Subunit structure of the phycobiliproteins of blue-green algae

Sustainable Production of Pigments from Cyanobacteria

Pigments are intensely coloured compounds used in many industries to colour other materials. The demand for naturally synthesised pigments is increasing and their production can be incorporated into circular bioeconomy approaches. Natural pigments are produced by bacteria, cyanobacteria, microalgae, macroalgae, plants and animals. There is a huge unexplored biodiversity of prokaryotic cyanobacteria which are microscopic phototrophic microorganisms that have the ability to capture solar energy and CO2 and use it to synthesise a diverse range of sugars, lipids, amino acids and biochemicals including pigments. This makes them attractive for the sustainable production of a wide range of high-value products including industrial chemicals, pharmaceuticals, nutraceuticals and animal-feed supplements. The advantages of cyanobacteria production platforms include comparatively high growth rates, their ability to use freshwater, seawater or brackish water and the ability to cultivate them on non-arable land. The pigments derived from cyanobacteria and microalgae include chlorophylls, carotenoids and phycobiliproteins that have useful properties for advanced technical and commercial products. Development and optimisation of strain-specific pigment-based cultivation strategies support the development of economically feasible pigment biorefinery scenarios with enhanced pigment yields, quality and price. Thus, this chapter discusses the origin, properties, strain selection, production techniques and market opportunities of cyanobacterial pigments.

Preparative continuous flow electrophoretic instrumentation for purification of biological samples

We constructed preparative instrumentation and developed the method that are based on separation of the samples by bidirectional isotachophoresis/moving boundary electrophoresis in continuous divergent flow. The described instrumentation can be used for variety of the samples, however, it can be easily optimized and tailored for the specific sample. The trapezoid separation bed from non-woven textile exhibited minimum adsorption effect for sample and it can be used repeatedly. By addition of different spacers via separation space inlets, the sections of pH gradient can be modified to enhance the separation. The liquid flow from two inlets positioned on each side of the sample inlet prevented the contact of the sample with anolyte and catholyte at the analysis beginning. One pair of thin electrodes (graphite and stainless-steel) was placed at the separation space output. The electrode products were washed out into drains without disturbing the focusing process. The influence of EOF was managed by tilting the separation bed in direction from cathodic to anodic side. The components of spirulina supernatant and color pI markers were separated in the pH gradient from 3.9 to 10.1. pH gradient was stable for at least 4.5 hours and spirulina supernatant from about 0.12 g of dry powder was processed. Compared to other preparative methods used for spirulina separation, the presented method/instrumentation working with continuous divergent flow had essential advantages. The efficient separation was fast, and no intermediate steps were necessary to obtain liquid fractions with separated components compatible with further biological experiments. This article is protected by copyright. All rights reserved.

The Arabidopsis CRUMPLED LEAF protein, a homolog of the cyanobacterial bilin lyase, retains the bilin-binding pocket for a yet unknown function

The photosynthetic bacterial phycobiliprotein lyases, also called CpcT lyases, catalyze the biogenesis of phycobilisome, a light-harvesting antenna complex, through the covalent attachment of chromophores to the antenna proteins. The Arabidopsis CRUMPLED LEAF (CRL) protein is a homolog of the cyanobacterial CpcT lyase. Loss of CRL leads to multiple lesions, including localized foliar cell death, constitutive expression of stress-related nuclear genes, abnormal cell cycle, and impaired plastid division. Notwithstanding the apparent phenotypes, the function of CRL still remains elusive. To gain insight into the function of CRL, we examined whether CRL still retains the capacity to bind with the bacterial chromophore phycocyanobilin (PCB) and its plant analog phytochromobilin (PΦB). The revealed structure of the CpcT domain of CRL is comparable to that of the CpcT lyase, despite the low sequence identity. The subsequent in vitro biochemical assays found, as shown for the CpcT lyase, that PCB/PΦB binds to the CRL dimer. However, some mutant forms of CRL, substantially compromised in their bilin-binding ability, still restore the crl-induced multiple lesions. These results suggest that although CRL retains the bilin-binding pocket, it seems not functionally associated with the crl-induced multiple lesions.

