.R. and Inoue, Y. (1982) Biochim. Biophys. Acta 682, 457–465). Further investigation of this thermoluminescence confirms this assignment and provides information on the function of PS II. The following data are reported: (1) Washing of chloroplasts with ferricyanide lowers the concentration of Q B− in the dark and predictable changes in the extent of the thermoluminescence band are observed. (2) The thermoluminescence intensity arising from S 2Q B− is approximately one half of that arising from S 3Q B−. (3) Preflash treatment followed by dark adaptation results in changes in the intensity of the thermoluminescence band recorded after a series of flashes. These changes can be explained according to the above assignments for the origin of the thermoluminescence and if Q B− provides an important source of deactivating electrons for the S states. Computer simulations of the preflash data are reported using the above assumptions. Previously unexplained data already in the literature (Läufer, A. and Inoue, Y. (1980) Photobiochem. Photobiophys. 1, 339–346) can be satisfactorily explained and are simulated using the above assumptions. (4) Lowering the pH to pH 5.5 results in a shift of the S 2Q B− thermoluminescence band to higher temperatures while that arising from S 3Q B− does not shift. This effect is interpreted as indicating that Q B− is protonated and the S 2 to S 3 reaction involves deprotonation while the S 1 to S 2 reaction does not. 相似文献
6.
Charge-transfer reactions to secondary electron donors (Z, M) and acceptors (Q A, Q B) in Photosystem II particles isolated from a thermophilic cyanobacterium Synechococcus sp. (Schatz, G.H. and Witt H.T. (1984) Photobiochem. Photobiophys. 7, 1–14) were analyzed by measurements of fluorescence yield and absorbance changes in the millisecond time domain induced by repetitive flashes. (1) The electron-transfer reaction Q −AQ B → Q AQ −B was found to occur with kinetic phases of 0.2 ± 0.1 ms and 1.5 ± 0.5 ms half-time. At 10 ms after flashes an equilibrium distribution of Q −AQ B/Q AQ −B of about 15/85 in oxygen-evolving and of about 25/75 in Tris-treated PS II particles was reached. (2) The absorbance difference spectra were determined for (Q −A - Q A), (Q −B - Q B), (Z + - Z) and for (S 4 - S 0), the transition associated with oxygen evolution. In the ultraviolet region they show that these electron-acceptors and -donors are the same as in spinach PS II. In the visible region all the difference spectra contain major contributions by electrochromic bandshifts due to electrostatic interaction of the reduced acceptors or oxidized donors with nearby reaction center pigments. Upon electron transfer from Q −A to Q B electrochromic bandshifts due to interaction with pheophytin a disappeared almost completely. Bandshifts observed in the (Z + - Z) and (S 4 - S 0) spectra were attributed to chlorophyll a. 相似文献
7.
The kinetics of deactivation of the S 3 state in Chlorella have been observed under a variety of conditions. The S 3 state appears to decline in a dark period coming after a sequence of 30 saturating flashes in a second-order reaction, the rate constant of which is 0.132/[S* 3] s −1 and which involves an electron donor, D 1, of concentration 1.25[S* 3] where [S* 3] is the concentration of the S 3 state when the oxygen yield of the light flashes is constant. If a 1 min period of 650 nm illumination is employed after the sequence of flashes, the subsequent S 3 state deactivation kinetics are more complex. There is an initial phase of S 3 state deactivation, accounting for about 35% of the original S 3 state, which is complete in less than 100 ms. The remaining 65% of the S 3 state appears to deactivate in a second-order reaction, the rate constant of which is 1.36/[S* 3] s −1 and which involves an electron donor of initial concentration 0.58[S* 3]. If a 1 min period of 710 nm illumination comes after the 30 flashes, at least 98% of the S 3 state deactivates according to first-order kinetics. It is shown that this can be explained using a second-order model if there is an electron donor present of which the concentration is large compared with [S* 3]. However, S 3 state deactivation observed after 5 min of dark and two saturating flashes can be described neither by a first-order model nor a second-order model. Deactivation of the S 2 state after a 5 min dark period and one saturating flash follows second-order kinetics with a rate constant of 0.2/[S* 3] s −1 and appears to involve an electron donor of initial concentration 1.3[S* 3]. Arguments are presented which tend to rule out the primary electron acceptor to Photosystem II as being any of the electron donors but it appears quite possible that the large plastoquinone pool is involved. 相似文献
8.
