Kinetics and pH Dependence of Light-Induced Deprotonation of the Schiff Base of Rhodopsin: Possible Coupling to Proton Uptake and Formation of the Active Form of Meta II

In this paper we first review what is known about the kinetics of Meta II formation, the role and stoichiometry of protons in Meta II formation, the kinetics of the light-induced changes of proton concentration, and the site of proton uptake. We then go on to compare the processes that lead to the deprotonation of the Schiff base in bacteriorhodopsin with rhodopsin. We point out that the similarity of the signs of the light-induced electrical signals from the two kinds of oriented pigment molecules could be explained by bacteriorhodopsin releasing a proton from its extracellular side while rhodopsin taking up a proton on its cytoplasmic side. We then examined the pH dependence of both the absorption spectrum of the unphotolyzed state and the amplitude and kinetics of Meta II formation in bovine rhodopsin. We also measured the effect of deuteration and azide on Meta II formation. We concluded that the pK _a of the counter-ion to the Schiff base of bovine rhodopsin and of a surface residue that takes up a proton upon photolysis are both less than 4 in the unphotolyzed state. The data on pH dependence of Meta II formation indicated that the mechanisms involved are more complicated than just two sequential, isospectral forms of Meta II in the bleaching sequence. Finally we examined the evidence that, like in bacteriorhodopsin, the protonation of the Schiff bases's counter-ion (Glu113) is coupled to the changing of the pK _a of a protonatable surface group, called Z for rhodopsin and tentatively assigned to Glu134. We conclude that there probably is such a coupling, leading to the formation of the active form of Meta II.     

The herbicide-resistant D1 mutant L275F of Chlamydomonas reinhardtii fails to show the bicarbonate-reversible formate effect on chlorophyll a fluorescence transients

Herbicide-resistant mutants of the eukaryotic green alga Chlamydomonas reinhardtii, that are altered in specific amino acids in their D-1 protein, show differential bicarbonate-reversible formate effects. These results suggest the involvement of D1 protein in the bicarbonate effect. A 25 mM formate treatment of mixotrophically or photoautotrophically grown wild type cells results in a slower rise of chlorophyll a fluorescence transient followed by a dramatically slowed decline during measurements in continuous light. These effects are fully reversed upon addition of 10 mM bicarbonate. The mutant BR-202 [L275F] is, however, highly insensitive to 25 mM formate suggesting that a significant change in formate (bicarbonate) binding has occurred in helix V of the D1 protein near histidine involved in Fe binding. With the exception of DCMU-4 [S264A], which is considerably more sensitive to formate than the wild type, five other different [V219I, A25IV, F255Y, G256D and cell-wall deficient CW-15] mutants display a relatively similar response to formate as wild type. Absence of formate effect on a PS II-lacking [FuD-7] mutant confirms the sole involvement of PS II in the bicarbonate effect.     

Effects of Cations and Abscisic Acid on Chlorophyll a Fluorescence in Guard Cells of Vicia faba

The effects of cations and abscisic acid on chloroplast activity in guard cells of Vicia faba were investigated by analysis of the transient of chlorophyll a fluorescence. When epidermal strips containing guard cells as the only living cells were incubated in water and illuminated with strong light, chlorophyll a fluorescence rose rapidly to a high intensity and then declined slowly to a stationary level. The rate of this decline was enhanced by K⁺ or Na⁺, and the effect of these cations was greater when added with phosphate than with chloride as the anion. Ca²⁺ suppressed the enhancement by Na⁺ and, to a lesser extent, that by K⁺. Abscisic acid also suppressed the enhancement by K⁺ and Na⁺. Since the fluorescence decline reflects the increase of intrathylakoid H⁺ concentration necessary for photophosphorylation, the acceleration of the decline by K⁺ (or Na⁺ in the absence of Ca²⁺) implicates chloroplast activity in ion accumulation by guard cells in the light. The differential effects of phosphate and chloride suggest that chloroplast activity may be involved in malate formation in guard cells in the light.     