A thermophilic C-phycocyanin with unprecedented biophysical and biochemical properties

C-phycoyanins are abundant light-harvesting pigments which have an important role in the energy transfer cascade of photosystems in prokaryotic cyanobacteria and eukaryotic red algae. These proteins have important biotechnological applications, since they can be used in food, cosmetics, nutraceutical, pharmaceutical industries and in biomedical research. Here, C-phycocyanin from the extremophilic red alga Galdieria phlegrea (GpPC) has been purified and characterized from a biophysical point of view by SDS-PAGE, mass spectrometry, UV-Vis absorption spectroscopy, circular dichroism and intrinsic fluorescence. Stability against pH variations, addition of the oxidizing agent hydrogen peroxide and the effects of temperature have been also investigated, together with its in cell antioxidant potential and antitumor activity. GpPC is stable under different pHs and unfolds at a temperature higher than 80 °C within the pH range 5.0-7.0. Its fluorescence spectra present a maximum at 650 nm, when excited at 589 nm. The protein exerts interesting in cell antioxidant properties even after high temperature treatments, like the pasteurization process, and is cytotoxic for A431 and SVT2 cancer cells, whereas it is not toxic for non-malignant cells. Our results assist in the development of C-phycocyanin as a multitasking protein, to be used in the food industry, as antioxidant and anticancer agent.

Targeted delivery of fluorogenic peptide aptamers into live microalgae by femtosecond laser photoporation at single-cell resolution

Microalgae-based metabolic engineering has been proven effective for producing valuable substances such as food supplements, pharmaceutical drugs, biodegradable plastics, and biofuels in the past decade. The ability to accurately visualize and quantify intracellular metabolites in live microalgae is essential for efficient metabolic engineering, but remains a major challenge due to the lack of characterization methods. Here we demonstrate it by synthesizing fluorogenic peptide aptamers with specific binding affinity to a target metabolite and delivering them into live microalgae by femtosecond laser photoporation at single-cell resolution. As a proof-of-principle demonstration of our method, we use it to characterize Euglena gracilis, a photosynthetic unicellular motile microalgal species, which is capable of producing paramylon (a carbohydrate granule similar to starch). Specifically, we synthesize a peptide aptamer containing a paramylon-binding fluorescent probe, 7-nitrobenzofurazan, and introduce it into E. gracilis cells one-by-one by suppressing their mobility with mannitol and transiently perforating them with femtosecond laser pulses at 800 nm for photoporation. To demonstrate the method’s practical utility in metabolic engineering, we perform spatially and temporally resolved fluorescence microscopy of single live photoporated E. gracilis cells under different culture conditions. Our method holds great promise for highly efficient microalgae-based metabolic engineering.

Phycocyanin: A Potential Drug for Cancer Treatment

Phycocyanin isolated from marine organisms has the characteristics of high efficiency and low toxicity, and it can be used as a functional food. It has been reported that phycocyanin has anti-oxidative function, anti-inflammatory activity, anti-cancer function, immune enhancement function, liver and kidney protection pharmacological effects. Thus, phycocyanin has an important development and utilization as a potential drug, and phycocyanin has become a new hot spot in the field of drug research. So far, there are more and more studies have shown that phycocyanin has the anti-cancer effect, which can block the proliferation of cancer cells and kill cancer cells. Phycocyanin exerts anti-cancer activity by blocking tumor cell cell cycle, inducing tumor cell apoptosis and autophagy, thereby phycocyanin can serve as a promising anti-cancer agent. This review discusses the therapeutic use of phycocyanin and focuses on the latest advances of phycocyanin as a promising anti-cancer drug.

Availability and Utilization of Pigments from Microalgae

Microalgae are the major photosynthesizers on earth and produce important pigments that include chlorophyll a, b and c, β-carotene, astaxanthin, xanthophylls, and phycobiliproteins. Presently, synthetic colorants are used in food, cosmetic, nutraceutical, and pharmaceutical industries. However, due to problems associated with the harmful effects of synthetic colorants, exploitation of microalgal pigments as a source of natural colors becomes an attractive option. There are various factors such as nutrient availability, salinity, pH, temperature, light wavelength, and light intensity that affect pigment production in microalgae. This paper reviews the availability and characteristics of microalgal pigments, factors affecting pigment production, and the application of pigments produced from microalgae. The potential of microalgal pigments as a source of natural colors is enormous as an alternative to synthetic coloring agents, which has limited applications due to regulatory practice for health reasons.

