Matrix Selection Strategies for MALDI-TOF MS/MS Characterization of Cyclic Tetrapyrroles in Blood and Food Samples

Cyclic tetrapyrrole derivatives such as porphyrins, chlorins, corrins (compounds with a corrin core), and phthalocyanines are a family of molecules containing four pyrrole rings usually coordinating a metal ion (Mg, Cu, Fe, Zn, etc.). Here, we report the characterization of some representative cyclic tetrapyrrole derivatives by MALDI-ToF/ToF MS analyses, including heme b and c, phthalocyanines, and protoporphyrins after proper matrix selection. Both neutral and acidic matrices were evaluated to assess potential demetallation, adduct formation, and fragmentation. While chlorophylls exhibited magnesium demetallation in acidic matrices, cyclic tetrapyrroles with Fe, Zn, Co, Cu, or Ni remained steadfast against demetallation across all conditions. Phthalocyanines and protoporphyrins were also detectable without a matrix using laser desorption ionization (LDI); however, the incorporation of matrices achieved the highest ionization yield, enhanced sensitivity, and negligible fragmentation. Three standard proteins, i.e., myoglobin, hemoglobin, and cytochrome c, were analyzed either intact or enzymatically digested, yielding heme b and heme c ions along with accompanying peptides. Furthermore, we successfully detected and characterized heme b in real samples, including blood, bovine and cod liver, and mussel. As a result, MALDI MS/MS emerged as a powerful tool for straightforward cyclic tetrapyrrole identification, even in highly complex samples. Our work paves the way for a more comprehensive understanding of cyclic tetrapyrroles in biological and industrial settings, including the geochemical field, as these compounds are a source of significant geological and geochemical information in sediments and crude oils.

Meta-analysis of transcriptomic profiles in Dunaliella tertiolecta reveals molecular pathway responses to different abiotic stresses

Microalgae are photosynthetic organisms and a potential source of sustainable metabolite production. However, different stress conditions might affect the production of various metabolites. In this study, a meta-analysis of RNA-seq experiments in Dunaliella tertiolecta was evaluated to compare metabolite biosynthesis pathways in response to abiotic stress conditions such as high light, nitrogen deficiency and high salinity. Results showed downregulation of light reaction, photorespiration, tetrapyrrole and lipid-related pathways occurred under salt stress. Nitrogen deficiency mostly induced the microalgal responses of light reaction and photorespiration metabolism. Phosphoenol pyruvate carboxylase, phosphoglucose isomerase, bisphosphoglycerate mutase and glucose-6-phosphate-1-dehydrogenase (involved in central carbon metabolism) were commonly upregulated under salt, light and nitrogen stresses. Interestingly, the results indicated that the meta-genes (modules of genes strongly correlated) were located in a hub of stress-specific protein-protein interaction (PPI) network. Module enrichment of meta-genes PPI networks highlighted the cross-talk between photosynthesis, fatty acids, starch and sucrose metabolism under multiple stress conditions. Moreover, it was observed that the coordinated expression of the tetrapyrrole intermediated with meta-genes was involved in starch biosynthesis. Our results also showed that the pathways of vitamin B6 metabolism, methane metabolism, ribosome biogenesis and folate biosynthesis responded specifically to different stress factors. Since the results of this study revealed the main pathways underlying the abiotic stress, they might be applied in optimised metabolite production by the microalga Dunaliella in future studies. PRISMA check list was also included in the study.

Tolyporphins-Exotic Tetrapyrrole Pigments in a Cyanobacterium-A Review

Tolyporphins were discovered some 30 years ago as part of a global search for antineoplastic compounds from cyanobacteria. To date, the culture HT-58-2, comprised of a cyanobacterium-microbial consortium, is the sole known producer of tolyporphins. Eighteen tolyporphins are now known-each is a free base tetrapyrrole macrocycle with a dioxobacteriochlorin (14), oxochlorin (3), or porphyrin (1) chromophore. Each compound displays two, three, or four open β-pyrrole positions and two, one, or zero appended C-glycoside (or -OH or -OAc) groups, respectively; the appended groups form part of a geminal disubstitution motif flanking the oxo moiety in the pyrroline ring. The distinct structures and repertoire of tolyporphins stand alone in the large pigments-of-life family. Efforts to understand the cyanobacterial origin, biosynthetic pathways, structural diversity, physiological roles, and potential pharmacological properties of tolyporphins have attracted a broad spectrum of researchers from diverse scientific areas. The identification of putative biosynthetic gene clusters in the HT-58-2 cyanobacterial genome and accompanying studies suggest a new biosynthetic paradigm in the tetrapyrrole arena. The present review provides a comprehensive treatment of the rich science concerning tolyporphins.

Synthesis of a BC-Dihydrodipyrrin Building Block of Bacteriochlorophyll a

A strategy for the synthesis of bacteriochlorophyll a relies on joining AD and BC halves that contain the requisite stereochemical configurations of the target macrocycle. The BC half (1) is a dihydrodipyrrin bearing a dimethoxymethyl group at the 1-position, a β-ketoester at the 8-position, and (R)-2-methyl and (R)-3-ethyl substituents in the pyrroline ring. An established route to AD-dihydrodipyrrins (Pd-mediated coupling of a 2-halopyrrole with a chiral 4-pentynoic acid followed by Petasis methenylation, acidic hydrolysis, Paal-Knorr ring closure, and Riley oxidation) proved to be unviable for BC-dihydrodipyrrins given the presence of the β-ketoester unit. A route presented here entails Pd-mediated coupling of a 2-halopyrrole (2) with (3R,4R)-4-ethyl-1,1-dimethoxy-3-methylhex-5-yn-2-one (3), anti-Markovnikov hydration of the alkyne to give the 1,4-diketone, and Paal-Knorr ring closure. Compound 3 was prepared by Schreiber-modified Nicholas reaction beginning with (S)-4-isopropyl-3-propionyloxazolidin-2-one and the hexacarbonyldicobalt complex of (±) 3-methoxy-1-(trimethylsilyl)pentyne followed by transformation of the aldehyde derived therefrom to the 1,1-dimethoxymethylcarbonyl motif. The absolute stereochemical configuration of the Schreiber-Nicholas alkylation product was confirmed by single-crystal X-ray diffraction, whereas the BC half (1) by 1H NMR spectroscopy showed a J value of 2.9 Hz consistent with the trans-configuration. Taken together, the route provides a key chiral building block for the synthesis of photosynthetic tetrapyrroles and analogues.

Four Routes to 3-(3-Methoxy-1,3-dioxopropyl)pyrrole, a Core Motif of Rings C and E in Photosynthetic Tetrapyrroles

The photosynthetic tetrapyrroles share a common structural feature comprised of a β-ketoester motif embedded in an exocyclic ring (ring E). As part of a total synthesis program aimed at preparing native structures and analogues, 3-(3-methoxy-1,3-dioxopropyl)pyrrole was sought. The pyrrole is a precursor to analogues of ring C and the external framework of ring E. Four routes were developed. Routes 1-3 entail a Pd-mediated coupling process of a 3-iodopyrrole with potassium methyl malonate, whereas route 4 relies on electrophilic substitution of TIPS-pyrrole with methyl malonyl chloride. Together, the four routes afford considerable latitude. A long-term objective is to gain the capacity to create chlorophylls and bacteriochlorophylls and analogues thereof by facile de novo means for diverse studies across the photosynthetic sciences.

