The native forms of the phycobilin chromophores of algal biliproteins. A clarification

Enhancement of phycocyanobilin biosynthesis in Escherichia coli by strengthening the supply of precursor and artificially self-assembly complex

Phycocyanobilin (PCB) is widely used in healthcare, food processing, and cosmetics. Escherichia coli is the common engineered bacterium used to produce PCB. However, it still suffers from low production level, precursor deficiency, and low catalytic efficiency. In this study, a highly efficient PCB-producing strain was created. First, chassis strains and enzyme sources were screened, and copy numbers were optimized, affording a PCB titer of 9.1 mg/L. Most importantly, the rate-limiting steps of the PCB biosynthetic pathway were determined, and the supply of precursors necessary for PCB synthesis was increased from endogenous sources, affording a titer of 21.4 mg/L. Then, the key enzymes for PCB synthesis, HO1 and PcyA, were assembled into a multi-enzyme complex using the short peptide tag RIAD-RIDD, and 23.5 mg/L of PCB was obtained. Finally, the basic conditions for PCB fermentation were initially determined in 250 mL shake flasks and a 5-L bioreactor to obtain higher titers of PCB. The final titer of PCB reached 147.0 mg/L, which is the highest reported titer of PCB so far. This research provided the foundation for the industrial production of PCB and its derivatives.

Biosynthesis of 2-methylisoborneol is regulated by chromatic acclimation of Pseudanabaena

Cyanobacteria can sense different light color by adjusting the components of photosynthetic pigments including chlorophyll a (Chl a), phycoerythrin (PE), and phycocyanin (PC), etc. Filamentous cyanobacteria are the main producer of 2-methylisoborneol (MIB) and many can increase their PE levels so that they are more competitive in subsurface layer where green light is more abundant, and have caused extensive odor problems in drinking water reservoirs. Here, we identified the potential correlation between MIB biosynthesis and ambient light color induced chromatic acclimation (CA) of a MIB-producing Pseudanabaena strain. The results suggest Pseudanabaena regulates the pigment proportion through Type III CA (CA3), by increasing PE abundance and decreasing PC in green light. The biosynthesis of MIB and Chl a share the common precursor, and are positively correlated with statistical significance regardless of light color (R²=0.68; p<0.001). Besides, the PE abundance is also positively correlated with Chl a in green light (R²=0.57; p=0.019) since PE is the antenna that can only transfer the energy to PC and Chl a. In addition, significantly higher MIB production was observed in green light since more Chl a was synthesized.

The 42.1 and 53.7 kDa bands in SDS-PAGE of R-phycoerythrin from Polysiphonia urceolata

In SDS-PAGE gels of three purified R-phycoerythrins (R-PEs) isolated from three species of red algae, two bands whose molecular weights were about 40 kDa and 50 kDa can stably be found when the sample loading amount was enough. It is important for structure study of R-PE to clarify what these bands represent and how they are formed. According to results of the second SDS-PAGE, as well as molecular weights, fluorescences under UV and abundance, the 42.1 kDa and 53.7 kDa bands in SDS-PAGE gels of R-PE from Polysiphonia urceolata were believed to be complexes of αβ and βγ1, respectively. Formation of these bands may be related to light and phycourobilins (PUB) in subunits; and appearance of these two bands provided some proofs on position of chromophores and directions of energy transfer in R-PE. R-PE containing γ1 subunit was obviously more stable than R-PE containing γ2 subunit when they were exposed to protein denaturants, so γ subunits of R-PE may play important roles in structural stability of R-PE aggregates and the main forces that maintain the stability of R-PE may be interactions between γ subunit and β subunits.

