Magnesium Protoporphyrin Monoester and Protoporphyrin Monomethyl Ester in Chlorophyll Biosynthesis

Erythropoietic protoporphyrias: Pathogenesis, diagnosis and management

The erythropoietic protoporphyrias consist of three ultra-rare genetic disorders of the erythroid heme biosynthesis, including erythropoietic protoporphyria (EPP1), X-linked protoporphyria (XLEPP) and CLPX-protoporphyria (EPP2), which all lead to the accumulation of protoporphyrin IX (PPIX) in erythrocytes. Affected patients usually present from early childhood with episodes of severe phototoxic pain in the skin exposed to visible light. The quantification of PPIX in erythrocytes with a metal-free PPIX ≥3 times the upper limit of normal confirms the diagnosis. Protoporphyria-related complications include liver failure, gallstones, mild anaemia and vitamin D deficiency with reduced bone mineral density. The management is focused on preventing phototoxic reactions and treating the complications. Vitamin D should be supplemented, and DEXA scans in adults should be considered. In EPP1, even in cases of biochemically determined iron deficiency, supplementation of iron may stimulate PPIX production, resulting in an increase in photosensitivity and the risk of cholestatic liver disease. However, for patients with XLEPP, iron supplementation can reduce PPIX levels, phototoxicity and liver damage. Because of its rarity, there is little data on the management of EPP-related liver disease. As a first measure, any hepatotoxins should be eliminated. Depending on the severity of the liver disease, phlebotomies, exchange transfusions and ultimately liver transplantation with subsequent haematopoietic stem cell transplantation (HSCT) are therapeutic options, whereby multidisciplinary management including porphyria experts is mandatory. Afamelanotide, an alpha-melanocyte-stimulating hormone analogue, is currently the only approved specific treatment that increases pain-free sunlight exposure and quality of life.

Bioactive substances from marine fishes, shrimps, and algae and their functions: present and future

Marine fishes, shrimps, and algae have many important bioactive substances, such as peptides, unsaturated fatty acids, polysaccharides, trace elements, and natural pigments. The introduction of these substances contributes to a significant improvement in developing them in final processed products. In fact, the knowledge of these bioactive substances has experienced a rapid increase in the past 20 years and prompted the relevant technological revolution with a decisive contribution to the final application. The purpose of this review was to introduce critically and comprehensively the present knowledge of these bioactive substances and pointed out their future developmental situation.

Porphyrins promote the association of GENOMES UNCOUPLED 4 and a Mg-chelatase subunit with chloroplast membranes

In plants, chlorophylls and other tetrapyrroles are synthesized from a branched pathway that is located within chloroplasts. GUN4 (GENOMES UNCOUPLED 4) stimulates chlorophyll biosynthesis by activating Mg-chelatase, the enzyme that commits porphyrins to the chlorophyll branch. GUN4 stimulates Mg-chelatase by a mechanism that involves binding the ChlH subunit of Mg-chelatase, as well as a substrate (protoporphyrin IX) and product (Mg-protoporphyrin IX) of Mg-chelatase. We chose to test whether GUN4 might also affect interactions between Mg-chelatase and chloroplast membranes, the site of chlorophyll biosynthesis. To test this idea, we induced chlorophyll precursor levels in purified pea chloroplasts by feeding these chloroplasts with 5-aminolevulinic acid, determined the relative levels of GUN4 and Mg-chelatase subunits in soluble and membrane-containing fractions derived from these chloroplasts, and quantitated Mg-chelatase activity in membranes isolated from these chloroplasts. We also monitored GUN4 levels in the soluble and membrane-containing fractions derived from chloroplasts fed with various porphyrins. Our results indicate that 5-aminolevulinic acid feeding stimulates Mg-chelatase activity in chloroplast membranes and that the porphyrin-bound forms of GUN4 and possibly ChlH associate most stably with chloroplast membranes. These findings are consistent with GUN4 stimulating chlorophyll biosynthesis not only by activating Mg-chelatase but also by promoting interactions between ChlH and chloroplast membranes.

A new method for isolating physiologically active Mg-protoporphyrin monomethyl ester, the substrate of the cyclase enzyme of the chlorophyll biosynthetic pathway

Mg-protoporphyrin monomethyl ester (MPE) is a biosynthetic intermediate of chlorophyll and converted by MPE cyclase to protochlorophyllide. Limited availability of MPE has so far hampered cyclase research. In a new, simplified, method MPE was prepared from freeze dried bchE mutant Rhodobacter capsulatus DB575 cells by extraction with acetone/H(2)O/25% NH(3). Isolated MPE was identified by absorption and fluorescence spectroscopy, and its purity was analyzed by HPLC. The extracted MPE was dried and redissolved in buffered DMSO and its substrate activity is shown by enzymatic cyclase assays. A linear time course was observed for MPE conversion to protochlorophyllide by enzymes from barley etioplasts. Our innovation of freeze drying the R. capsulatus cells before extraction provides a high yield method for MPE, which is significantly faster and more reproducible than previous extraction methods.