PLGA-PVA-PEG Single Emulsion Method as a Candidate for Aminolevulinic Acid (5-ALA) Encapsulation: Laboratory Scaling up and Stability Evaluation

One of the most widely used molecules used for photodynamic therapy (PDT) is 5-aminolevulinic acid (5-ALA), a precursor in the synthesis of tetrapyrroles such as chlorophyll and heme. The 5-ALA skin permeation is considerably reduced due to its hydrophilic characteristics, decreasing its local bioavailability and therapeutic effect. For this reason, five different systems containing polymeric particles of poly [D, L–lactic–co–glycolic acid (PLGA)] were developed to encapsulate 5-ALA based on single and double emulsions methodology. All systems were standardized (according to the volume of reagents and mass of pharmaceutical ingredients) and compared in terms of laboratory scaling up, particle formation and stability over time. UV-VIS spectroscopy revealed that particle absorption/adsorption of 5-ALA was dependent on the method of synthesis. Different size distribution was observed by DLS and NTA techniques, revealing that 5-ALA increased the particle size. The contact angle evaluation showed that the system hydrophobicity was dependent on the surfactant and the 5-ALA contribution. The FTIR results indicated that the type of emulsion influenced the particle formation, as well as allowing PEG functionalization and interaction with 5-ALA. According to the ¹H-NMR results, the 5-ALA reduced the T1 values of polyvinyl alcohol (PVA) and PLGA in the double emulsion systems due to the decrease in molecular packing in the hydrophobic region. The results indicated that the system formed by single emulsion containing the combination PVA–PEG presented greater stability with less influence from 5-ALA. This system is a promising candidate to successfully encapsulate 5-ALA and achieve good performance and specificity for in vitro skin cancer treatment.

Interference between Franck-Condon and Herzberg-Teller Terms in the Condensed-Phase Molecular Spectra of Metal-Based Tetrapyrrole Derivatives

The commonly used Franck-Condon (FC) approximation is inadequate for explaining the electronic spectra of compounds that possess vibrations with substantial Herzberg-Teller (HT) couplings. Metal-based tetrapyrrole derivatives, which are ubiquitous natural pigments, often exhibit prominent HT activity. In this paper, we compare the condensed phase spectra of zinc-tetraphenylporphyrin (ZnTPP) and zinc-phthalocyanine (ZnPc), which exhibit vastly different spectral features in spite of sharing a common tetrapyrrole backbone. The absorption and emission spectra of ZnTPP are characterized by a lack of mirror symmetry and nontrivial temperature dependence. In contrast, mirror symmetry is restored, and the nontrivial temperature-dependent features disappear in ZnPc. We attribute these differences to FC-HT interference, which is less pronounced in ZnPc because of a larger FC component in the dipole moment that leads to FC-dominated transitions. A single minimalistic FC-HT vibronic model reproduces all the experimental spectral features of these molecules. These observations suggest that FC-HT interference is highly susceptible to chemical modification.

[Advances in the biosynthesis of tetrapyrrole compounds]

Tetrapyrrole compounds are a class of compounds with important functions. They exist in living organisms and have been widely used in agriculture, food, medicine, and other fields. The cumbersome process and high cost of chemical synthesis, as well as the shortcomings of unstable quality of animal and plant extraction methods, greatly hampered the industrial production and applications of tetrapyrrole compounds. In recent years, the rapid development of synthetic biology has provided new tools for microorganisms to efficiently synthesize tetrapyrrole compounds from renewable biomass resources. This article summarizes various strategies for the biosynthesis of tetrapyrrole compounds, discusses methods to improve its biosynthesis efficiency and future prospects, with the aim to facilitate the research on biosynthesis of tetrapyrrole compounds.

An Easily Prepared Monomeric Cobalt(II) Tetrapyrrole Complex That Efficiently Promotes the 4e-/4H+ Peractivation of O2 to Water

The selective 4e-/4H+ reduction of dioxygen to water is an important reaction that takes place at the cathode of fuel cells. Monomeric aromatic tetrapyrroles (such as porphyrins, phthalocyanines, and corroles) coordinated to Co(II) or Co(III) have been considered as oxygen reduction catalysts due to their low cost and relative ease of synthesis. However, these systems have been repeatedly shown to be selective for O2 reduction by the less desired 2e-/2H+ pathway to yield hydrogen peroxide. Herein, we report the initial synthesis and study of a Co(II) tetrapyrrole complex based on a nonaromatic isocorrole scaffold that is competent for 4e-/4H+ oxygen reduction reaction (ORR). This Co(II) 10,10-dimethyl isocorrole (Co[10-DMIC]) is obtained in just four simple steps and has excellent yield from a known dipyrromethane synthon. Evaluation of the steady state spectroscopic and redox properties of Co[10-DMIC] against those of Co porphyrin (cobalt 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, [Co(TPFPP)]) and corrole (cobalt 5,10,15-tris(pentafluorophenyl)corrole triphenylphosphine, Co[TPFPC](PPh3)) homologues demonstrated that the spectroscopic and electrochemical properties of the isocorrole are distinct from those displayed by more traditional aromatic tetrapyrroles. Further, the investigation of the ORR activity of Co[10-DMIC] using a combination of electrochemical and chemical reduction studies revealed that this simple, unadorned monomeric Co(II) tetrapyrrole is ∼85% selective for the 4e-/4H+ reduction of O2 to H2O over the more kinetically facile 2e-/2H+ process that delivers H2O2. In contrast, the same ORR evaluations conducted for the Co porphyrin and corrole homologues demonstrated that these traditional aromatic systems catalyze the 2e-/2H+ conversion of O2 to H2O2 with near complete selectivity. Despite being a simple, easily prepared, monomeric tetrapyrrole platform, Co[10-DMIC] supports an ORR catalysis that has historically only been achieved using elaborate porphyrinoid-based architectures that incorporate pendant proton-transfer groups or ditopic molecular clefts or that impose cofacially oriented O2 binding sites. Accordingly, Co[10-DMIC] represents the first simple, unadorned, monomeric metalloisocorrole complex that can be easily prepared and shows a privileged performance for the 4e-/4H+ peractivation of O2 to water as compared to other simple cobalt containing tetrapyrroles.

Expression and In Vivo Loading of De Novo Proteins with Tetrapyrrole Cofactors

Tetrapyrrole cofactors such as heme and chlorophyll imprint their intrinsic reactivity and properties on a multitude of natural proteins and enzymes, and there is much interest in exploiting their functional and catalytic capabilities within minimal, de novo designed protein scaffolds. Here we describe how, using only natural biosynthetic and post-translational modification pathways, de novo designed soluble and hydrophobic proteins can be equipped with tetrapyrrole cofactors within living Escherichia coli cells. We provide strategies to achieve covalent and non-covalent heme incorporation within the de novo proteins and describe how the heme biosynthetic pathway can be co-opted to produce the light sensitive zinc protoporphyrin IX for loading into proteins in vivo. In addition, we describe the imaging of hydrophobic proteins and cofactor-rich protein droplets by electron and fluorescence microscopy, and how cofactors can be stripped from the de novo proteins to aid in vitro identification.

An evergreen mind and a heart for the colors of fall

With the finest biochemical and molecular approaches, convincing explorative strategies, and long-term vision, Stefan Hörtensteiner succeeded in elucidating the biochemical pathway responsible for chlorophyll degradation. After having contributed to the identification of key chlorophyll degradation products in the course of the past 25 years, he gradually identified and characterized most of the crucial players in the PAO/phyllobilin degradation pathway of chlorophyll. He was one of the brightest plant biochemists of his generation, and his work opened doors to a better understanding of plant senescence, tetrapyrrole homeostasis, and their complex regulation. He sadly passed away on 5 December 2020, aged 57.

Stable (18) O-labeling method for stercobilin and other bilins for metabolomics

RATIONALE

Bilin tetrapyrroles including stercobilin are unique to mammalian waste; they have been used as markers of source water contamination and may have important diagnostic value in human health conditions. Unfortunately, commercial isotopomers for bilins are not available. Thus, there is a need for isotopomer standards of stercobilin and other bilins for quantification in environmental and clinical diagnostic applications.

METHODS

A procedure is described here using H2 (18) O to label the carboxylic acid groups of bilin tetrapyrroles. Reaction conditions as a function of temperature and reagent volume were found to produce a mixture of isotopomers, as assessed by electrospray ionization and Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS). Stability as a function of storage time and temperature and in conjunction with solid-phase extraction (SPE) was assessed.