Haematococcus pluvialis soluble proteins: Extraction, characterization, concentration/fractionation and emulsifying properties

A water-soluble matrix was extracted from green vegetative Haematococcus pluvialis through high-pressure cell disruption either at native pH (5.7) or with pH shifting to neutral (7). The resulting supernatant is mainly composed of carbohydrates and proteins, with the highest yield of proteins obtained at neutral pH (73±2% of total biomass proteins). The key emulsification properties of the proteins isolated in neutral supernatant (emulsification capacity (EC): 534±41mLoilg(-1) protein, emulsification stability (ES): 94±3% and emulsification activity index (EAI): 80±1m(2)g(-1)) were comparable to the native supernatant values (EC: 589±21mLoilg(-1) protein, ES: 84±3% and EAI: 75±1m(2)g(-1)). Confronted to sodium caseinate (EC: 664±30mLoilg(-1) protein, ES: 63±4%, and EAI: 56±4m(2)g(-1)) these results highlighted the strong potential of proteins isolated from H. pluvialis as emulsifier agent. Moreover, experiments have shown that the stability of emulsions obtained from supernatants is due to the proteins rather than the carbohydrates.

Extraction and purification of C-phycocyanin from Spirulina platensis (CCC540)

In this study a simple protocol was developed for purifying phycocyanin (PC) from Spirulina platensis (CCC540) by using ammonium sulphate precipitation, followed by a single step chromatography by using DEAE-Cellulose-11 and acetate buffer. Precipitation with 65 % ammonium sulphate resulted in 80 % recovery of phycocyanin with purity of 1.5 (A₆₂₀/A₂₈₀). Thro1ugh chromatography an 80 % recovery of phycocyanin with a purity of 4.5 (A₆₂₀/A₂₈₀) was achieved. In SDS_PAGE analysis, the purified PC showed the presence of two subunit α (16 kD) and β (17 kD).

Biliproteins of cyanobacteria and Rhodophyta: Homologous family of photosynthetic accessory pigments

Amino-terminal sequence determinations are reported of the subunits of biliproteins of prokaryotic unicellular and filamentous cyanobacteria and of eukaryotic unicellular red algae. The biliproteins examined, allophycocyanin, C-phycocyanin, R-phycocyanin, b-phycoerythrin, and phycoerythrocyanin, vary with respect to the chemical nature and the number and distribution of the bilin chromophores between the two dissimilar subunits. The amino-terminal sequences fall into two classes, "alpha-type" and "beta-type", with a high degree of homology within each class.In those biliproteins where the number of bilin chromophores on the two subunits is unequal, the subunit with the greater number of chromophores has the beta-type amino-acid sequence.Extensive homology also exists between alpha- and beta-type sequences, strongly supporting the view that these arose by gene duplication to give rise to the ancestral alpha- and beta-type genes early in the evolution of the biliproteins. The subsequent generation of the various classes of biliproteins appears to be the result of further gene duplication of the alpha- and beta-type genes, ultimately to give rise to families of polypeptide chains of similar sequence, but varying in the number of chromophore attachment sites and the structure of the chromophores.

Purification of c-phycocyanin from Spirulina fusiformis and its effect on the induction of urokinase-type plasminogen activator from calf pulmonary endothelial cells

c-Phycocyanin (c-pc), a blue coloured, fluorescent protein was purified from blue-green alga, Spirulina fusiformis and its effect on fibrinolytic system in vascular endothelial cells was investigated. The c-pc consisted of two subunits, alpha and beta, whose molecular masses were 16 and 17 kDa, respectively. N-terminal sequences of both subunits were well conserved compared with other blue green algal phycobiliproteins. Fibrinolytic activity in the medium conditioned by calf pulmonary arterial endothelial cells was measured by the fibrin plate method. The c-pc increased the fibrinolytic activity in dose- and time-dependent manners. Fibrin zymographic studies indicated that c-pc-induced urokinase-type plasminogen activator in the cells. These in vitro results suggest that c-pc from S. fusiformis is a potent profibrinolytic protein in the vascular endothelial system.