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.

Spectroscopic study of protoporphyrin IX zinc(II) encapsulated in sol-gel glass

Previous studies indicated that the organization of native porphyrins and their intentionally designated derivatives in solid substrates is of current interest because of the biological and practical importance of these compounds. In this paper, we report herein for the first time the incorporation of a functionalized diacid, protoporphyrin IX Zn(II) (Zn-pp-IX), successfully in a silica based transparent organic-inorganic hybrid material by choosing proper alkoxy silane containing amino-group via the sol-gel method. The entrapped guest was diagnosed using UV-vis sectrophotometry, emission spectroscopy, and infrared spectrometry; the properties of the encapsulated porphyrin were compared to those of the compound in solution. The results indicate that Zn-pp-IX is well distributed and homogeneously in the glass.

Anaerobic chlorophyll isocyclic ring formation in Rhodobacter capsulatus requires a cobalamin cofactor

The isocyclic ring of bacteriochlorophyll (BChl) is formed by the conversion of Mg-protoporphyrin monomethyl ester (MPE) to protochlorophyllide (PChlide). Similarities revealed by blast searches with the putative anaerobic MPE-cyclase BchE suggested to us that this protein also uses a cobalamin cofactor. We found that vitamin B(12) (B(12))-requiring mutants of the bluE and bluB genes of Rhodobacter capsulatus, grown without B(12), accumulated Mg-porphyrins. Laser desorption/ionization time-of-flight (LDI-TOF) MS and NMR spectroscopy identified them as MPE and its 3-vinyl-8-ethyl (mvMPE) derivative. An in vivo assay was devised for the cyclase converting MPE to PChlide. Cyclase activity in the B(12)-dependent mutants required B(12) but not protein synthesis. The following reaction mechanism is proposed for this MPE-cyclase reaction. Adenosylcobalamin forms the adenosyl radical, which leads to withdrawal of a hydrogen atom and formation of the benzylic-type 13(1)-radical of MPE. Withdrawal of an electron gives the 13(1)-cation of MPE. Hydroxyl ion attack on the cation gives 13(1)-hydroxy-MPE. Withdrawal of three hydrogen atoms leads successively to 13(1)-keto-MPE, its 13(2)-radical, and cyclization to PChlide.

Genetic analysis of chlorophyll biosynthesis

During this decade, there have been major advancements in the understanding of genetic loci involved in synthesis of the family of Mg-tetrapyrroles known as chlorophylls and bacteriochlorophylls. Molecular genetic analysis of Mg-tetrapyrrole biosynthesis was initiated by the performance of detailed sequence and mutational analysis of the photosynthesis gene cluster from Rhodobacter capsulatus. These studies provided the first detailed understanding of genes involved in bacteriochlorophyll a biosynthesis. In the short time since these studies were initiated, most of the chlorophyll biosynthesis genes have been identified by virtue of their ability to complement bacteriochlorophyll a biosynthesis mutants as well as by sequence homology comparisons. This review is centered on a discussion of our current understanding of bacterial, algal, and plant genes that code for enzymes in the Mg-branch of the tetrapyrrole biosynthetic pathway that are responsible for synthesis of chlorophylls and bacteriochlorophylls.

Genetic analyses of photopigment biosynthesis in eubacteria: a guiding light for algae and plants

Images

Biosynthesis of a chlorophyllide b-like pigment in phenanthroline-treated Chlamydomonas reinhardtii y-1

Incubation of degreened Chlamydomonas reinhardtii y-1 cells in the dark with m-phenanthroline induced de novo synthesis of a chlorophyllide b-like pigment. The rate of synthesis of this pigment in the dark was greater than that of total chlorophyll in illuminated cells. Most of the newly synthesized pigment was excreted into the culture medium. The product was extracted from the medium as the metal-free pheophorbide, which had a fluorescence excitation maximum at 428 +/- 1 nm and an emission maximum at 657 +/- 1 nm (E428F657) in ethyl acetate (E427F657 in diethyl ether). Three pheophorbide species were extracted from the medium of green cells treated in the dark, a minor component with a spectrum (E410F670) identical to demetallated chlorophyll a, and two major species with spectral values of E428F657 and E433F657. The latter, predominant form had a spectrum identical to demetallated chlorophyll b, which was purified from the algal cells. E428F657 and E433F657 reacted with hydroxylamine and Girard's T-reagent, which caused a shift in the fluorescence emission maximum to 668 nm. Pheophytin b, which contains an aldehyde group, exhibited an identical spectral shift when treated in the same way, but pheophytin a or porphyrin biosynthetic intermediates did not. Proton NMR analysis of the E428F657 chlorin produced by yellow cells treated with m-phenanthroline confirmed the presence of an aldehydic proton. Chelating and nonchelating phenanthroline analogs equally stimulated synthesis of this product.