RESULTS

The highest labeling efficiency was achieved at 70 °C for 8 h, while a stable ratio of the isotopmers could be produced at 60 °C for 4 h. The stability of the isotopic distribution was maintained under storage (room temperature or frozen) for 20 days. It was also stable throughout SPE. The high mass accuracy and resolving power of FTICRMS enables clear distinction between (18) O-labeled bilins from other unlabeled bilins present, avoiding a potential interference in quantitation.

CONCLUSIONS

A procedure was developed to label bilins with (18) O. The final ratio of the (18) O-labeled bilin isotopomers was reproducible and highly stable for at least 20 days under storage. This ratio was not changed in any statistically significant way even after SPE. Thus a reliable method for producing stable isotopomer ratios for bilins has been achieved. Copyright © 2016 John Wiley & Sons, Ltd.

5-Aminolaevulinic acid synthase and uroporphyrinogen methylase: two key control enzymes of tetrapyrrole biosynthesis and modification

Two enzymes which play an important role in regulation and flux control through the tetapyrrole biosynthetic pathway are considered. The Rhodobacter sphaeroides 5-aminolaevulinic acid synthase isoenzymes are discussed and the progress being made on their recombinant expression and isolation is reported. The Escherichia coli uroporphyrinogen methylase, which is encoded by the cysG gene, is also examined. In this case evidence is provided which demonstrates that the gene product is responsible for the complete synthesis of sirohaem from uroporphyrinogen III. The enzyme is thus capable of performing two S-adenosylmethionine-dependent methylation reactions, an NADP(+)-dependent dehydrogenation and iron chelation. The uroporphyrinogen methylase is thus a small multifunctional enzyme.

Biosynthesis of open-chain tetrapyrroles in plants, algae, and cyanobacteria

Phycobilins are open-chain tetrapyrroles of plants and algae which act as the chromophores of phycobiliproteins where they function as light energy-harvesting pigments. Phytochromobilin, another open-chain tetrapyrrole, is the chromophore of phytochrome, which functions as a light-sensing pigment in plant development. These open-chain tetrapyrroles are biosynthetically derived from protohaem. Enzyme reactions that convert protohaem to biliverdin IX alpha, and biliverdin IX alpha to phycocyanobilin, have been detected and characterized in extracts of the unicellular rhodophyte Cyanidium caldarium. Algal haem oxygenase and algal biliverdin-IX alpha reductase are both soluble enzymes that use electrons derived from reduced ferredoxin. Biochemical intermediates in the conversion of biliverdin IX alpha to (3E)-phycocyanobilin were identified as 15, 16-dihydrobiliverdin IX alpha, (3Z)-phycoerythrobilin and (3Z)-phycocyanobilin. Separate enzymes catalyse the two two-electron reduction steps in the conversion of biliverdin IX alpha to (3Z)-phycoerythrobilin. Z-to-E isomerization of the phycobilin ethylidine group is catalysed by an enzyme that requires glutathione for activity. Protein-bound phycoerythrobilin can be chemically converted to phytochromobilin which can then be released from the protein by methanolysis. This procedure was used to produce phytochromobilin in quantities sufficient to allow its chemical characterization and use in phytochrome reconstitution experiments. The results indicate that (2R,3E)-phytochromobilin spontaneously condenses with recombinant oat apophytochrome to form photoreversible holoprotein that is spectrally identical to native phytochrome.

Chlorophyll a biosynthetic heterogeneity

Chlorophyll a biosynthesis is presently interpreted in terms of two different biochemical pathways. According to one pathway, chlorophyll a is made via a single linear chain of reactions starting with divinylprotoporphyrin IX and ending with monovinylchlorophyll a. The experimental evidence for this pathway is marred by incompletely characterized intermediates that were detected in Chlorella mutants. The second pathway considers chlorophyll a to be made via multiple and parallel biosynthetic routes that result in the formation and accumulation of monovinyl- and divinylchlorophyll a chemical species. Two of these routes, namely the di/monocarboxylic monovinyl and divinyl routes, are responsible for the biosynthesis of most of the chlorophyll a in green plants. The experimental evidence for these two routes consists of: (a) the detection and spectroscopic characterization of intermediates and end products; (b) the demonstration of precursor-product relationships between various intermediates in vivo and in vitro; and (c) the detection of 4-vinylreductases that appear to be mainly responsible for the observed biosynthetic heterogeneity. The biological significance of chlorophyll a biosynthetic heterogeneity is becoming better understood. On the basis of the prevalence of the di/monocarboxylic monovinyl-and divinylchlorophyll a biosynthetic routes, green plants have been classified into three different greening groups. It now appears that the major chlorophylls in the euphotic zone of tropical waters are divinylchlorophyll a and b. It also appears that the di/monocarboxylic monovinyl and divinyl biosynthetic routes lead to the formation of different pigment proteins in different greening groups of plants, and that the more highly evolved monovinylchlorophyll a biosynthetic route is associated with higher field productivity in wheat.

A green light-absorbing phycoerythrin is present in the high-light-adapted marine cyanobacterium Prochlorococcus sp. MED4

In the high-light-adapted unicellular marine cyanobacterium Prochlorococcus sp. MED4 the cpeB gene is the only gene coding for a structural phycobiliprotein. The absence of any other phycoerythrin gene in the fully sequenced genome of this organism, the previous inability to detect a gene product, and the mutation of two out of four cysteine residues, normally involved in binding chromophores, suggested that MED4-cpeB might not code for a functional protein. Here, transcription of MED4-cpeB at a low level was detected and the transcriptional start site was mapped. Enrichment of the protein identified phycoerythrobilin as its sole chromophore in vivo, which was confirmed by chromophorylation assays in vitro using the recombinant protein. Phycourobilin is the major chromophore in low-light-adapted Prochlorococcus ecotypes such as strain SS120. Therefore, spectrally tuned phycoerythrins are a characteristic feature of distinct Prochlorococcus ecotypes. Further in vitro mutagenesis experiments replacing one or both cysteines C61R/C82S by arginine or serine, respectively, revealed that only Cys82 is required for chromophore binding. Thus, an unusual green light-absorbing phycoerythrin evolved in the high-light-adapted ecotypes of Prochlorococcus, which potentially serves as a photoreceptor.

Light-induced Proton Release of Phytochrome Is Coupled to the Transient Deprotonation of the Tetrapyrrole Chromophore

The Pr --> Pfr phototransformation of the bacteriophytochrome Agp1 from Agrobacterium tumefaciens and the structures of the biliverdin chromophore in the parent states and the cryogenically trapped intermediate Meta-R(C) were investigated with resonance Raman spectroscopy and flash photolysis. Strong similarities with the resonance Raman spectra of plant phytochrome A indicate that in Agp1 the methine bridge isomerization state of the chromophore is ZZZasa in Pr and ZZEssa in Pfr, with all pyrrole nitrogens being protonated. Photoexcitation of Pr is followed by (at least) three thermal relaxation components in the formation of Pfr with time constants of 230 micros and 3.1 and 260 ms. H2O/D2O exchange reveals kinetic isotope effects of 1.9, 2.6, and 1.3 for the respective transitions that are accompanied by changes of the amplitudes. The second and the third relaxation correspond to the formation and decay of Meta-R(C), respectively. Resonance Raman measurements of Meta-R(C) indicate that the chromophore adopts a deprotonated ZZE configuration. Measurements with a pH indicator dye show that formation and decay of Meta-R(C) are associated with proton release and uptake, respectively. The stoichiometry of the proton release corresponds to one proton per photoconverted molecule. The coupling of transient chromophore deprotonation and proton release, which is likely to be an essential element in the Pr --> Pfr photocon-version mechanism of phytochromes in general, may play a crucial role for the structural changes in the final step of the Pfr formation that switch between the active and the inactive state of the photoreceptor.

