Light-induced changes of absorbance and electron spin resonance in small photosystem II particles

Diagnostic and treatment implications of psychosis secondary to treatable metabolic disorders in adults: a systematic review

Objective

It is important for psychiatrists to be aware of certain inborn errors of metabolism (IEMs) as these rare disorders can present as psychosis, and because definitive treatments may be available for treating the underlying metabolic cause. A systematic review was conducted to examine IEMs that often present with schizophrenia-like symptoms.

Data sources

Published literature on MEDLINE was assessed regarding diseases of homocysteine metabolism (DHM; cystathionine beta-synthase deficiency [CbS-D] and homocysteinemia due to methyltetrahydrofolate reductase deficiency [MTHFR-D]), urea cycle disorders (UCD), acute porphyria (POR), Wilson disease (WD), cerebrotendinous-xanthomatosis (CTX) and Niemann-Pick disease type C (NP-C).

Study selection

Case reports, case series or reviews with original data regarding psychiatric manifestations and cognitive impairment published between January 1967 and June 2012 were included based on a standardized four-step selection process.

Data extraction

All selected articles were evaluated for descriptions of psychiatric signs (type, severity, natural history and treatment) in addition to key disease features.

Results

A total of 611 records were identified. Information from CbS-D (n = 2), MTHFR-D (n = 3), UCD (n = 8), POR (n = 12), WD (n = 11), CTX (n = 14) and NP-C publications (n = 9) were evaluated. Six non-systematic literature review publications were also included. In general, published reports did not provide explicit descriptions of psychiatric symptoms. The literature search findings are presented with a didactic perspective, showing key features for each disease and psychiatric signs that should trigger psychiatrists to suspect that psychotic symptoms may be secondary to an IEM.

Conclusion

IEMs with a psychiatric presentation and a lack of, or sub-clinical, neurological signs are rare, but should be considered in patients with atypical psychiatric symptoms.

Determination of the primary charge separation rate in isolated photosystem II reaction centers with 500-fs time resolution

We have measured directly the rate of formation of the oxidized chlorophyll a electron donor (P680(+)) and the reduced electron acceptor pheophytin a(-) (Pheoa(-)) following excitation of isolated spinach photosystem II reaction centers at 4 degrees C. The reaction-center complex consists of D(1), D(2), and cytochrome b-559 proteins and was prepared by a procedure that stabilizes the protein complex. Transient absorption difference spectra were measured from 440 to 850 nm as a function of time with 500-fs resolution following 610-nm laser excitation. The formation of P680(+)-Pheoa(-) is indicated by the appearance of a band due to P680(+) at 820 nm and corresponding absorbance changes at 505 and 540 nm due to formation of Pheoa(-). The appearance of the 820-nm band is monoexponential with tau = 3.0 +/- 0.6 ps. The time constant for decay of (1*)P680, the lowest excited singlet state of P680, monitored at 650 nm, is tau = 2.6 +/- 0.6 ps and agrees with that of the appearance of P680(+) within experimental error. Treatment of the photosystem II reaction centers with sodium dithionite and methyl viologen followed by exposure to laser excitation, conditions known to result in accumulation of Pheoa(-), results in formation of a transient absorption spectrum due to (1*)P680. We find no evidence for an electron acceptor that precedes the formation of Pheoa(-).

Ligated chlorophyll cation radicals: Their function in photosystem II of plant photosynthesis

Magnesium tetraphenylchlorin, a synthetic model for chlorophyll, exhibits significant variations in the unpaired spin densities of its cation radicals with concomitant changes in oxidation potentials as a function of solvent and axial ligand. Similar effects are observed for chlorophyll (Chl) a and its cation radicals. Oxidation potentials for Chl --> Chl(+.) as high as +0.9 V (against a normal hydrogen electrode) are observed in nonaqueous solvents, with linewidths of the electron spin resonance signals of monomeric Chl(+.) ranging between 9.2 and 7.8 G in solution. These changes in electronic configuration and ease of oxidation are attributed to mixing of two nearly degenerate ground states of the radicals theoretically predicted by molecular orbital calculations. Comparison of the properties of chlorophyll in vitro with the optical, redox, and magnetic characteristics attributed to P-680, the primary donor of photosystem II which mediates oxygen evolution in plant photosynthesis, leads us to suggest that P-680 may be a ligated chlorophyll monomer whose function as a phototrap is determined by interactions with its (protein?) environment.