The dominance of diatoms in turbulent waters suggests special adaptations to the wide fluctuations in light intensity that
phytoplankton must cope with in such an environment. Our recent demonstration of the unusually effective photoprotection by
the xanthophyll cycle in diatoms [Lavaud et al. (2002) Plant Physiol 129 (3) (in press)] also revealed that failure of this
protection led to inactivation of oxygen evolution, but not to the expected photoinhibition. Photo-oxidative damage might
be prevented by an electron transfer cycle around Photosystem II (PS II). The induction of such a cycle at high light intensity
was verified by measurements of the flash number dependence of oxygen production in a series of single-turnover flashes. After
a few minutes of saturating illumination, the oxygen flash yields are temporarily decreased. The deficit in oxygen production
amounts to at most 3 electrons per PS II, but continues to reappear with a half time of 2 min in the dark until the total
pool of reducing equivalents accumulated during the illumination has been consumed by (chloro)respiration. This is attributed
to an electron transfer pathway from the plastoquinone pool or the acceptor side of PS II to the donor side of PS II that
is insignificant at limiting light intensity but is accelerated to milliseconds at excess light intensity. Partial filling
of the 3-equivalents capacity of the cyclic electron transfer path in PS II may prevent both acceptor-side photoinhibition
in oxygen-evolving PS II and donor-side photoinhibition when the oxygen-evolving complex is temporarily inactivated.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
9.
Analyses of chlorophyll fluorescence induction kinetics from DCMU-poisoned thylakoids were used to examine the contribution of the light-harvesting chlorophyll a/ b protein complex (LHCP) to Photosystem II (PS II) heterogeneity. Thylakoids excited with 450 nm radiation exhibited fluorescence induction kinetics characteristic of major contributions from both PS II and PS II β centres. On excitation at 550 nm the major contribution was from PS II β centres, that from PS II centres was only minimal. Mg 2+ depletion had negligible effect on the induction kinetics of thylakoids excited with 550 nm radiation, however, as expected, with 450 nm excitation a loss of the PS II component was observed. Thylakoids from a chlorophyll- b-less barley mutant exhibited similar induction kinetics with 450 and 550 nm excitation, which were characteristic of PS II β centres being the major contributors; the PS II contribution was minimal. The fluorescence induction kinetics of wheat thylakoids at two different developmental stages, which exhibited different amounts of thylakoid appression but similar chlorophyll a/ b ratios and thus similar PS II:LHCP ratios, showed no appreciable differences in the relative contributions of PS II and PS II β centres. Mg 2+ depletion had similar effects on the two thylakoid preparations. These data lead to the conclusion that it is the PS II:LHCP ratio, and probably not thylakoid appression, that is the major determinant of the relative contributions of PS II and PS II β to the fluorescence induction kinetics. PS II characteristics are produced by LHCP association with PS II, whereas PS II β characteristic can be generated by either disconnecting LHCP from PS II or by preferentially exciting PS II relative to LHCP. 相似文献
10.
Treatment of the oxygen-evolving photosystem II preparation from the thermophilic cyanobacterium Synechococcus sp. with EDTA inhibited electron flow from Z to P680 and consequently induced a back electron flow from Q −a to P680 +. The inhibition was reversed fully by Ca 2+and partially by Mn 2+ and Mg 2+ when EDTA-treated preparations had been incubated with respective divalent metal cations for several minutes, whereas diphenylcarbazide had no effect on the recombination between q −a and P680 + in EDTA-treated preparations. It is concluded that Ca 2+ is essential for electron transport from Z to P680. Oxygen evolution Electron transport Photosystem II Ca2+ Thermophilic cyanobacterium 相似文献
11.
The temperature dependence of S-state transitions in Photosystem II was measured by means of thermoluminescence using two different protocols for low-temperature flash excitation: protocol A, “last flash at low temperature”, and protocol B, “all flashes at low temperature”. Comparison of the temperature-dependence curves obtained by these two protocols revealed a marked difference particular for the three-flash experiments. The difference was attributed to the formation of a low-temperature sensitive precursor state between S 2 and S 3. The state is formed by two flash illumination given at −5 to −50°C, spontaneously transforms to normal S 3 on dark warming, and is not converted to S 0 by the 3rd flash. The precursor state was tentatively assigned to an S 3 in which H + release is not completed. 相似文献
12.