Agrobacterium phytochrome as an enzyme for the production of ZZE bilins

Photoconversion of phytochrome from the red-absorbing form Pr to the far-red-absorbing form Pfr is initiated by a Z to E isomerization around the ring C-ring D connecting double bond; the chromophore undergoes a ZZZ to ZZE isomerization. In vivo, phytochrome chromophores are covalently bound to the protein, but several examples of noncovalent in vitro adducts have been reported which also undergo Pr to Pfr photoconversion. We show that free biliverdin or phycocyanobilin, highly enriched in the ZZE isomer, can easily be obtained from chromophores bound in a noncovalent manner to Agrobacterium phytochrome Agp1, and used for spectral assays. Photoconversion of free biliverdin in a methanol/HCl solution from ZZE to ZZZ proceeded with a quantum yield of 1.8%, but was negligible in neutral methanol solution, indicating that this process is proton-dependent. The ZZE form of biliverdin and phycocyanobilin were tested for their ability to assemble with Agp1 and cyanobacterial phytochrome Cph1, respectively. In both cases, a Pfr-like adduct was formed but the chromophore was bound in a noncovalent manner to the protein. Agp1 Pfr undergoes dark reversion to Pr; the same feature was found for the noncovalent ZZE adduct. After dark reversion, the chromophore became covalently bound to the protein. In analogy, the PCB chromophore became covalently bound to Cph1 upon irradiation with strong far-red light which initiated ZZE to ZZZ isomerization. Agrobacterium Agp2 belongs to a yet small group of phytochromes which also assemble in the Pr form but convert from Pr to Pfr in darkness. When the Agp2 apoprotein was assembled with the ZZE form of biliverdin, the formation of the final adduct was accelerated compared to the formation of the ZZZ control, indicating that the ZZE chromophore fits directly into the chromophore pocket of Agp2.

Amino acid residues associated with enzymatic activities of the isomerizing phycoviolobilin-lyase PecE/F

PecE and PecF jointly catalyze the covalent attachment of phycocyanobilin to Cys-alpha84 of PecA and its concomitant isomerization to phycoviolobilin. (a) An Eschertchia coli supernatant expressing pecF has a residual activity of 6%; compared to the holoenzyme, this activity is lost upon purification. (b) Functional domains of both subunits from the cyanobacterium Mastigocladus laminosus were evaluated by mutageneses and chemical modification of amino acids. When in PecE the two motifs Y29YAAWWL and D263DLL were deleted, the holoenzyme lost its activity; it is also inactivated upon deletion of a central part (R111 to A122). The three conserved cysteines C48, C91, and C161 have only minor effects on catalysis. When in PecF the 20 C-terminal and 56 N-terminal amino acids were truncated, the lyase-isomerase activity in combination with PecE decreased to 12% and 15%, respectively, compared to the native enzyme. The catalytic efficiency (k(cat)/K(m)) decreased 16-fold when the unique four histidine residues in PecF beginning at H53 were deleted. H121 and C122 of PecF are essential for the enzyme activity; they are part of a unique stretch extending from A104 to N125 which is absent in the beta-subunit of related but nonisomerizing lyases. A single histidine and a single tryptophan are required for activity in both PecE and PecF, as judged from diethyl pyrocarbonate and N-bromosuccinimide modification and statistical analyses. Inactivation of PecE and PecF is also possible by arginine-specific reagents, while modifications of lysine, glutamate, and aspartate retained activity. (c) PecE and PecF, as well as most of the mutants, bind PCB covalently in substoichiometric amounts, as assayed by Zn(2+)-induced fluorescence on denaturing gels.

Heteronuclear Solution-State NMR Studies of the Chromophore in Cyanobacterial Phytochrome Cph1

Precise structural information regarding the chromophore binding pocket is essential for an understanding of photochromicity and photoconversion in phytochrome photoreceptors. To this end, we are studying the 59 kDa N-terminal module of the cyanobacterial phytochrome Cph1 from Synechocystis sp. PCC 6803 in both thermally stable forms (Pr and Pfr) using solution-state NMR spectroscopy. The protein is deuterated, while the chromophore, phycocyanobilin (PCB), is isotopically labeled with (15)N or (13)C and (15)N. We have established a simple approach for preparing labeled PCB based on BG11 medium supplemented with an appropriate buffer and NaH(13)CO(3) and Na(15)NO(3) as sole carbon and nitrogen sources, respectively. We show that structural details of the chromophore binding pocket in both Pr and Pfr forms can be obtained using multidimensional heteronuclear solution-state NMR spectroscopy. Using one-dimensional (15)N NMR spectra, we show unequivocally that the chromophore is protonated in both Pr and Pfr states.

Reconstitution of phycobilisome core-membrane linker, LCM, by autocatalytic chromophore binding to ApcE

The core-membrane linker, LCM, connects functionally the extramembraneous light-harvesting complex of cyanobacteria, the phycobilisome, to the chlorophyll-containing core-complexes in the photosynthetic membrane. Genes coding for the apoprotein, ApcE, from Nostoc sp. PCC 7120 and for a C-terminally truncated fragment ApcE(1-240) containing the chromophore binding cysteine-195 were overexpressed in Escherichia coli. Both bind covalently phycocyanobilin (PCB) in an autocatalytic reaction, in the presence of 4M urea necessary to solubilize the proteins. If judged from the intense, red-shifted absorption and fluorescence, both products have the features of the native core-membrane linker LCM, demonstrating that the lyase function, the dimerization motif, and the capacity to extremely red-shift the chromophore are all contained in the N-terminal phycobilin domain of ApcE. The red-shift is, however, not the result of excitonic interactions: Although the chromoprotein dimerizes, the circular dichroism shows no indication of excitonic coupling. The lack of homologies with the autocatalytically chromophorylating phytochromes, as well as with the heterodimeric cysteine-alpha84 lyases, indicates that ApcE constitutes a third type of bilin:biliprotein lyase.

High-throughput single-cell fluorescence spectroscopy

A high-throughput method for measuring single-cell fluorescence spectra is presented. Upon excitation with a 488 nm argon-ion laser many bacterial cells were imaged by a 20x microscope objective while they moved through a capillary tube. Fluorescence was dispersed by a transmission diffraction grating, and an intensified charge-coupled device (ICCD) camera simultaneously recorded the zero and the first orders of the fluorescence from each cell. Single-cell fluorescence spectra were reconstructed from the distance between zero-order and first-order maxima as well as the length and the pixel intensity distribution of the first-order images. By using this approach, the emission spectrum of E. coli cells expressing green fluorescent protein (GFP) was reconstructed. Also, fluorescence spectra of E. coli cells expressing non-fluorescent apo-subunits of R-phycoerythrin (R-PE) were recorded after incubation of the cells with phycoerythrobilin (PEB) chromophore. The fluorescence spectra are in good agreement with results obtained on the same cells using a fluorescence spectrometer or a fluorescence microscope. When spectra are to be acquired, this approach has a higher throughput, better sensitivity, and better spectral resolution compared to flow cytometry.

Harnessing phytochrome's glowing potential

Directed evolution of a cyanobacterial phytochrome was undertaken to elucidate the structural basis of its light sensory activity by remodeling the chemical environment of its linear tetrapyrrole prosthetic group. In addition to identifying a small region of the apoprotein critical for maintaining phytochrome's native spectroscopic properties, our studies revealed a tyrosine-to-histidine mutation that transformed phytochrome into an intensely red fluorescent biliprotein. This tyrosine is conserved in all members of the phytochrome superfamily, implicating direct participation in the primary photoprocess of phytochromes. Fluorescent phytochrome mutants also hold great promise to expand the present repertoire of genetically encoded fluorescent proteins into the near infrared.

The mobility of phytochrome within protonemal tip cells of the moss Ceratodon purpureus, monitored by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is a versatile tool for investigating the mobilities of fluorescent molecules in cells. In this article, we show that it is possible to distinguish between freely diffusing and membrane-bound forms of biomolecules involved in signal transduction in living cells. Fluorescence correlation spectroscopy was used to measure the mobility of phytochrome, which plays a role in phototropism and polarotropism in protonemal tip cells of the moss Ceratodon purpureus. The phytochrome was loaded with phycoerythrobilin, which is fluorescent only in the phytochrome-bound state. Confocal laser scanning microscopy was used for imaging and selecting the xy measuring position in the apical zone of the tip cell. Fluorescence correlation was measured at ancient z-positions in the cell. Analysis of the diffusion coefficients by nonlinear least-square fits showed a subcellular fraction of phytochrome at the cell periphery with a sixfold higher diffusion coefficient than in the core fraction. This phytochrome is apparently bound to the membrane and probably controls the phototropic and polarotropic response.