Identification of the special pair of photosystem II in a chlorophyll d-dominated cyanobacterium

The composition of photosystem II (PSII) in the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina MBIC 11017 was investigated to enhance the general understanding of the energetics of the PSII reaction center. We first purified photochemically active complexes consisting of a 47-kDa Chl protein (CP47), CP43' (PcbC), D1, D2, cytochrome b(559), PsbI, and a small polypeptide. The pigment composition per two pheophytin (Phe) a molecules was 55 +/- 7 Chl d, 3.0 +/- 0.4 Chl a, 17 +/- 3 alpha-carotene, and 1.4 +/- 0.2 plastoquinone-9. The special pair was detected by a reversible absorption change at 713 nm (P713) together with a cation radical band at 842 nm. FTIR difference spectra of the specific bands of a 3-formyl group allowed assignment of the special pair. The combined results indicate that the special pair comprises a Chl d homodimer. The primary electron acceptor was shown by photoaccumulation to be Phe a, and its potential was shifted to a higher value than that in the Chl a/Phe a system. The overall energetics of PSII in the Chl d system are adjusted to changes in the redox potentials, with P713 as the special pair using a lower light energy at 713 nm. Taking into account the reported downward shift in the potential of the special pair of photosystem I (P740) in A. marina, our findings lend support to the idea that changes in photosynthetic pigments combine with a modification of the redox potentials of electron transfer components to give rise to an energetic adjustment of the total reaction system.

Chloroplast-encoded Polypeptide PsbT Is Involved in the Repair of Primary Electron Acceptor QA of Photosystem II during Photoinhibition in Chlamydomonas reinhardtii

PsbT is a small chloroplast-encoded hydrophobic polypeptide associated with the D1/D2 heterodimer of the photosystem II (PSII) reaction center and is required for the efficient post-translational repair of photodamaged PSII. Here we addressed that role in detail in Chlamydomonas reinhardtii wild type and DeltapsbT cells by analyzing the activities of PSII, the assembly of PSII proteins, and the redox components of PSII during photoinhibition and repair. Strong illumination of cells for 15 min decreased the activities of electron transfer through PSII and Q(A) photoreduction by 50%, and it reduced the amount of atomic manganese by 20%, but it did not affect the steady-state level of PSII proteins, photoreduction of pheophytin (pheo(D1)), and the amount of bound plastoquinone (Q(A)), indicating that the decrease in PSII activity resulted mainly from inhibition of the electron transfer from pheo(D1) to Q(A). In wild type cells, we observed parallel recovery of electron transfer activity through PSII and Q(A) photoreduction, suggesting that the recovery of Q(A) activity is one of the rate-limiting steps of PSII repair. In DeltapsbT cells, the repairs of electron transfer activity through PSII and of Q(A) photoreduction activity were both impaired, but PSII protein turnover was unaffected. Moreover, about half the Q(A) was lost from the PSII core complex during purification. Since PsbT is intimately associated with the Q(A)-binding region on D2, we propose that this polypeptide enhances the efficient recovery of Q(A) photoreduction by stabilizing the structure of the Q(A)-binding region.

Destruction of a single chlorophyll is correlated with the photoinhibition of photosystem II with a transiently inactive donor side

Pigments destroyed during photoinhibition of water-splitting photosystem II core complexes from the green alga Chlamydomonas reinhardtii were studied. Under conditions of a transiently inactivated donor side, illumination leads to an irreversible inhibition of the electron transfer at the donor side that is paralleled by the destruction of chlorophylls a absorbing maximally around 674 and 682 nm. The observed stochiometry of 1 +/- 0.1 destroyed chlorophyll per inhibited photosystem II suggests that chlorophyll destruction could be the primary photodamage causing the inhibition of photosystem II under these conditions.