The 33-kDa manganese-stabilizing protein stabilizes the manganese cluster in the oxygen-evolving complex. There has been, however, a considerable amount of controversy concerning the stoichiometry of this photosystem II (PS II) component. In this paper, we have verified the extinction coefficient of the manganese-stabilizing protein by amino acid analysis, determined the manganese content of oxygen-evolving photosystem II membranes and reaction center complex using inductively coupled plasma spectrometry, and determined immunologically the amount of the manganese-stabilizing protein associated with photosystem II. Oxygen-evolving photosystem II membranes and reaction center complex preparations contained 258 +/- 11 and 67 +/- 3 chlorophyll, respectively, per tetranuclear manganese cluster. Immunoquantification of the manganese-stabilizing protein using mouse polyclonal antibodies on "Western blots" demonstrated the presence of 2.1 +/- 0.2 and 2.0 +/- 0.3 molecules of the manganese-stabilizing protein/tetranuclear manganese cluster in oxygen-evolving PS II membranes and highly purified PS II reaction center complex, respectively. Since the manganese-stabilizing protein co-migrated with the D2 protein in our electrophoretic system, accurate immunoquantification required the inclusion of CaCl2-washed PS II membrane proteins or reaction center complex proteins in the manganese-stabilizing protein standards to compensate for the possible masking effect of the D2 protein on the binding of the manganese-stabilizing protein to Immobilon-P membranes. Failure to include these additional protein components in the manganese-stabilizing protein standards leads to a marked underestimation of the amount of the manganese-stabilizing protein associated with these photosystem II preparations. 相似文献
13.
We used two different techniques to measure the recovery time of Photosystem II following the transfer of a single electron from P-680 to Q A in thylakoid membranes isolated from spinach. Electron transfer in Photosystem II reaction centers was probed first by spectroscopic measurements of the electrochromic shift at 518 nm due to charge separation within the reaction centers. Using two short actinic flashes separated by a variable time interval we determined the time required after the first flash for the electrochromic shift at 518 nm to recover to the full extent on the second flash. In the second technique the redox state of Q A at variable times after a saturating flash was monitored by measurement of the fluorescence induction in the absence of an inhibitor and in the presence of ferricyanide. The objective was to determine the time required after the actinic flash for the fluorescence induction to recover to the value observed after a 60 s dark period. Measurements were done under conditions in which (1) the electron donor for Photosystem II was water and the acceptor was the endogenous plastoquinone pool, and (2) Q 400, the Fe 2+ near Q A, remained reduced and therefore was not a participant in the flash-induced electron-transfer reactions. The electrochromic shift at 518 nm and the fluorescence induction revealed a prominent biphasic recovery time for Photosystem II reaction centers. The majority of the Photosystem II reaction centers recovered in less than 50 ms. However, approx. one-third of the Photosystem II reaction centers required a half-time of 2–3 s to recover. Our interpretation of these data is that Photosystem II reaction centers consist of at least two distinct populations. One population, typically 68% of the total amount of Photosystem II as determined by the electrochromic shift, has a steady-state turnover rate for the electron-transfer reaction from water to the plastoquinone pool of approx. 250 e − / s, sufficiently rapid to account for measured rates of steady-state electron transport. The other population, typically 32%, has a turnover rate of approx. 0.2 e − / s. Since this turnover rate is over 1000-times slower than normally active Photosystem II complexes, we conclude that the slowly turning over Photosystem II complexes are inconsequential in contributing to energy transduction. The slowly turning over Photosystem II complexes are able to transfer an electron from P-680 to Q A rapidly, but the reoxidation of Q −A is slow ( t1/2 = 2 s). The fluorescence induction measurements lead us to conclude that there is significant overlap between the slowly turning over fraction of Photosystem II complexes and PS II β reaction centers. One corollary of this conclusion is that electron transfer from P-680 to Q A in PS II β reaction centers results in charge separation across the membrane and gives rise to an electrochromic shift. 相似文献
14.
Dinuclear manganese(II) complexes [Mn 2(bomp)(PhCO 2) 2]BPh 4 (1), [Mn 2(bomp)(MeCO 2) 2]BPh 4 (2), and [Mn 2(bomp)(PhCO 2) 2]PF 6 (3) were synthesized with a dinucleating ligand 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol [H(bomp)]. Dinuclear zinc complex [Zn 2(bomp)(PhCO 2) 2]PF 6 (4) was also synthesized for the purpose of comparison. X-ray analysis revealed that the complex 1·CHCl 3 contains two manganese ions bridged by the phenolic oxygen and two benzoate groups, forming a μ-phenoxo-bis(μ-benzoato)dimanganese(II) core. Magnetic susceptibility measurements of 1–3 over the temperature range 1.8–300 K indicated antiferromagnetic interaction ( J=−4 to −6 cm −1). Cyclic voltammograms of 3 showed a quasi-reversible oxidation process at +0.9 V versus a saturated sodium chloride calomel reference electrode, assigned to Mn IIMn II/Mn IIMn III. 相似文献
15.