Identification and characterization of a new class of bilin lyase: the cpcT gene encodes a bilin lyase responsible for attachment of phycocyanobilin to Cys-153 on the beta-subunit of phycocyanin in Synechococcus sp. PCC 7002

Synechococcus sp. PCC 7002 and all other cyanobacteria that synthesize phycocyanin have a gene, cpcT, that is paralogous to cpeT, a gene of unknown function affecting phycoerythrin synthesis in Fremyella diplosiphon. A cpcT null mutant contains 40% less phycocyanin than wild type and produces smaller phycobilisomes with red-shifted absorbance and fluorescence emission maxima. Phycocyanin from the cpcT mutant has an absorbance maximum at 634 nm compared with 626 nm for the wild type. The phycocyanin beta-subunit from the cpcT mutant has slightly smaller apparent molecular weight on SDS-PAGE. Purified phycocyanins from the cpcT mutant and wild type were cleaved with formic acid, and the products were analyzed by SDS-PAGE. No phycocyanobilin chromophore was bound to the peptide containing Cys-153 derived from the phycocyanin beta-subunit of the cpcT mutant. Recombinant CpcT was used to perform in vitro bilin addition assays with apophycocyanin (CpcA/CpcB) and phycocyanobilin. Depending on the source of phycocyanobilin, reaction products with CpcT had absorbance maxima between 597 and 603 nm as compared with 638 nm for the control reactions, in which mesobiliverdin becomes covalently bound. After trypsin digestion and reverse phase high performance liquid chromatography, the CpcT reaction product produced one major phycocyanobilin-containing peptide. This peptide had a retention time identical to that of the tryptic peptide that includes phycocyanobilin-bound, cysteine 153 of wild-type phycocyanin. The results from characterization of the cpcT mutant as well as the in vitro biochemical assays demonstrate that CpcT is a new phycocyanobilin lyase that specifically attaches phycocyanobilin to Cys-153 of the phycocyanin beta-subunit.

Celebrating the millennium: historical highlights of photosynthesis research, part 3

This paper introduces the third and final part of the 'millennium celebrations of historical highlights of photosynthesis research.' Part 1 (308 pages) was published in October 2002 as Vol. 73 of the journal Photosynthesis Research, and Part 2 (458 pages) was published in July 2003 as Vol. 76. Here, we recognize particularly the work of three major contributors to our understanding of photosynthesis: Roger Stanier (1916-1982); Germaine Cohen-Bazire (Stanier) (1920-2001); and William Arnold (1904-2001). We also introduce the historical papers contained in this volume; consider the legacy of Alfred Nobel (1833-1896); and identify Nobel prizes of special relevance to understanding the capture, conversion, and storage of light energy in both anoxygenic and oxygenic photosynthesis.

Phycobiliproteins and phycobilisomes: the early observations

The purpose of this minireview is to highlight the early observations that led to the discovery of the physico-chemical properties of the phycobiliproteins, their structure and function, and to their architectural organization in supramolecular complexes, the phycobilisomes. Generally attached on the stromal surface of the thylakoid membranes in both prokaryotic (cyanobacteria) and eukaryotic cells (cyanelles, red algae and cryptomonads), these complexes represent the most abundant soluble proteins and the major light-harvesting antennae for photosynthesis. This review mainly focuses on the years prior to the development of the molecular biology of cyanobacteria that flourished in the 1980s. We refer the reader to the comprehensive and excellent review by Sidler (1994) for more recent discoveries and more detailed literature on this topic. [-6pt]'It would be difficult to find another series of colouring matters of greater beauty or with such remarkable and instructive chemical and physical peculiarities.' -H. Sorby, 1877.