A well resolved ODMR triplet minus singlet spectrum of P680 from PSII particles

An ADMR T-S spectrum of the primary donor (P680) of photosystem II (PSII) was obtained from anaerobically photoreduced particles. The spectrum is the best resolved obtained so far having a main bleaching band at 684 nm with a linewidth of only 100 cm-1. The view that this spectrum is produced by native homogeneous P680 unlike those obtained before is defended. A small bleaching observed at 678 nm is discussed in terms of the reaction center structure. One possible interpretation of the observations is that P680 is a very loose dimer with an exciton splitting of only 144 cm-1 corresponding to a dimer center-to-center distance of roughly 11.5 A.

Light-induced Fourier transform infrared spectrum of the cation radical P680+

The structure of the primary electron donor of photosystem II, P680, is still under debate. It is not decided if it is composed of a chlorophyll (Chl) monomer or dimer. In this study, Fourier transform infrared (FTIR) spectroscopy was used to analyze the changes in the vibration modes occurring upon photooxidation of P680 in a Mn-depleted PS II preparation. It is demonstrated that illumination of the above in the presence of artificial electron acceptors results in a light-minus-dark absorbance change typical of the formation of P680+. The light-minus-dark difference FTIR spectrum obtained under similar conditions is characterized by two negative peaks located at 1694 and 1652 or 1626 cm-1 that can be assigned to the 9-keto groups of the P680 Chl, the latter band being indicative of a strongly associated group. These vibrations are shifted to 1714 and 1676 cm-1, respectively, in the positive features of the difference spectrum attributed to P680+. The occurrence of two pairs of bands attributed to 9-keto groups is discussed in terms of P680 being formed of a Chl dimer.

Rate of oxidation of P680 in isolated photosystem 2 reaction centers monitored by loss of chlorophyll stimulated emission

We have continued our studies of the primary photochemistry of isolated photosystem 2 reaction centers using femtosecond transient absorption spectroscopy. Experiments were performed over a wide range of excitation and probe wavelengths, using several data collection time scales. This has enabled us to resolve five different lifetimes ranging between 100 fs and 200 ps plus a nanosecond component. We demonstrate here and elsewhere [e.g., Durrant, J.R., Hastings, G., Joseph, D. M., Barber, J., Porter, G., & Klug, D. R. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11632-11636] that the kinetic spectra associated with all but two of these lifetimes are clearly distinguishable. We have previously reported that a 21-ps lifetime is associated with pheophytin reduction [Hastings, G., Durrant, J. R., Hong, Q., Barber, J., Porter, G., & Klug, D. R. (1992) Biochemistry 31, 7638-7647]. In this paper, we show that it is possible to spectrally and temporally resolve stimulated emission from PS2 reaction centers with great accuracy and that this stimulated emission is largely unaffected by those kinetic components which are faster than 21 ps. The observation of a distinct stimulated emission band allows us to distinguish charge-separated states from chlorin singlet states. In this way, we are able to show that the proportion of charge-separated states prior to the 21-ps component is between 0% and 25%. We also show that the shape of the spectrum which we obtain for the state P680+Ph- is essentially invariant between 100 ps and 9 ns, and is the same as that previously reported for P680+Ph- by other researchers.(ABSTRACT TRUNCATED AT 250 WORDS)

Subpicosecond equilibration of excitation energy in isolated photosystem II reaction centers

Photosystem II reaction centers have been studied by femtosecond transient absorption spectroscopy. We demonstrate that it is possible to achieve good photoselectivity between the primary electron donor P680 and the majority of the accessory chlorins. Energy transfer can be observed in both directions between P680 and these accessory chlorins depending on which is initially excited. After excitation of either P680 or the other chlorins, the excitation energy is observed to equilibrate between the majority of these pigments at a rate of 100 +/- 50 fs-1. This energy-transfer equilibration takes place before any electron-transfer reactions and must therefore be taken into account in studies of primary electron-transfer reactions in photosystem II. We also show further evidence that the initially excited P680 excited singlet state is delocalized over at least two chlorins and that this delocalization lasts for at least 200 fs.