The Excited-State Dynamics of Phycocyanobilin in Dependence on the Excitation Wavelength

The primary light-induced processes of phycocyanobilin were studied by means of transient-grating spectroscopy, whereby the excitation wavelength was varied over the spectral region of the ground-state absorption. On the basis of the results obtained, both the rate of the photoreaction in phycocyanobilin and the ratio of the decay of different excited-state species via two decay channels depend on the excitation wavelength. Furthermore, the formation of the photoreaction product is also dependent on the pump color. These data support a recently established model for the primary photoprocesses in phycocyanobilin. In addition, phycocyanobilin protonated at the basic pyrrolenine-type nitrogen atom was included in the transient absorption study. The decay behavior was found to be almost unchanged when compared with the unprotonated form, and this suggests that protonation of the tetrapyrrole ring structure has no effect on the overall photochemistry.

Photochromic Biliproteins from the Cyanobacterium Anabaena sp. PCC 7120: Lyase Activities, Chromophore Exchange, and Photochromism in Phytochrome AphA

Photochromic biliproteins can be switched by light between two states, initiated by Z/E photoisomerization of the linear tetrapyrrole chromophore. The cyanobacterium Anabaena sp. PCC 7120 contains three genes coding for such biliproteins, two coding for phytochromes (aphA/B) and one for the alpha subunit of phycoerythrocyanin (pecA). (a) aphA was overexpressed in Escherichia coli with N-terminal His and S tags, and the protein was reconstituted by an optimized protocol with phycocyanobilin (PCB), to yield the photochromic chromoprotein, PCB-AphA, carrying the PCB chromophore. (b) AphA chromophorylation is autocatalytic such as in other phytochromes. (c) AphA chromophorylation is also possible by chromophore transfer from the PCB-carrying biliprotein, phycocyanin (CPC). The autocatalytic transfer is very slow, and it is enhanced more than 100-fold by catalysis of PCB:CpcA lyase and alpha-CPC as donor. (d) Through deletion mutations of aphA, a short sequence IQPHGV [amino acids (aa) 26-31] was found essential for the lyase activity of AphA, indicating an interaction of the N terminus with the chromophore-binding domain around cysteine 259. (e) A motif of at least 23 aa, starting with this sequence and located approximately 250 aa N terminal of the chromophore-binding cysteine, is proposed to relate to the lyase function in plant and most prokaryotic phytochromes. (f) Long-range interactions in AphA are further supported by blue-shifted absorptions (<or=12 nm) of both the Pr and Pfr forms of truncated chromoproteins.

Differing involvement of sulfoquinovosyl diacylglycerol in photosystem II in two species of unicellular cyanobacteria

Sulfoquinovosyl diacylglycerol (SQDG) is involved in the maintenance of photosystem II (PSII) activity in Chlamydomonas reinhardtii[Minoda, A., Sato, N., Nozaki, H., Okada, K., Takahashi, H., Sonoike, K. & Tsuzuki, M. et al. (2002) Eur. J. Biochem.269, 2353-2358]. To understand the spread of the taxa in which PSII interacts with SQDG, especially in cyanobacteria, we produced a mutant defective in the putative sqdB gene responsible for SQDG synthesis from two cyanobacteria, Synechocystis sp. PCC6803 and Synechococcus sp. PCC7942. The mutant of PCC6803, designated SD1, lacked SQDG synthetic ability and required SQDG supplementation for its growth. After transfer from SQDG-supplemented to SQDG-free conditions, SD1 showed decreased net photosynthetic and PSII activities on a chlorophyll (Chl) basis with a decrease in the SQDG content. Moreover, the sensitivity of PSII activity to 3-(3,4-dichlorophenyl)-1,1-dimethylurea and atrazine was increased in SD1. However, SD1 maintained normal amounts of cytochrome b559 and D1 protein (the subunits comprising the PSII complex) on a Chl basis, indicating that the PSII complex content changed little, irrespective of a decrease in the SQDG content. These results suggest that the role of SQDG is the conservation of the PSII properties in PCC6803, consistent with the results obtained with C. reinhardtii. In contrast, the SQDG-null mutant of PCC7942 showed the normal level of PSII activity with little effect on its sensitivity to PSII herbicides. Therefore, the difference in the SQDG requirement for PSII is species-specific in cyanobacteria; this could be of use when investigating the molecular evolution of the PSII complex.

Chromophore selectivity in bacterial phytochromes

Bacterial phytochromes (Bphs) are ancestors of the well characterized plant photoreceptors. Whereas plant phytochromes perform their photoisomerization exclusively via a covalently bound bilin chromophore, Bphs are variable in their chromophore selection. This is demonstrated in the cyanobacterium Calothrix PCC7601 that expresses two Bphs, CphA and CphB. CphA binds phycocyanobilin (PCB) covalently, whereas CphB, lacking the covalently binding cysteine of the plant phytochromes, carries biliverdin IXalpha (BV) as the chromophore. Our experiments elucidate the different modes of chromophore-protein interaction in CphA and CphB and offer a rationale for their chromophore selectivity. The tight binding of BV by CphB prevents PCB from competing for the binding cavity. Even when the chromophore-binding cysteine has been inserted (CphB-mutant L266C), PCB replaces BV very slowly, indicating the tight, but not irreversible binding of BV. The mutant CphB L266C showed a redox-sensitivity with respect to its PCB binding mode: under reducing conditions, the chromoprotein assembly leads to spectra indicative for a covalent binding, whereas absence of dithiothreitol or its removal prior to assembly causes spectra indicative for noncovalent binding. Regarding the CphB-type Bphs lacking the covalently binding cysteine, our results support the involvement of the succeeding histidine residue in chromophore fixation via a Schiff base-like bond between the bilin A-ring carbonyl and the histidine imidazole group. The assembly process and the stability of the holo-proteins were strongly influenced by the concentration of added imidazole (mimicking the histidine side-chain), making the attachment of the chromophore via the histidine more likely than via another cysteine of the protein.

Energy transfer in monomeric phycoerythrocyanin

Phycoerythrocyanin (PEC) is part of the phycobilisome of cyanobacteria. Its monomer carries one phycoviolobilin and two phycocyanobilins (PCB) as chromophores. For an understanding of the complicated energy transfer in phycobilisomes, a detailed knowledge of the processes in the constituting building proteins is indispensable. We report the experimental data necessary for the description of Förster energy transfer in monomeric PEC, including fluorescence lifetimes and quantum yields of the two subunits. The bulk experiments are complemented by studies of single PEC molecules. Förster energy calculations and Monte Carlo simulations based on the bulk data are presented. They reveal that earlier experimental findings of energy transfer heterogeneities in single PEC molecules originate in spectral shifts between the contributing chromophores.

TIGRINA d, required for regulating the biosynthesis of tetrapyrroles in barley, is an ortholog of the FLU gene of Arabidopsis thaliana

Regulation of tetrapyrrole biosynthesis in higher plants has been attributed to negative feedback control of steps prior to delta-aminolevulinic acid (ALA) formation. One of the first mutants with a defect in this control had been identified in barley. The tigrina (tig) d mutant accumulates 10-15-fold higher amounts of protochlorophyllide than wild type, when grown in the dark. The identity of the TIGRINA d protein and its mode of action are not known yet. Initially this protein had been proposed to act as a repressor of genes that encode enzymes involved in early steps of ALA formation, but subsequent attempts to confirm this experimentally failed. Here we demonstrate that the TIGRINA d gene of barley is an ortholog of the FLU gene of Arabidopsis thaliana. The FLU protein is a nuclear-encoded plastid protein that plays a key role in negative feedback control of chlorophyll biosynthesis in higher plants. Sequencing of the FLU gene of barley revealed a frame shift mutation in the FLU gene of the tig d mutant that results in the loss of two tetratricopeptide repeats that in the FLU protein of Arabidopsis are essential for its biological activity. This mutation cosegregates strictly with the tigrina phenotype within the F1 population of a heterozygous tig d mutant, thus providing additional support for the flu gene being responsible for the tigrina phenotype of barley.