A molecular understanding of complementary chromatic adaptation

Photosynthetic activity and the composition of the photosynthetic apparatus are strongly regulated by environmental conditions. Some visually dramatic changes in pigmentation of cyanobacterial cells that occur during changing nutrient and light conditions reflect marked alterations in components of the major light-harvesting complex in these organisms, the phycobilisome. As noted well over 100 years ago, the pigment composition of some cyanobacteria is very sensitive to ambient wavelengths of light; this sensitivity reflects molecular changes in polypeptide constituents of the phycobilisome. The levels of different pigmented polypeptides or phycobiliproteins that become associated with the phycobilisome are adjusted to optimize absorption of excitation energy present in the environment. This process, called complementary chromatic adaptation, is controlled by a bilin-binding photoreceptor related to phytochrome of vascular plants; however, many other regulatory elements also play a role in chromatic adaptation. My perspectives and biases on the history and significance of this process are presented in this essay.

Aspects of the relation between Cyanophora paradoxa (Korschikoff) and its endosymbiotic cyanelles Cyanocyta korschikoffiana (Hall & Claus) II. The photosynthetic pigments

All the photosynthetically active pigments in Cyanophora paradoxa are located within the endosymbiotic cyanelles Cyanocyta korschikoffiana . These pigments include β-carotene, chlorophyll a , zeaxanthin and the two bilichromoproteins allo - and c -phycocyanin. The dimer of allo -phycocyanin has a molecular mass of 28 × 10 3 , and is composed of two subunits of 12.6 × 10 3 each. The dimer of c -phycocyanin has a molecular mass of 31 x 10 3 , and is composed of two subunits of 13.2 x 10 3 and 14.5 × 10 3 respectively.

The phycobilisome, a light-harvesting complex responsive to environmental conditions

Photosynthetic organisms can acclimate to their environment by changing many cellular processes, including the biosynthesis of the photosynthetic apparatus. In this article we discuss the phycobilisome, the light-harvesting apparatus of cyanobacteria and red algae. Unlike most light-harvesting antenna complexes, the phycobilisome is not an integral membrane complex but is attached to the surface of the photosynthetic membranes. It is composed of both the pigmented phycobiliproteins and the nonpigmented linker polypeptides; the former are important for absorbing light energy, while the latter are important for stability and assembly of the complex. The composition of the phycobilisome is very sensitive to a number of different environmental factors. Some of the filamentous cyanobacteria can alter the composition of the phycobilisome in response to the prevalent wavelengths of light in the environment. This process, called complementary chromatic adaptation, allows these organisms to efficiently utilize available light energy to drive photosynthetic electron transport and CO2 fixation. Under conditions of macronutrient limitation, many cyanobacteria degrade their phycobilisomes in a rapid and orderly fashion. Since the phycobilisome is an abundant component of the cell, its degradation may provide a substantial amount of nitrogen to nitrogen-limited cells. Furthermore, degradation of the phycobilisome during nutrient-limited growth may prevent photodamage that would occur if the cells were to absorb light under conditions of metabolic arrest. The interplay of various environmental parameters in determining the number of phycobilisomes and their structural characteristics and the ways in which these parameters control phycobilisome biosynthesis are fertile areas for investigation.

Absence of glycosylation on cyanobacterial phycobilisome linker polypeptides and rhodophytan phycoerythrins

The 27-, 30-, and 33-kDa rod linker polypeptides and the 75-kDa core linker of phycobilisomes from the cyanobacterium Synechococcus sp. strain PCC 7942 have been reported to be glycoproteins with carbohydrate contents ranging from 3.2 to 18.8% and composed of N-acetylgalactosamine and glucose (H.C. Riethman, T.P. Mawhinney, and L.A. Sherman, J. Bacteriol. 170:2433-2440, 1988). Synechococcus sp. strain PCC 7942 phycobilisomes were purified extensively, and the linker polypeptides were separated from the phycobiliproteins by precipitation in 1 M NaSCN. Upon hydrolysis, the linker fraction yielded 0.037% glucose and 0.015% galactosamine by weight and no other carbohydrate. Phycobilisome polypeptides separated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate were subjected to various glycoprotein-specific staining procedures. Linker polypeptides showed very weak concanavalin A binding and no staining by the Schiff-periodate method or by a much more sensitive periodate oxidation-based method. These results indicated that the linker polypeptides are not glycosylated. An earlier report (T. Fujiwara, J. Biochem. 49:361-367, 1961) contended, on the basis of the isolation of sugar-containing peptic chromopeptides from Porphyra tenera R-phycoerythrin, that this red algal phycobiliprotein is a glycoprotein. Analysis of Gastroclonium coulteri R-phycoerythrin and Porphyridium cruentum B-phycoerythrin revealed only traces of carbohydrate in these two proteins, 0.36 and 0.14%, respectively. Results of glycoprotein staining of gels suggested that the carbohydrate in the R-phycoerythrin preparation is due to a glycoprotein contaminant and that neither red algal phycoerythrin is glycosylated.