Determination of the primary charge separation rate in Photosystem II reaction centers at 15 K

We have measured the rate constant for the formation of the oxidized chlorophyll a electron donor (P680(+)) and the reduced electron acceptor pheophytin a (-) (Pheo a (-)) following excitation of isolated Photosystem II reaction centers (PS II RC) at 15 K. This PS II RC complex consists of D1, D2, and cytochrome b-559 proteins and was prepared by a procedure which stabilizes the protein complex. Transient absorption difference spectra were measured from 450-840 nm as a function of time with 500fs resolution following 610 nm laser excitation. The formation of P680(+)-Pheo a (-) is indicated by the appearance of a band due to P680(+) at 820 nm and corresponding absorbance changes at 490, 515 and 546 nm due to the formation of Pheo a (-). The appearance of the 490 nm and 820 nm bands is monoexponenital with τ=1.4±0.2 ps. Treatment of the PS II RC with sodium dithionite and methyl viologen followed by exposure to laser excitation results in accumulation of Pheo a (-). Laser excitation of these prereduced RCs at 15 K results in formation of a transient absorption spectrum assigned to (1*)P680. We observe wavelength-dependent kinetics for the recovery of the transient bleach of the Qy absorption bands of the pigments in both untreated and pre-reduced PS II RCs at 15K. This result is attributed to an energy transfer process within the PS II RC at low temperature that is not connected with charge separation.

Photoreduction of QA, QB, and cytochrome b-559 in an oxygen-evolving photosystem II preparation from the thermophilic cyanobacterium Synechococcus sp

Light-induced absorption changes in an oxygen-evolving photosystem II (PS II) preparation from the thermophilic cyanobacterium Synechococcus sp. were analyzed using continuous illumination which caused the reduction of both QA (first stable quinone electron acceptor) and QB (second quinone electron acceptor of photosystem II). In this photosystem II preparation in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) the amount of QA was estimated to be 1 per 42 chlorophylls. In the absence of DCMU, plastoquinone (1.68 per QA) was photoreduced to plastohydroquinone within a few seconds, indicating that QB is reduced and protonated during this period. An electrochromic band shift centered around 685 nm was observed with and without DCMU. The extent of this band shift caused by QB reduction per electron was about a third or half of that caused by QA reduction. A significant amount of cytochrome b-559 (0.86 per QA) was photoreduced. Only 60% of the photoreduction of cytochrome b-559 was inhibited by a DCMU concentration that inhibited electron transfer beyond QB, indicating that the site of the reduction of cytochrome b-559 is located before the QB site and possibly on the donor side of PS II.

The fractionation of plant photoactive pigment-protein complexes I and II

The pigment-protein complexes enriched with Photosystem I (PPC-I) and Photosystem II (PPC-II) were obtained using sievorptive chromatography on DEAE-Sephadex column. Both types of complexes contain Chlorophyll a, beta-carotene and minor quantities of Chl b. Red absorbance maxima are located at 676 nm and 673 nm for PPC-I and PPC-II, respectively. The degrees of reaction centre enrichment were measured by the method of differential spectrophotometry: PPC-I has one P-700 per 35 bulk Chl a molecules, PPC-II contains one P-680 per 18 bulk Chl a molecules. The yield of PPC-II is 7--10 times lower than that of PPC-I. After one chromatographic procedure the amount of P-680 in PPC-I preparation does not exceed 7% of that of P-700, the amount of P-700 in PPC-II preparation 2% of that of P-680. The product of PPC-II degradation was studied.