Identification of singlet oxygen photosensitizes in lambs drinking water in an alveld risk area in West Norway

Alveld is a hepatogenous photosensitivity disorder in lambs. Although alveld has been known in Norway for more than 100years, there are still questions related to the cause of the disease. Phytoporphyrin has long been incriminated as the photosensitizer in hepatogenous photosensitivity diseases but previous findings suggest that the photosensitizing mechanism in alveld is more complex, possibly involving other co-factors. The current work investigates the presence of non-hepatogenous photosensitizers originating in lamb's drinking water from various sources. In addition samples of two of the predominent cyanobacteria found in a representative biofilm (i.e. aggregates of microbes) were identified and isolated in axenic (i.e. pure) cultures. Information from the absorption-, fluorescence emission-, and -excitation spectra and the action spectrum for the formation of singlet oxygen was combined in order to identify the chromophores responsible for the formation of singlet oxygen, e.g. phycocyanins from the cyanobacteria. The highest level of singlet oxygen formation was detected in lotic (i.e. flowing) water in the period consistent with the outbreak of the alveld disease in the area. Meteorological data indicate a warm and wet May with a high radiation exposure leading up to a colder and wet June with an even higher solar irradiance. The seasonal variation in the amount of photosensitizers in lamb's drinking water combined meteorological data can be important to predict the outbreak of alveld.

Structure and apoprotein linkages of phycourobilin

R-Phycoerythrin contains two covalently bound bilin prosthetic groups, phycoerythrobilin and phycourobilin. The two chromophore types were separated as their peptide-bound derivatives by subjecting tryptic digests of R-phycoerythrin to adsorption chromatography on Sephadex G-25. The structure and apoprotein linkages of the bound phycoerythrobilin were found to be identical with those previously reported for this phycobilin [Killilea, O'Carra & Murphy (1980) Biochem. J. 187, 311-320]. Phycourobilin is a tetrapyrrole, containing no oxo bridges and has the same order of side chains as IX alpha bilins. The chromophore is linked to the peptide through two and possibly three of its pyrrole rings. One linkage possibly consists of an ester bond between the hydroxy group of a serine residue and the propionic acid side chain of one of the inner rings. The second linkage is a labile thioether bond between a cysteine residue and the C2 side chain of pyrrole ring A. The third linkage is a stable thioether bond between a cysteine residue and the alpha-carbon atom of the C2 side chain of pyrrole ring D. Ring D is unsaturated and is attached to ring C through a saturated carbon bridge. Rings B and C have a conjugated system of five bonds, as found in other urobilinoid pigments. Ring A is attached to ring B via a saturated carbon bridge. Both of the alpha-positions of ring A are in the reduced state, but the ring does contain an unsaturated centre (probably a double bond between the beta-carbon and the ring nitrogen atom). The presence of this double bond and its isomerization into the bridge position between rings A and B would explain the extension of the conjugated system of phycourobilin to that of a phycoerythrobilinoid/rhodenoid pigment in acid or alkali.

Enzymic Transformation of Biliverdin to Phycocyanobilin by Extracts of the Unicellular Red Alga Cyanidium caldarium

Cell-free extracts of the unicellular red alga Cyanidium caldarium catalyze the transformation of biliverdin to a product indistinguishable from phycocyanobilin, the free bilin derived from phycocyanin by methanolysis. Crude cell-free extract requires biliverdin as the only substrate, but after removal of low molecular weight components by gel filtration, the reaction shows an additional requirement for a reduced pyridine nucleotide. Boiled extract is enzymically inactive, activity is not sedimented by high-speed centrifugation, and mesobiliverdin cannot serve as a substrate.Incubation of cell extracts with biliverdin yields two products with very similar spectrophotometric properties in acidic methanol, but which are separable by reverse-phase high pressure liquid chromatography. The same two products are formed by methanolysis of protein-bound phycocyanin chromophore, with the late-eluting one predominating. The two products derived from either phycocyanin methanolysis or cell extract incubation with biliverdin are partially interconvertible and they form the same ethylidine-free isomeric derivative, mesobiliverdin. Their absorption spectra correspond to those of the Z- and E-ethylidine isomers of phycocyanobilin. Based on previous work showing that the major methanolysis product has the E-ethylidine configuration, the other product of methanolysis and enzymic biliverdin transformation is therefore the Z-ethylidine isomer. The time course for formation of the two products during incubation suggests that the early-eluting product is the precursor of the late-eluting one. These results suggest that Z-ethylidine phycocyanobilin is the precursor of the E-ethylidine isomer, and that the latter may be a normal cellular precursor to protein-bound phycocyanin chromophore.