Light-induced currents from oriented purple membrane: II. Proton and cation contributions to the photocurrent

The sign of B2, the micro-second component of the photocurrent from oriented purple membrane, is that of positive charge moving away from the purple membrane in the direction of proton release. B2 could be due to internal dipole or proton movement, proton release, or metal cation release. We found that the waveform of B2 is virtually insensitive to changes in the salt concentration as long as it is >40 mM KCl, >5 mM CaCl₂, or >0.5 mM LaCl₃. However, below these limits, B2's apparent rate of decay increases as the salt concentration decreases without any change in the initial amplitude. This salt dependence suggests that B2 is due to a positive charge, either a metal cation or a proton, moving from the membrane into the solution. That the positive charge is not a metal cation is suggested by the waveform of B2 remaining unchanged upon replacing the cations both in solution and in the binding sites of the purple membrane. Direct evidence that the positive charge movement is due to protons was obtained by examining the correlation of B2 with the proton dependent processes of bacteriorhodopsin in buffers and dyes. Based on these observations, we suggest that most, if not all, of the intrinsic B2 component of the photocurrent at moderate salt concentration is due to proton release.

The photocurrents from purple membranes whose surface potential has been reduced by delipidation or chemical modification of carboxyl groups with methyl esters were found to be only modestly changed. This suggests that the salt effect is not through its modulation of the surface potential. Rather, we propose that in low salt B2 represents the sum of a proton release from the surface of the purple membrane and a second current component, due to cations moving back towards the membrane, which is only important in low salt. The cation counter current is induced by proton release which creates a transient uncompensated negative charge on the membrane.

     

Photosynthetic glow peaks and their relationship with the free energy changes

This paper is concerned with relating thermoluminescence to the total free-energy change, G, involved in detrapping a particular electron-hole pair as a photosynthetic sample is warmed from an initial low temperature. It extends a mathematical discussion of four possible mechanisms introduced in an earlier paper [DeVault, Govindjee and Arnold, Proc Nat'l Acad Sci USA 80: 983–987 (1983)]; here, particular attention is paid to the dependence of the absolute temperature of the maximum of a glow-peak, T _m , on the total free-energy change, G. The conclusion from the cases studied is that T _m =G/(k _B W) where G is evaluated at T _m , W is a complicated function of temperature and of thermodynamic parameters in the steps of the mechanism, and k _B is the Boltzmann constant. If the rate limiting step in the mechanism of detrapping is not preceded by any step in which G is appreciably negative, W is likely to have a value of about 33 and T _m is approximately proportional to G. Otherwise W can become much smaller and more strongly dependent on temperature and T _m is no longer proportional to G. These conclusions are of significance in lending theoretical support to the practice of inferring redox midpoint potential changes from shifts in T _m .     

Light-induced changes in the fluorescence yield of chlorophyll a in Anacystis nidulans II. The fast changes and the effect of photosynthetic inhibitors on both the fast and slow fluorescence induction

1. The intensity dependence and spectral variations during thefast transient of chlorophyll a (Chl a) fluorescence have beenanalyzed in the blue-green alga Anacystis nidulans. (Unlikethe case of eukaryotic unicellular green or red algae, the fastfluorescence induction characteristics of the prokaryotic blue-greenalgae had not been documented before.)
2. Dark adapted cellsof Anacystis exhibit two types of fluctuationsin the fluorescenceyield when excited with bright orange light(absorbed mainlyin phycocyanin). The first kinetic patterncalled the fast (sec)fluorescence transient exhibits a characteristicoriginal levelO, intermediary hump I, a pronounced dip D, peakP and a transitorysmall decline to a quasi steady state S.After attaining S,fluorescence yield slowly rises to a maximumlevel M. From M,the decline in fluorescence yield to a terminalT level is extremelyslow as shown earlier by Papageorgiou andGovindjee (8). Ascompared with green and red algae, blue-greenalgae seem tohave a small PS decline and a very characteristicslow SM rise,with a M level much higher than the peak P.
3. A prolonged darkadaptation and relatively high intensity ofexciting illuminationare required to evoke DPS type yield fluctuationsin Anacystis.At low to moderate intensities of exciting light,the time forthe development of P depends on light doses, butfor M, thisremains constant at these intensities.
4. Fluorescence emissionwas heterogeneous during the inductionperiod in Anacystis;the P and the M levels were relativelyenriched in short-wavelengthsystem II Chi a emission as comparedto D and S levels.
5. Thefast DPS transient was found to be affected by electrontransportcofactor (methyl viologen), and inhibitors (e.g.,DCMU, NH₂OH)in a manner suggesting that these changes are mostlyrelatedto the oxido-reduction level of intermediates betweenthe twophotosystems. On the other hand, the slow SM changesin fluorescenceyield, as reported earlier (5, 15), paralleloxygen evolution.These changes were found to be resistant toa variety of electrontransport inhibitors (O-phenanthroline,HOQNO, salicylaldoxime,DCMU, NH₂OH and Antimycin a). It issuggested that, in Anacystis,even in the presence of so-calledinhibitors of cyclic electronflow, a "high energy state" isstill produced.
6. Measurementsof Chlorophyll a fluorescence and delayed lightemission inthe presence of both DCMU and NH₂OH indicate thatthe slow SMchanges are not due to the recovery of the reactioncenter IIin darkness preceeding illumination.
7. Our results, thus, suggestthat in Anacystis a net electrontransport supported oxidation-reductionstate of the quencherQ regulates only partially the developmentof the DPS transient,but the development of the slow fluorescenceyield changes seemsnot to be regulated by these reactions.It appears, from datapresented elsewhere, that the slow risein the yield resultsdue to a structural modification of thethylakoid membrane.