Flash-induced charge separation and dark recombination in a photosystem-II subchloroplast particle

The decay time of flash-induced absorption changes in a Photosystem-II subchloroplast fragment is very temperature sensitive down to 210 K, below which it remains constant at 1.25 +/- 0.05 ms. The difference spectrum from the near-infra-red to the ultraviolet regions indicates that the monophasic decay represents charge recombination between P-680+ and the reduced primary acceptor. The charge recombination proceeds by electron tunneling. The P-680 concentration in the TSF-IIa fragment was estimated to be one in 30 +/- 5 total chlorophyll molecules.

Polypeptide profiles of chlorophyll . protein complexes and thylakoid membranes of spinach chloroplasts

In addition to the major chlorophyll . protein complexes I and II, two minor chlorophyll proteins have been observed in sodium dodecyl sulfate (SDS))-polyacrylamide gels of spinach chloroplast membranes. These minor pigmented zones appeared to be derived from the light-harvesting chlorophyll a/b . protein and from the reaction centre complex of Photosystem II. Data are presented on the polypeptide profiles of purified digitonin-subschloroplast particles, with special regard to the effect of solubilization temperature and extraction of lipids. The results are compared with the SDS-polypeptide pattern of spinach thylakoids obtained under exactly the same conditions with respect to electrophoresis technique, solubilization method and presence of lipid. In addition, the effects of temperature and lipid extraction on the distinct chlorophyll . protein complexes appearing in SDS gel electrophoretograms of chloroplast membranes were studied by slicing the chlorophyll-containing regions and subjecting them to a second run with or without heating or extraction with acetone. By supplementing these data with an examination of the polypeptide composition of cytochrome f and coupling factor, it has been possible to identify most of the major chloroplast membrane polypeptides.

Flash-induced absorption changes of the primary donor of photosystem II at 820 nm in chloroplasts inhibited by low pH or tris-treatment

A comparative study is made, at 15 degrees C, of flash-induced absorption changes around 820 nm (attributed to the primary donors of Photosystems I and II) and 705 nm (Photosystem I only), in normal chloroplasts and in chloroplasts where O2 evolution was inhibited by low pH or by Tris-treatment. At pH 7.5, with untreated chloroplasts, the absorption changes around 820 nm are shown to be due to P-700 alone. Any contribution of the primary donor of Photosystem II should be in times shorter than 60 mus. When chloroplasts are inhibited at the donor side of Photosystem II by low pH, an additional absorption change at 820 nm appears with an amplitude which, at pH 4.0, is slightly higher than the signal due to oxidized P-700. This additional signal is attributed to the primary donor of Photosystem II. It decays (t 1/2 about 180 mus) mainly by back reaction with the primary acceptor and partly by reduction by another electron donor. Acid-washed chloroplasts resuspended at pH 7.5 still present the signal due to Photosystem II (t 1/2 about 120 mus). This shows that the acid inhibition of the first secondary donor of Photosystem II is irreversible. In Tris-treated chloroplasts, absorption changes at 820 nm due to the primary donor of Photosystem II are also observed, but to a lesser extent and only after some charge accumulation at the donor side. They decay with a half-time of 120 mus.

Primary reactions of photosystem II at low pH. 2. Light-induced changes of absorbance and electron spin resonance in spinach chloroplasts

The effects of lowering the pH on Photosystem II has been studied by measuring changes in absorbance and electron spin resonance in spinach chloroplasts. At pH values around 4 a light-induced dark-reversible chlorophyll oxidation by Photosystem II was observed. This chlorophyll is presumably the primary electron donor of system II. At pH values between 5 and 4 steady state illumination induced an ESR signal, similar in shape and amplitude to signal II, which was rapidly reversed in the dark. This may reflect the accumulation of the oxidized secondary donor upon inhibition of oxygen evolution. Near pH 4 the rapidly reversible signal and the stable and slowly decaying components of signal II disappeared irreversibly concomitant with the release of bound manganese. The results are discussed in relation to the effects of low pH on prompt and delayed fluorescence reported earlier (van Gorkom, H.J., Pulles, M.P.J., Haveman, J. and den Haan, G.A. (1976) Biochim. Biophys, Acta 423, 217-226).