Algal phycocyanins promote growth of human cells in culture

The growth-promoting substances in a non-dialyzable extract of Synechococcus elongatus var. on RPMI 8226 cells (a human myeloma cell line) were separated by gel filtration and ion exchange chromatography. By gel filtration with Sepharose 4B, the dialyzate was separated into two fractions. One fraction was green-colored (P-1) and the other was blue-colored (P-2). The P-2 fraction had a higher growth-promoting activity than P-1. By ion exchange chromatography, the P-2 fraction was separated into two blue-colored fractions of phycocyanin and allophycocyanin. Both biliproteins promoted the growth of RPMI 8226 cells; however, allophycocyanin was more active than phycocyanin.

The photoregulated expression of multiple phycocyanin species. A general mechanism for the control of phycocyanin synthesis in chromatically adapting cyanobacteria

The regulation of phycocyanin synthesis in response to growth in chromatic illumination was studied in 69 strains of cyanobacteria. Cyanobacteria (24 of 31 strains examined), which chromatically adapt by modulating the synthesis of both phycocyanin and phycoerythrin, controlled phycocyanin synthesis through the differential, photoregulated expression of two phycocyanin species (two alpha-type and two beta-type subunits). For these strains the expression of one pair of phycocyanin subunits was constitutive (i.e. irrespective of the light wavelength in which the cells were grown); the expression of the second pair of phycocyanin subunits occurred specifically during growth in red light. Two facultatively heterotrophic cyanobacteria, Calothrix strains 7101 and 7601, synthesized both the constitutive and the inducible pairs of phycocyanin subunits when grown heterotrophically in the dark after transfer from either red or green light. No evidence for the existence of multiple and/or photoregulated phycocyanin species was found for cyanobacteria (25 strains) incapable of chromatic adaptation, nor for cyanobacteria (13 strains) which chromatically adapt by modulating the synthesis of phycoerythrin alone.

Control of phycobiliprotein proteolysis and heterocyst differentiation in Anabaena

Phycobiliprotein degradation can be initiated in cultures of the cyanobacterium Anabaena by removal of combined nitrogen from the medium. Certain strains of Anabaena differentiate cells specialized for aerobic nitrogen fixation (heterocysts) under such conditions. We describe here a procedure for the preparation of extracts from heterocysts or vegetative cells that contain an activity capable of degrading only the phycobiliproteins in a mixture of soluble Anabaena proteins in vitro. This activity increased under nitrogen starvation conditions or in ammonia-replete cultures treated with the glutamine synthetase inhibitor methionine sulfoximine. The increase in activity induced by nitrogen starvation was prevented by chloramphenicol or by carbon starvation. Under all these conditions, phycobiliprotein degradative activity assayed in vitro was correlated with the loss of phycobiliprotein absorbance in vivo. Finally, starvation of a met auxotroph of Anabaena for methionine (in the presence of ammonia) did not induce phycobiliprotein degradation in vivo or the increase in proteinase activity. Together with direct measurements of ppGpp, these results indicate that proteolysis in Anabaena is not controlled by compounds associated with the stringent response in Escherichia coli. Since the increase in proteinase activity appears to be regulated by the same variables that control heterocyst differentiation, the activity should provide a useful biochemical marker for the early events of differentiation.