Mechanism of Cph1 Phytochrome Assembly from Stopped-Flow Kinetics and Circular Dichroism

The kinetics and mechanism of the autocatalytic assembly of holo-Cph1 phytochrome (from Synechocystis) from the apoprotein and the bilin chromophores phycocyanobilin (PCB) and phycoerythrobilin (PEB) were investigated by stopped flow and circular dichroism. At 1:1 stoichiometry, pH 7.9, and 10 degrees C, SVD analysis of the kinetic data for PCB revealed three spectral components involving three transitions with time constants tau(1) approximately 150 ms, tau(2) approximately 2.5 s, and tau(3) approximately 50 s. Tau(1) was associated with a major red shift and transfer of oscillator strength from the Soret region to the 680 nm region. When the sulfhydryl group of cysteine 259 was blocked with iodoacetamide, preventing the formation of a covalent adduct, a noncovalent red-shifted complex (680 nm) was formed with a time constant of 200 ms. Tau(1) could thus be assigned to the formation of a noncovalent complex. The absorption changes during tau(1) are due to the formation of the extended conformation of the linear tetrapyrrole and to its protonation in the binding pocket. From the concentration and pH dependence of the kinetics we obtained a value of 1.5 microM for the K(D) of this noncovalent complex and a value of 8.4 for the pK(a) of the proton donor. The tau(2) component was associated with a blue shift of about 25 nm and was attributed to the formation of the covalent bond (P(r)), accompanied with the loss of the 3-3' double bond to ring A. Tau(3) was due to photoconversion to P(fr). For PEB, which is not photochromic, the formation of the noncovalent complex is faster (tau(1) = 70 ms), but the covalent bond formation is about 80 times slower (tau(2) = 200 s) than with the natural chromophore PCB. The CD spectra of the PCB adduct in the 250-800 nm range show that the chromophore geometries in P(r) and P(fr) are similar to those in plant phytochrome. The opposite rotational strengths of P(r) and P(fr) in the longest wavelength band suggest that the photoisomerization induces a reversal of the chirality. The Cph1 complex with noncovalently bound PCB was still photochromic when cysteine 259 was blocked with IAA or with the bulkier IAF. The covalent linkage to cysteine 259 is thus not required for photoconversion. The CD spectra of the noncovalently bound PCB in P(r)- and P(fr)-like states are qualitatively similar to those of the covalent adducts, suggesting analogous structures in the binding pocket. The noncovalent interactions with the binding pocket are apparently sufficient to hold the chromophore in the appropriate geometry for photoisomerization.

Control of tetrapyrrole biosynthesis by alternate quaternary forms of porphobilinogen synthase

Porphobilinogen synthase (PBGS) catalyzes the first common step in the biosynthesis of tetrapyrroles (such as heme and chlorophyll). Although the predominant oligomeric form of this enzyme, as inferred from many crystal structures, is that of a homo-octamer, a rare human PBGS allele, F12L, reveals the presence of a hexameric form. Rearrangement of an N-terminal arm is responsible for this oligomeric switch, which results in profound changes in kinetic behavior. The structural transition between octamer and hexamer must proceed through an unparalleled equilibrium containing two different dimer structures. The allosteric magnesium, present in most PBGS, has a binding site in the octamer but not in the hexamer. The unprecedented structural rearrangement reported here relates to the allosteric regulation of PBGS and suggests that alternative PBGS oligomers may function in a magnesium-dependent regulation of tetrapyrrole biosynthesis in plants and some bacteria.

Metabolic control of the tetrapyrrole biosynthetic pathway for porphyrin distribution in the barley mutant albostrians

The barley line albostrians exhibits a severe block in chloroplast development as a result of a mutationally induced lack of plastid ribosomes. White leaves of this mutant contain undifferentiated plastids, possess only traces of chlorophyll (Chl), and are photosynthetically inactive. Chl deficiency, combined with a continuous heme requirement, should lead to drastic changes in the tetrapyrrole metabolism in white versus green leaves. We analyzed the extent to which the synthesis rate of the pathway and the porphyrin distribution toward the Chl- and heme-synthesizing bifurcation is altered in the white tissue of albostrians. Expression and activity of several distinctively regulated enzymes, such as glutamyl-tRNAglu reductase, glutamate 1-semialdehyde aminotransferase, Mg- and Fe-chelatase, and Chl synthetase, were altered in white mutant leaves in comparison to control leaves. A drastic loss in the rate-limiting formation of 5-aminolevulinate and in the Mg-chelatase and Mg-protoporphyrin IX methyltransferase activity, as well as an increase in Fe-chelatase activity, accounts for a decrease in the metabolic flux and the re-direction of metabolites. It is proposed that the tightly balanced control of activities in the pathway functions by different metabolic feedback loops and in response to developmental state and physiological requirements. This data supports the idea that the initial steps of Mg-porphyrin synthesis contribute to plastid-derived signaling toward the nucleus. The barley mutant albostrians proved to be a valuable system for studying regulation of tetrapyrrole biosynthesis and their involvement in the bi-directional communication between plastids and nucleus.

Photochromic chromopeptides derived from phycoerythrocyanin: biophysical and biochemical characterization

Truncated chromopeptides have been prepared from the small photo- and redox-switchable biliprotein alpha-phycoerythrocyanin (alpha-PEC). The native chromoprotein consists of a C-terminal globin domain containing the chromophore and the regulatory cysteins 98 and 99, and a two-helix (X,Y) N-terminal domain responsible for aggregation. Digestion with chymotrypsin-free trypsin leads to three chromopeptides, (N-30, N-33 and N-35), basically lacking the two N-terminal helices X and Y. The photo- and redox chemistry of the major product (N-33) is identical, qualitatively and quantitatively, to that of native alpha-PEC. A series of N- and C-terminally truncated polypeptides were expressed in E. coli and subjected to autocatalytic and enzymatic reconstitution with phycocyanobilin. Enzymatic reconstitution was possible with N-terminally truncated polypeptides up to 45 aa, while neither a more extensively shortened (N-63) peptide, nor two C-terminally shortened polypeptides could be reconstituted. All chromopeptides recovered from enzymatic reconstitution contained the native phycoviolobilin chromophore and showed the photochemical and redox reactivity of alpha-PEC, albeit quantitatively reduced in the N-45 chromopeptide.

Pathways of intracellular communication: tetrapyrroles and plastid-to-nucleus signaling

Retrograde plastid-to-nucleus signaling plays a central role in coordinating nuclear and plastid gene expression. The gun (genomes uncoupled) mutants of Arabidopsis have been used to demonstrate that Mg-protoporphyrin (Mg-Proto) acts as a plastid signal to repress the transcription of nuclear photosynthesis genes (1). It is unclear how Mg-Proto triggers repression, but several components of this pathway have been recently identified. These include the products of GUN4 and GUN5. GUN5 is the ChlH subunit of Mg-chelatase, which produces Mg-Proto, and GUN4 is a regulator of ChlH activity (2). GUN4 might also play a role in photoprotection and in the trafficking of Mg-Proto.