Effect of acetic acid on phycocyanin-phycoerythrocyanin mixture extracted from Anabaena variabilis

Exposure of a phycocyanin-phycoerythrocyanin mixture extracted from Anabaena variabilis to sodium acetate, pH 3.8, ionic strength of 0.1, results in dissociation of the phycoerythrocyanin's beta subunit from its alpha subunit. The alpha subunit obtained by this method has a strong absorption transition at 508 nm. This transition is a consequence of the subunit's specific conformation, rather than of a new chromophore. The behavior of the phycocyanin-phycoerythrocyanin mixture in acetate buffers of variable compositions suggests that interactions which involve carboxylic amino acid residues play an important role, along with hydrophobic associations, in the association of phycoerythrocyanin subunits into monomers (alpha beta) and between this protein and phycocyanin. This work also indicates that the linkage between alpha and beta subunits of phycoerythrocyanin is labile and may be weaker than the association of these subunits with phycocyanin under acidic conditions.

Bile pigment synthesis in plants. Incorporation of haem into phycocyanobilin and phycobiliproteins in Cyanidium caldarium

A procedure was developed whereby haem was taken up by dark-grown cells of the unicellular rhodophyte Cyanidium caldarium. These cells were subsequently incubated either in the dark with 5-aminolaevulinate, which results in excretion of phycocyanobilin into the suspending medium or incubated in the light, which results in synthesis and accumulation of phycocyanin and chlorophyll a within the cells. Phycocyanobilin was isolated from phycocyanin by cleavage from apoprotein in methanol. Phycocyanobilin prepared from phycocyanin or excreted from cells given 5-aminolaevulinate was methylated and purified by t.l.c. By using 14C labelling either in the haem or in 5-aminolaevulinate administered, haem incorporation into phycocyanobilin was demonstrated in both dark and light systems. Since chlorophyll a synthesized in the light in the presence of labelled haem contained no radioactivity, it was clear that haem was directly incorporated into phycocyanobilin and not first converted into protoporphyrin IX. These results clearly demonstrate phycocyanobilin synthesis via haem and not via magnesium protoporphyrin IX as has also been postulated.

Structures and apoprotein linkages of phycoerythrobilin and phycocyanobilin

Phycoerythrobilin and phycocyanobilin are covalently attached to the apoproteins of phycoerythrins and phycocyanins. One linkage consists of an ester bond between the hydroxy group of a serine residue and the propionate side chain on one of the inner pyrrole rings (probably ring C). The other linkage is a labile thioether bond between a cysteine residue and the two-carbon side chain on pyrrole ring A. This side chain and both of the alpha-positions of the ring A are in the reduced state. This constitutes an important structural revision, since, in the structures currently accepted for the phycobilins, the two-carbon side chain on ring A is depicted as an ethylidene grouping and this has been regarded not only as a very characteristic feature of the phycobilins, but also as a probable structural feature of the chromophore of phytochrome, largely on the basis of other analogies with the phycobilins. The ethylidene-containing structures apply instead to artefact forms of the pigments released from the apoproteins by treatment with hot methanol. Cleavage of the ring-A linkage involves an elimination reaction releasing the cysteine residue and generating a double bond in the ring-A side chain. During cleavage in methanol the direction of the elimination is towards the ring, generating the ethylidene double bond. Since this is linked to the conjugated system, the methanol-released pigments differ spectrally from the native phycobilins. During acid-catalysed release of the pigments, the elimination apparently goes in the opposite direction, generating a double bond at the outer position of the side chain. Since this double bond is not linked to the conjugated system, the acid-released pigments remain spectrally identical with their protein-bound counterparts.