Properties of allophycocyanin II and its alpha- and beta-subunits from the thermophilic blue--green alga Mastigocladus laminosus

Purified allophycocyanin II and its subunits have been examined with respect to spectroscopic properties, sedimentation, reconstitution and isoelectric behaviour. In 0.02m-potassium phosphate buffer, pH8.0, and at a concentration of 0.25mg/ml, allophycocyanin II and its alpha- and beta-subunits show visible absorption maxima at 650, 615 and 615nm respectively, whereas the fluorescence emission maxima were determined to be at 662, 640 and 630nm respectively. The absorption difference spectrum (dilution difference) of allophycocyanin II displays maxima at 650 and 590nm with a minimum at 610nm. The c.d. spectrum of allophycocyanin II showed only one positive-ellipticity band at 635nm, and a major negative-ellipticity band at 340nm. Oxidation of allophycocyanin II, low- and high-pH solutions (pH3.0 and 11.0), various ethanol concentrations as well as dialysis against distilled water induce a spectral change leading to phycocyanin-like characteristics. In most cases these shifts are reversible. Allophycocyanin II is thermostable over a period of 60min at temperatures up to 60 degrees C. The isoelectric points of allophycocyanin II and its alpha- and beta-subunits are 4.65, 4.64 and 4.82 respectively. Estimated molecular weights from sedimentation-equilibrium analyses were 102500 for allophycocycanin II, 16000 for the alpha- and 31500 for the beta-subunit. Recombination of alpha- and beta-subunits leads to allophycocyanin II, which is indistinguishable from native allophycocyanin with respect to its spectral form, to its gel-filtration and to its electrophoretic behaviour.

Studies on plant bile pigments, VII. Preparation and characterization of phycobiliproteins with chromophores chemically modified by reduction

The reversible denaturation and reduction with dithionite has been studied for the phycobiliproteins, C-phycocyanin (1) and allophycocyanin (2) from Spirulina platensis, and C-phycoerythrin (4) from Fremyella diplosiphon (both cyanobacteria). By treatment with sodium dithionite, the chromophores are selectively reduced at the central (C-10) methine bridge, producing pigments with bilirubinoid (lambda max = 418 nm from 1 and 2), and vinylpyrroloc (lambda max= 300 nm from 4) chromophores. The extent of reduction is dependent on the state of the protein. The chromophores of denatured biliproteins are completely reduced at 0.5 mM dithionite. In the native pigments, dithionite concentrations up to 0.5 mM lead only to partial reduction, thus forming products containing both reduced and oxidized chromophores (e.g. "phycocyanorubins" from 1 and 2). The reduction is non-statistical with respect to the different chromophores present in 1 and 4, the chromophores absorbing at shorter wavelengths being preferentially reduced. Renaturation of the proteins containing reduced chromophores is accompanied by their reoxidation. This oxidation is complete in the absence of dithionite or at concentrations up to 0.5 mM. At higher dithionite concentrations, the reoxidation is incomplete, and the products are spectroscopically identical to those obtained by reduction of the native pigments at similar concentrations of reductant. The results are interpreted by a model in which the protein is "transparent" to the reducing agent, dithionite. The difference in the extent of reduction of the native and denatured pigments can only be due to thermodynamic (viz. stability) differences in the susceptibility of the chromophores to reduction. Specifically, the (extended) chromophore present in the native pigment is much more difficult to reduce than the chromophore (present in a cyclic conformation) in the denatured pigment. The energetics of the process of refolding both the protein and the chromophores are discussed.

Effect of levulinic acid on pigment biosynthesis in Agmenellum quadruplicatum

When levulinic acid was added to a growing culture of the cyanobacterium (blue-green alga) Agmenellum quadruplicatum PR-6, delta-aminoelevulinic acid accumulated in the medium and chlorophyll a synthesis and cell growth were inhibited, but there was a small amount of c-phycocyanin synthesis. The amount of delta-aminolevulinic acid produced in the treated culture did not fully account for the amount of pigment synthesized in the untreated control. Levulinic acid and either sodium nitrate or ammonium chloride were added to nitrogen-starved cultures of PR-6, and delta-aminolevulinic acid production and chlorophyll a and c-phycocyanin content were monitored. When ammonium chloride was added as a nitrogen source after nitrogen starvation, the cells recovered more rapidly than when sodium nitrate was added as a nitrogen source. In cultures recovering from nitrogen starvation, synthesis of c-phycocyanin occurred before synthesis of chlorophyll a.