¹We are grateful to the National Science Foundation for financialsupport. (Received November 21, 1972; )     

Photochemical properties of mesophyll and bundle sheath chloroplasts of maize

Several photochemical and spectral properties of maize (Zea mays) bundle sheath and mesophyll chloroplasts are reported that provide a better understanding of the photosynthetic apparatus of C₄ plants. The difference absorption spectrum at 298 K and the fluorescence excitation and emission spectra of chlorophyll at 298 K and 77 K provide new information on the different forms of chlorophyll a in bundle sheath and mesophyll chloroplasts: the former contain, relative to short wavelength chlorophyll a forms, more long wavelength chlorophyll a form (e.g. chlorophyll a 693 and chlorophyll a 705) and less chlorophyll b than the latter. The degree of polarization of chlorophyll a fluorescence is 6% in bundle sheath and 4% in mesophyll chloroplasts. This result is consistent with the presence of relatively high amounts of oriented long wavelength forms of chlorophyll a in bundle sheath compared to mesophyll chloroplasts. The relative yield of variable, with respect to constant, chorophyll a fluorescence in mesophyll chloroplasts is more than twice that in bundle sheath chloroplast. Furthermore, the relative yield of total chlorophyll a fluorescence is 40% lower in bundle sheath compared to that in mesophyll chloroplasts. This is in agreement with the presence of the higher ratio of the weakly fluorescent pigment system I to pigment system II in bundle sheath than in mesophyll chloroplast. The efficiency of energy transfer from chlorophyll b and carotenoids to chlorophyll a are calculated to be 100 and 50%, respectively, in both types of chloroplasts. Fluorescence quenching of atebrin, reflecting high energy state of chloroplasts, is 10 times higher in mesophyll chloroplasts than in bundle sheath chloroplasts during noncyclic electron flow but is equal during cyclic flow. The entire electron transport chain is shown to be present in both types of chloroplasts, as inferred from the antagonistic effect of red (650 nm) and far red (710 nm) lights on the absorbance changes at 559 nm and 553 nm, and the photoreduction of methyl viologen from H₂O. (The rate of methyl viologen photoreduction in bundle sheath chloroplasts was 40% of that of mesophyll chloroplasts.)     

The Photoreduction of Nicotinamide-Adenine Dinucleotide by Chromatophore Fractions from Rhodospirillum rubrum

The photoreduction of nicotinamide-adenine dinucleotide (NAD⁺), catalyzed by chromatophore fractions from young (1 day) and old (4-5 days) cultures of Rhodospirillum rubrum, was measured in the presence of either succinate or 2,6-dichlorophenol indophenol (DPIP) and an excess of ascorbate. The time-course of photoreduction in the succinate system suggested a “reversed electron flow” from the donor to NAD⁺ mediated by a high energy intermediate produced by a light-induced, cyclic electron transport in the chromatophore fractions. The effects of the uncoupler carbonyl cyanide [p-(trifluoromethoxy)phenyl]hydrazone (FCCP) and of the inhibitors antimycin A and 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO) were consistent with this interpretation. The time-course of NAD⁺ photoreduction in the presence of DPIP and ascorbate suggested a direct, light-induced electron transport from the donor to the acceptor. We cannot yet distinguish between a model in which the same reaction center is utilized in the photoreduction by both donor systems (the reaction center component P-870 may relate to two primary acceptors at different redox potential levels) and a model in which each photoreducing system is driven by its own reaction center component.