Biochemical properties of CikA, an unusual phytochrome-like histidine protein kinase that resets the circadian clock in Synechococcus elongatus PCC 7942

We recently described the cikA (circadian input kinase A) gene, whose product supplies environmental information to the circadian oscillator in the cyanobacterium Synechococcus elongatus PCC 7942. CikA possesses three distinct domains: a GAF, a histidine protein kinase (HPK), and a receiver domain similar to those of the response regulator family. To determine how CikA functions in providing circadian input, we constructed modified alleles to tag and truncate the protein, allowing analysis of each domain individually. CikA covalently bound bilin chromophores in vitro, even though it lacks the expected ligand residues, and the GAF domain influenced but did not entirely account for this function. Full-length CikA and truncated variants that carry the HPK domain showed autophosphorylation activity. Deletion of the GAF domain or the N-terminal region adjacent to GAF dramatically reduced autophosphorylation, whereas elimination of the receiver domain increased activity 10-fold. Assays to test phosphorelay from the HPK to the cryptic receiver domain, which lacks the conserved aspartyl residue that serves as a phosphoryl acceptor in response regulators, were negative. We propose that the cryptic receiver is a regulatory domain that interacts with an unknown protein partner to modulate the autokinase activity of CikA but does not work as bona fide receiver domain in a phosphorelay.

Dimerization and inter-chromophore distance of Cph1 phytochrome from Synechocystis, as monitored by fluorescence homo and hetero energy transfer

We investigated the dimerization of phytochrome Cph1 from the cyanobacterium Synechocystis by fluorescence resonance energy transfer (FRET). As donor we used the chromophore analogue phycoerythrobilin (PEB) and as acceptor either the natural chromophore phycocyanobilin (PCB; hetero transfer) or PEB (homo transfer). Both chromophores bind in a 1:1 stoichiometry to apo-monomers expressed in Escherichia coli. Energy transfer was characterized by time-resolved fluorescence intensity and anisotropy decay after excitation of PEB by picosecond pulses from a tunable Ti-sapphire laser system. ApoCph1 was first assembled with PEB at a low stoichiometry of 0.1. The remaining sites were then sequentially titrated with PCB. In the course of this titration, the mean lifetime of PEB decreased from 3.33 to 1.25 ns in the P(r) form of Cph1, whereas the anisotropy decay was unaffected. In the P(fr)/P(r) photoequilibrium (about 65% P(fr)), the mean lifetime decreased significantly less, to 1.67 ns. These observations provide strong support for inter-chromophore hetero energy transfer in mixed PEB/PCB dimers. The reduced energy transfer in P(fr) may be due to a structural difference but is at least in part due to the difference in spectral overlap, which was 4.1 x 10(-13) and 1.6 x 10(-13) cm(3) M(-1) in P(r) and P(fr), respectively. From the changes in the mean lifetime, rates of hetero energy transfer of 0.68 and 0.37 ns(-1) were calculated for the P(r) form and the P(fr)/P(r) photoequilibrium, respectively. Sequential titration of apo Cph1 with PEB alone to full occupancy did not affect the intensity decay but led to a substantial increase in depolarization. This is the experimental signature of homo energy transfer. Values for the rate of energy transfer k(HT) (0.47 ns(-1)) and the angle 2theta between the transition dipole moment directions (2theta = 45 +/- 5 degrees) were determined from an analysis of the concentration dependence of the anisotropy at five different PEB/Cph1 stoichiometries. The independently determined rates of hetero and homo energy transfer are thus of comparable magnitude and consistent with the energy transfer interpretation. Using these results and exploiting the 2-fold symmetry of the dimer, the chromophore-chromophore distance R(DA) was calculated and found to be in the range 49 A < R(DA) < 63 A. Further evidence for energy transfer in Cph1 dimers was obtained from dilution experiments with PEB/PEB dimers: the lifetime was unchanged, but the anisotropy increased as the dimers dissociated with increasing dilution. These experiments allowed a rough estimate of 5 +/- 3 microM for the dimer dissociation constant. With the deletion mutant Cph1Delta2 that lacks the carboxy terminal histidine kinase domain less energy transfer was observed suggesting that in this mutant dimerization is much weaker. The carboxy terminal domain of Cph1 that is involved in intersubunit trans-phosphorylation and signal transduction thus plays a dominant role in the dimerization. The FRET method provides a sensitive assay to monitor the association of Cph1 monomers.

Green or red: what stops the traffic in the tetrapyrrole pathway?

Regulation of tetrapyrrole biosynthesis is crucial to plant metabolism. The two pivotal control points are formation of the initial precursor, 5-aminolaevulinic acid (ALA), and the metal-ion insertion step: chelation of Fe(2+) into protoporphyrin IX leads to haem and phytochromobilin, whereas insertion of Mg(2+) is the first step to chlorophyll. Recent studies with mutants and transgenic plants have demonstrated that perturbation of the branch point affects ALA formation. Moreover, one of the signals that controls the expression of genes for nuclear-encoded chloroplast proteins has been shown to be Mg-protoporphyrin-IX. Here, we discuss the regulation of branch-point flux and the relative contributions of the haem and chlorophyll branches to the regulation of ALA synthesis and thus to flow through the tetrapyrrole pathway.

Enantioselective syntheses of ring-C precursors of vit. B(12). Reagent control

Enelactones of the general structure S-(-)-I were prepared in three steps from alcohol 21 and acids 22 (ee approximately 85%). Lactones S-(-)-I are versatile precursors to enelactams II of the type found in Vitamin B(12).

Expression and characterization of cyanobacterium heme oxygenase, a key enzyme in the phycobilin synthesis. Properties of the heme complex of recombinant active enzyme

An efficient bacterial expression system of cyanobacterium Synechocystis sp. PCC 6803 heme oxygenase gene, ho-1, has been constructed, using a synthetic gene. A soluble protein was expressed at high levels and was highly purified, for the first time. The protein binds equimolar free hemin to catabolize the bound hemin to ferric-biliverdin IX alpha in the presence of oxygen and reducing equivalents, showing the heme oxygenase activity. During the reaction, verdoheme intermediate is formed with the evolution of carbon monoxide. Though both ascorbate and NADPH-cytochrome P450 reductase serve as an electron donor, the heme catabolism assisted by ascorbate is considerably slow and the reaction with NADPH-cytochrome P450 reductase is greatly retarded after the oxy-heme complex formation. The optical absorption spectra of the heme-enzyme complexes are similar to those of the known heme oxygenase complexes but have some distinct features, exhibiting the Soret band slightly blue-shifted and relatively strong CT bands of the high-spin component in the ferric form spectrum. The heme-enzyme complex shows the acid-base transition, where two alkaline species are generated. EPR of the nitrosyl heme complex has established the nitrogenous proximal ligand, presumably histidine 17 and the obtained EPR parameters are discriminated from those of the rat heme oxygenase-1 complex. The spectroscopic characters as well as the catabolic activities strongly suggest that, in spite of very high conservation of the primary structure, the heme pocket structure of Synechocystis heme oxygenase isoform-1 is different from that of rat heme oxygenase isoform-1, rather resembling that of bacterial heme oxygenase, H mu O.

Detection of protoporphyrin IX in envelope membranes of pea chloroplasts

Envelope membranes were prepared from mature pea chloroplasts. The tetrapyrrole contents of envelope membranes were analysed. The envelope membranes of pea chloroplasts contained substantial amounts of protoporphyrin IX and trace amounts of Mg-protoporphyrin IX and its monoester in addition to protochlorophyllide. The protoporphyrin IX content of envelope membranes was 89.25 pmol (mg protein)(-1). Its content in pea envelope membrane was higher than that of protochlorophyllide. The proportion of monovinyl and divinyl forms of protochlorophyllide present in pea chloroplast envelope membrane was 3:7. The significance of the presence of protoporphyrin IX in the envelope membrane is discussed in relation to plastidic Chl biosynthesis.

Interaction of FLU, a negative regulator of tetrapyrrole biosynthesis, with the glutamyl-tRNA reductase requires the tetratricopeptide repeat domain of FLU

Regulation of tetrapyrrole biosynthesis in plants has been attributed to feedback control of glutamyl-tRNA reductase (GLU-TR) by heme. Recently, another negative regulator, the FLU protein, has been discovered that operates independently of heme. A truncated form of FLU that contains two domains implicated in protein-protein interaction was co-expressed in yeast with either GLU-TR or glutamate-1-semialdehyde-2-1-aminotransferase (GSA-AT), the second enzyme involved in delta-aminolevulinic acid (ALA) biosynthesis. FLU interacts strongly with GLU-TR but not with GSA-AT. Two variants of FLU that carry single amino acid exchanges within their coiled coil and tetratricopeptide repeat (TPR) domains, respectively, were also tested. Only the FLU variant with the mutated TPR motif lost the capacity to interact with GLU-TR.