Manganese in the oxygen-evolving complex is a physiological electron donor to Photosystem II. PS II depleted of manganese may oxidize exogenous reductants including benzidine and Mn 2+. Using flash photolysis with electron spin resonance detection, we examined the room-temperature reaction kinetics of these reductants with Y z
+, the tyrosine radical formed in PS II membranes under illumination. Kinetics were measured with membranes that did or did not contain the 33 kDa extrinsic polypeptide of PS II, whose presence had no effect on the reaction kinetics with either reductant. The rate of Y z
+ reduction by benzidine was a linear function of benzidine concentration. The rate of Y z
+ reduction by Mn 2+ at pH 6 increased linearly at low Mn 2+ concentrations and reached a maximum at the Mn 2+ concentrations equal to several times the reaction center concentration. The rate was inhibited by K +, Ca 2+ and Mg 2+. These data are described by a model in which negative charge on the membrane causes a local increase in the cation concentration. The rate of Y z
+ reduction at pH 7.5 was biphasic with a fast 400 s phase that suggests binding of Mn 2+ near Y z
+ at a site that may be one of the native manganese binding sites.Abbreviations PS II
Photosystem II
- Y D
tyrosine residue in Photosystem II that gives rise to the stable Signal II EPR spectrum
- Y z
tyrosine residue in Photosystem II that mediates electron transfer between the reaction center chlorophyll and the site of water oxidation
- ESR
electron spin resonance
- DPC
diphenylcarbazide
- DCIP
dichlorophenolindophenol 相似文献
16.
The effects of Mn 2+ on aerobic photobleaching of carotenoids, on photoreduction of 2,6-dichlorophenolindophenol (DCIP) and on fluorescence above 600 mμ of spinach chloroplasts washed with 0.8 M Tris-HC1 buffer were investigated. Carotenoids (mostly carotenes, lutein and violaxanthin) in the Tris-washed chloroplasts were irreversibly bleached by illumination with red light, while carotenoids in normal chloroplasts prepared with a low concentration of Tris-HC1 underwent no bleaching upon illumination. The photobleaching of carotenoids observed with Tris-washed chloroplasts was inhibited by Mn 2+ (MnCl 2 or MnSO 4) as well as by some inhibitors of the Hill reaction such as dichlorophenyl-1,1-dimethylurea (DCMU), methylthio-4,6-bis-isopropylamino- s-triazine and o-phenanthroline or by reducing agents such as ascorbate plus tetramethyl- p-phenylene diamine (TMPD). DCIP photoreduction, which was deactivated by Tris, was reactivated to 50–80% of the rate for normal chloroplasts upon addition of Mn 2+. The restored photoreduction of DCIP was inhibited by DCMU and carbonylcyanide m-chlorophenylhydrazone (CCCP). The steady-state fluorescence yield of normal chloroplasts measured at room temperature was lowered by Tris treatment, and the decreased yield was restored by adding Mn 2+ as well as ascorbate plus TMPD. CCCP also lowered the yield; the yield was recovered by adding ascorbate plus TMPD. Determination of manganese in normal and Tris-washed chloroplasts showed that 30% of the manganese in chloroplast was removed with Tris. It was postulated that Mn 2+ functions in the electron transport on the oxidizing side of Photosystem II at a site between water and an electron carrier ( Y). CCCP as well as Tris inhibits the reduction of Y+ by Mn 2+, and carotenoids are oxidized by Y+ which is reduced by ascorbate plus TMPD. 相似文献
17.