13C nuclear magnetic resonance studies on bacteriochlorophyll a biosynthesis in Rhodopseudomonas spheroides S

The 13C NMR spectra were analyzed in bacteriochlorophyll a and magnesium protoporphyrin methyl ester formed in Rhodopseudomonas spheroides S. in the presence of L-[1-13C]glutamate and [2-13C]glycine. After reassignment of three alpha-pyrrolic carbons (C-9, -14 and -16) of bacteriochlorophyll a, the spectra showed that C-2 of glycine was preferentially incorporated into the eight-carbon atoms in these tetrapyrrole macrocycles derived from C-5 of 5-aminolevulinic acid (ALA). C-2 of glycine was also incorporated specifically into methyl ester carbon of magnesium protoporphyrin IX methyl ester and methoxyl carbon of methoxycarbonyl group attached to isocyclic ring of bacteriochlorophyll a. No enrichment of these nine-carbon atoms was observed in the spectrum of bacteriochlorophyll formed in the presence of L-[1-13C]glutamate, showing exclusive operation of ALA synthase on bacteriochlorophyll biosynthesis.

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.

Biosynthesis of delta-aminolevulinic acid by blue-green algae (cyanobacteria)

When levulinic acid, a competitive inhibitor of delta-aminolevulinic acid dehydratase, was added to growing cultures of blue-green algae (cyanobacteria), delta-aminolevulinic acid was excreted into the medium and cell growth was inhibited.

Mechanism and stereochemistry of vinyl-group formation in haem biosynthesis

5-Aminolaevulinate containing tritium at C-3 and C-5 was converted into haem using a preparation of anaemic chicken blood. The biosynthetic haem was degraded to ethylmethyl maleimide and haematinic acid which had relative tritium radioactivity of 0.58 and 1.0 respectively. These results indicated that in the formation of the vinyl group of haem only one of the hydrogen atoms from the beta-positions of two propionate side chains of coproporphyrinogne III was removed. Haem was also biosynthesised from [(3R)-3H1]2-oxoglutarate. The determination of relative radioactivity in ethylmethyl maleimide and haematinic acid endorsed the above conclusion and further indicated that the pro-R hydrogen atoms located at the beta-positions of the propionate side chains are retained in haem biosynthesis. In order to explore the status of hydrogen atoms located at the alpha-positions of propionate side chains haem was biosynthesised using [2RS)-3H2]succinate, [(2R)-3H1]succinate and [(2S)-3H1]succinate. Degradation of the three samples of haem into ethylmethyl maleimide and haematinic acid showed that both the vinyl groups of haem are formed through the loss of pro-S hydrogen atoms located at the beta-positions of the propionic acid side chains of coproporphyrinogen III. The results further showed that the hydrogen atoms located at the alpha-positions of the side chains are not involved in the biosynthesis of haem. Various mechanisms for the formation of vinyl groups in the biosynthesis are discussed.

Effect of vanadium on growth, chlorophyll formation and iron metabolism in unicellular green algae

In the presence of vanadium, growth of Scenedesmus obliquus and Chlorella pyrenoidosa was increased five to sixfold as determined by dry weight, when cultured under autotrophic conditions for 7 days. The stimulation by vanadium decreased with increasing stability towards hydrolysis of the iron(III)-compounds added. Pentavalent vanadium (20 mug V/1 as NH4VO3) was able to overcome completely a limited iron-deficiency in the algae following growth in presence of 1.8 - 10(-5) m ferric chloride. Vanadium did not alter the iron uptake into the algal cells. 90% of offered 48V was taken up by Scenedesmus obliquus during 5 days of growth, and 21% thereof were found in the chloroplast fraction. In presence of vanadium, the chlorophyll formation was stimulated in Scenedesmus obliquus. This stimulation by vanadium was found to be light-dependent but occurred to a certain extent in the dark also. The main porphyrin of the yellow mutant 211-11h/20 of Chlorella vulgaris was identified as protoporphyrin-IX. The formation of this compound was stimulated by vanadium within 10 days up to 83%. The role of vanadium in the biosynthesis of chlorophylls is discussed.