Properties and N-terminal sequence of allophycocyanin from the unicellular rhodophyte Cyanidium caldarium

Allophycocyanin from the unicellular rhodophyte Cyanidium caldarium was purified by (NH4)2SO4 fractionation and ion-exchange chromatography on brushite (calcium phosphate) columns and on DEAE-Sephadex A-25 columns. The specific absorption coefficient (A0.1%1cm) at 650nm of purified allophycocyanin was 6.35 in 0.05M-potassium phosphate buffer, pH7.0. Absorption maxima of allophycocyanin occurred at 650, 618 (shoulder), 350 and 275 nm. Circular-dichroic spectra displayed positive-ellipticity bands at 658 and 630 nm and a major negative-ellipticity band at 340nm. Computer analysis of the circular-dichroic spectrum of allophycocyanin from 207 to 243 nm indicated 42% alpha-helix and 58% beta-form. The estimated molecular weight of purified allophycocyanin on calibrated Sephadex G-200 columns at pH7.0. was 196000. Electrophoretic examination of allophycocyanin on sodium dodecyl sulphate/polyacrylamide gels revealed a single band with apparent mol.wt. 16000. The presence of two polypeptide subunits, with nearly the same molecular weight, was revealed on polyacrylamide gels by using a modified electrophoresis buffer. Spectral analysis of the allophycocyanin subunits resolved by ion-exchange chromatography on Bio-Rex 70 columns indicated that a single phycocyanobilin chromophore was present on each polypeptide chain. Treatment of allophycocyanin with 8M-urea (pH3.0) and subsequent removal of urea by dialysis against water yielded a derivative phycobiliprotein with spectroscopic characteristics similar to those of phycocyanin. The original allophycocyanin spectrum was regenerated after incubation in phosphate buffer, pH7.0. Automated sequences analysis of the N-terminus of allophycocyanin showed that (a) the sequences of the two subunits were different from one another and were different from the subunits of phycocyanin from the same alga, (b) the subunits occurred in a molar ratio of 1:1 and (c) the sequences homology at the N-terminus among alpha- and beta-subunits of allophycocyanin from blue-green and red algae approached 90%.

Isolation and Function of Allophycocyanin B of Porphyridium cruentum

Allophycocyanin B was purified to homogeneity from the eukaryotic red alga Porphyridium cruentum. This biliprotein is distinct from the allophycocyanin of P. cruentum with respect to subunit molecular weights, and spectroscopic and immunological properties. The purified allophycocyanin B has a long wavelength absorption maximum at 669 nm at room temperature and at 675 nm at -196 C while the fluorescence emission maximum is at 673 nm at room temperature and 679 nm at -196 C. The emission spectrum of allophycocyanin shifted only 1 nm, from 659 to 660 nm, on cooling to -196 C, and was the same with allophycocyanin crystals as it was with pure solutions of the pigment. Phycobilisomes from P. cruentum have a major fluorescence emission band at 680 nm at -196 C which emanates from the small amount of allophycocyanin B present in the phycobilisomes. Light energy absorbed by the bulk of the biliprotein pigments is transferred to allophycocyanin B with high efficiency.

Physico-chemical and immunological properties of allophycocyanins

Allophycocyanins were purified from diverse cyanobacteria and one rhodophytan alga (Cyanidium caldarium). The native proteins are trimeric molecules with the structure (alpha beta)3. Representative native allophycocyanins and their alpha and beta subunits were characterized with respect to molecular weight, amino acid composition, isoelectric point, absorption and fluorescence spectra and immunological properties. All of the allophycocyanins studied were strikingly similar with respect to each of these properties. Renatured alpha and beta subunits of allophycocyanin were distinct immunologically from each other, and both cross-reacted with the antiserum to the native protein. Trimeric allophycocyanin was readily reconstituted from the purified alpha and beta subunits. Formation of hybrid allophycocyanins was demonstrated by direct isolation and characterization of the hybrid proteins and by immunological techniques. The results support the view that allophycocyanins are a highly conserved group of proteins.