Characterization of a human and mouse tetrapyrrole-binding protein

The cDNA for p22HBP has been cloned from human and mouse, and the protein expressed, purified, and characterized. Both mouse and human proteins bind heme and porphyrins with micromolar K(d)s, are highly homologous, monomeric, and soluble, and have a cytoplasmic location. The proteins bind metalloporphyrins, free porphyrins, and N-methylprotoporphyrin with similar affinities, and mutations of a selected set of putative metal ligating residues did not have any significant effect on the measured K(d)s. That the presence or absence of metal in the porphyrin has no effect on the binding constants and the observation that the EPR signal for heme does not change upon binding to the protein strongly suggest that p22HBP is a generic tetrapyrrole-binding protein rather than a dedicated heme-binding protein. A role for p22HBP in cellular porphyrin metabolism is discussed.

The presence of chlorophyll b in Synechocystis sp. PCC 6803 disturbs tetrapyrrole biosynthesis and enhances chlorophyll degradation

Both chlorophyll (Chl) a and b accumulate in the light in a Synechocystis sp. PCC 6803 strain that expresses higher plant genes coding for a light-harvesting complex II protein and Chl a oxygenase. This cyanobacterial strain also lacks photosystem (PS) I and cannot synthesize Chl in darkness because of the lack of chlL. When this PS I-less/chlL(-)/lhcb(+)/cao(+) strain was grown in darkness, small amounts of two unusual tetrapyrroles, protochlorophyllide (PChlide) b and pheophorbide (pheide) b, were identified. Accumulation of PChlide b trailed that of PChlide a by several days, suggesting that PChlide a is an inefficient substrate of Chl a oxygenase. The presence of pheide b in this organism suggests a breakdown of Chl b via a pathway that does not involve conversion to a-type pigments. When the PS I-less/chlL(-) control strain was grown in darkness, Chl degradation was much slower than in the PS I-less/chlL(-)/lhcb(+)/cao(+) strain, suggesting that the presence of Chl b leads to more rapid turnover of Chl-binding proteins and/or a more active Chl degradation pathway. Levels and biosynthesis kinetics of Chl and of its biosynthetic intermediates are very different in the PS I-less/chlL(-)/lhcb(+)/cao(+) strain versus in the control. Moreover, when grown in darkness for 14 days, upon the addition of delta-aminolevulinic acid, the level of magnesium-protoporphyrin IX increased 60-fold in the PS I-less/chlL(-)/lhcb(+)/cao(+) strain (only approximately 2-fold in the PS I-less/chlL(-) control strain), whereas the PChlide and protoheme levels remained fairly constant. We propose that a b-type PChlide, Chl, or pheide in the PS I-less/chlL(-)/lhcb(+)/cao(+) strain may bind to tetrapyrrole biosynthesis regulatory protein(s) (for example, the small Cab-like proteins) and thus affect the regulation of this pathway.

Regulation of the distribution of chlorophyll and phycobilin-absorbed excitation energy in cyanobacteria. A structure-based model for the light state transition

The light state transition regulates the distribution of absorbed excitation energy between the two photosystems (PSs) of photosynthesis under varying environmental conditions and/or metabolic demands. In cyanobacteria, there is evidence for the redistribution of energy absorbed by both chlorophyll (Chl) and by phycobilin pigments, and proposed mechanisms differ in the relative involvement of the two pigment types. We assayed changes in the distribution of excitation energy with 77K fluorescence emission spectroscopy determined for excitation of Chl and phycobilin pigments, in both wild-type and state transition-impaired mutant strains of Synechococcus sp. PCC 7002 and Synechocystis sp. PCC 6803. Action spectra for the redistribution of both Chl and phycobilin pigments were very similar in both wild-type cyanobacteria. Both state transition-impaired mutants showed no redistribution of phycobilin-absorbed excitation energy, but retained changes in Chl-absorbed excitation. Action spectra for the Chl-absorbed changes in excitation in the two mutants were similar to each other and to those observed in the two wild types. Our data show that the redistribution of excitation energy absorbed by Chl is independent of the redistribution of excitation energy absorbed by phycobilin pigments and that both changes are triggered by the same environmental light conditions. We present a model for the state transition in cyanobacteria based on the x-ray structures of PSII, PSI, and allophycocyanin consistent with these results.

An in ovo chicken model to study the systemic and localized teratogenic effects of valproic acid

BACKGROUND

The antiepileptic valproic acid (VPA) is a teratogen whose embryopathic mechanism(s) remain uncertain. Elucidating potential cellular and molecular effects of VPA is complicated by systemic application paradigms. We developed an in ovo model to reproduce the teratogenic effects of VPA and a localized VPA application procedure to determine whether VPA can selectively effect abnormal development in one region of the embryo.

METHODS

VPA was applied topically to chicken embryos in ovo at different embryonic stages. Embryos were later evaluated for gross and skeletal anomalies. Pax-2 and Pax-6 protein expression in the developing eye was also evaluated because VPA-induced eye anomalies are similar to those seen by the disruption of Pax-2 and Pax-6. For localized application, a thin sheet of the synthetic polymer Elvax was impregnated with VPA. A small piece of the VPA-impregnated polymer was applied directly to the presumptive wing bud region in Stage 10-17 embryos. Embryos were examined for gross and skeletal anomalies. Sham controls were employed for all experiments.

RESULTS

Chicken embryos exposed to VPA in ovo demonstrated increased mortality, growth delay and anomalies similar to ones previously seen in humans: neural tube, cardiovascular, craniofacial, limb and skeletal. Pax-2 and Pax-6 protein expression was qualitatively diminished in the eye. Localized wing bud VPA exposure caused structural abnormalities in the developing wing in the absence of other anomalies in the embryos. These wing defects were similar to those observed after topical whole-embryo VPA application.

CONCLUSIONS

These results indicate that at least one mechanism for the teratogenicity of VPA involves a direct effect on developing tissue. The nature of the abnormalities observed implies that this effect may be mediated by disruption of genes that regulate pattern formation.

Taxonomic revision of water-bloom-forming species of oscillatorioid cyanobacteria

A polyphasic approach was used to clarify the taxonomy of the water-bloom-forming oscillatorioid cyanobacteria. Seventy-five strains of oscillatorioid cyanobacteria were characterized by 16S rDNA sequence analysis, DNA base composition, DNA-DNA hybridization, fatty acid composition, phycobilin pigment composition, complementary chromatic adaptation, morphological characters, growth temperature and salinity tolerance. Phylogenetic analysis based on 16S rDNA sequences divided the strains into six groups, all of which were clearly separated from the type species of the genus Oscillatoria, Oscillatoria princeps Gomont NIVA CYA 150. Therefore, these strains should be classified into genera other than Oscillatoria. Groups I-III were closely related to one another and groups IV-VI were distinct from one another and from groups I to III. Group I was further divided into two subgroups, group I-pc, which includes strains containing only phycocyanin (PC), and group I-pe, which includes strains containing large amounts of phycoerythrin (PE) in addition to PC. This phenotypic distinction was supported by DNA-DNA hybridization studies. Based on the properties examined herein and data from traditional, botanical taxonomic studies, the groups and subgroups were classified into single species and we propose either emended or new taxonomic descriptions for Planktothrix agardhii (type strain NIES 204T), Planktothrix rubescens (type strain CCAP 1459/22T), Planktothrix pseudagardhii sp. nov. (type strain T1-8-4T), Planktothrix mougeotii (type strain TR1-5T), Planktothricoides raciborskii gen. nov., comb. nov. (type strain NIES 207T), Tychonema bourrellyi (type strain CCAP 1459/11BT) and Limnothrix redekei (type strain NIVA CYA 277/1T).