1. By using dibromothymoquinone as the electron acceptor, it is possible to isolate functionally that segment of the chloroplast electron transport chain which includes only Photosystem II and only one of the two energy conservation sites coupled to the complete chain (Coupling Site II, observed P/e 2 = 0.3–0.4). A light-dependent, reversible proton translocation reaction is associated with the electron transport pathway: H 2O → Photosystem II → dibromothymoquinone. We have studied the characteristics of this proton uptake reaction and its relationship to the electron transport and ATP formation associated with Coupling Site II. 2. The initial phase of H+ uptake, analyzed by a flash-yield technique, exhibits linear kinetics (0–3 s) with no sign of transient phenomena such as the very rapid initial uptake (“pH gush”) encountered in the overall Hill reaction with methylviologen. Thus the initial rate of H+ uptake obtained by the flash-yield method is in good agreement with the initial rate estimated from a pH change tracing obtained under continuous illumination. 3. Dibromothymoquinone reduction, observed as O2 evolution by a similar flash-yield technique, is also linear for at least the first 5 s, the rate of O2 evolution agreeing well with the steady-state rate observed under continuous illumination. 4. Such measurements of the initial rates of O2 evolution and H+ uptake yield an H+/e− ratio close to 0.5 for the Photosystem II partial reaction regardless of pH from 6 to 8. (Parallel experiments for the methylviologen Hill reaction yield an H+/e− ratio of 1.7 at pH 7.6.) 5. When dibromothymoquinone is being reduced, concurrent phosphorylation (or arsenylation) markedly lowers the extent of H+ uptake (by 40–60%). These data, unlike earlier data obtained using the overall Hill reaction, lend themselves to an unequivocal interpretation since phosphorylation does not alter the rate of electron transport in the Photosystem II partial reaction. ADP, Pi and hexokinase, when added individually, have no effect on proton uptake in this system. 6. The involvement of a proton uptake reaction with an H+/e− ratio of 0.5 in the Photosystem II partial reaction H2O → Photosystem II → dibromothymoquinone strongly suggests that at least 50% of the protons produced by the oxidation of water are released to the inside of the thylakoid, thereby leading to an internal acidification. It is pointed out that the observed efficiencies for ATP formation (P/e2) and proton uptake (H+/e−) associated with Coupling Site II can be most easily explained by the chemiosmotic hypothesis of energy coupling. 相似文献
18.
A Fourier transform infrared (FTIR) difference spectrum upon photooxidation of the accessory chlorophyll (Chl z) of photosystem II (PS II) was obtained at 210 K with Mn-depleted PS II membranes in the presence of fericyanide and silicomolybdate. The observed Chl z+/Chl z spectrum showed two differential bands at 1747/1736 and 1714/1684 cm −. The former was assigned to the free carbomethoxy C = 0 and the latter to the keto C = 0 that is hydrogen-bonded or in a highly polar environment. Also, the negative 1614 cm − band assignable to the macrocycle mode indicated 5-coordination of the central Mg. The negative 1660 cm −1 band, possibly due to the strongly hydrogen-bonded keto C = 0, may suggest oxidation of one more Chl z, although an alternative assignment, the amide I mode of proteins perturbed by Chl z oxidation, is also possible. 相似文献
19.
Inhibition of electron transport through photosystem II (PS II) by formate (HCO −2) or nitrite (NO −2) in the presence or absence of chloride ions was studied. The inhibition induced by HCO −2 or NO −2 is overcome by HCO −3 more in the presence, than in the absence of Cl −. The data on electron transport are supported by chlorophyll a fluorescence measurements. In experiments. In experiments in which water oxidation was blocked. Cl − was found to facilitate electron transport between bound quinone A (Q −A) and the plastoquinone (PQ) pool. It can thus be concluded that in addition to the well known site of action of Cl − on water oxidation, another site of Cl − action is between Q −A and the PQ pool. 相似文献
20.
The 33 kDa manganese-stabilizing extrinsic protein binds to the lumenal side of photosystem II (PS II) close to the Mn(4)Ca cluster of the oxygen-evolving complex, where it limits access of small molecules to the metal site. Our previous finding that the removal of this protein did not alter the magnetic coupling regime within the manganese cluster, measured by electron spin-echo envelope modulation [Gregor, W., and Britt, R. D. (2000) Photosynth. Res. 65, 175-185], prompted us to examine whether this accessibility control is also true for substrate water, using the same pulsed EPR technique. Comparing the deuteron modulation of the S(2)-state multiline signal of PS II membranes, equilibrated with deuterated water (D(2)O) after removal or retention of the 33 kDa protein, we observed no change in the number and the distance of deuterons magnetically coupled to manganese, indicating that the number and distance of water molecules bound to the manganese cluster are independent of bound 33 kDa protein in the S(1) state, in which the sample was poised prior to cryogenic illumination. A simple modulation depth analysis revealed a distance of 2.5-2.6 A between the closest deuteron and manganese. These results are in agreement with our refined X-ray absorption analysis. The manganese K-edge positions, reflecting their oxidation states, and the extended X-ray absorption fine structure amplitudes and distances between the manganese ions and their oxygen and nitrogen ligands (1.8, 2.7, and 3.3-3.4 A) were independent of bound 33 kDa protein. 相似文献
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