Protoheme extraction from plant tissue

Role of Musa paradisiaca ascorbate peroxidase in the transformation of methyl phenyl sulfide to its sulfoxide

An ascorbate peroxidase from a new source Musa paradisiaca leaf juice has been purified to homogeneity using a simple procedure involving concentration by ultra filtration and anion exchange chromatography on diethyl amino ethyl [DEAE] cellulose column. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis [SDS-PAGE] analysis of the purified enzyme has shown a single protein band of molecular mass 208.9 kDa which has been confirmed by native-PAGE and intact mass analysis by mass spectrometry. The Km and k_cat values of the enzyme using ascorbate and H₂O₂ as the variable substrates were 0.13 m mol L^-1, 40.42 s^-1 and 0.23 m mol L^-1, 27.24 s^-1, respectively. The pH and temperature optima of the enzyme were 7.0 and 298 K, respectively. The enzyme transformed approximately 97% methyl phenyl sulfide to its sulfoxide. The product was racemic mixture.

Determination of heme in microorganisms using HPLC-MS/MS and cobalt(III) protoporphyrin IX inhibition of heme acquisition in Escherichia coli

One of the main threats to the achievements in modern medicine is antimicrobial resistance. Molecular targeting of bacterial acquisition mechanisms of heme has been suggested to be an alternative to antibiotics. In the present study, HPLC-MS/MS combined with a simple clean-up based on liquid-liquid extraction has been developed and evaluated for simultaneous determination of heme and porphyrin heme precursors in microorganisms. Experimental design was used to optimize the extraction parameters, to obtain a method with high recovery, low matrix effects, and high precision. The effects of additives in the culture medium on the biosynthesis of heme were studied using Escherichia coli as a model microorganism. 5-Aminolaevulinic acid and hemin increased the heme concentration in E. coli by a factor of 1.5 and 4.5, respectively. Addition of 5-aminolaevulinic acid bypassed the E. coli negative feedback control of heme biosynthesis, which led to high amounts of intracellular porphyrins. The high heme concentration obtained when hemin was used as a culture additive shows that E. coli has an uptake of heme from its surroundings. In contrast, addition of cobalt protoporphyrin IX to the growth medium reduced the amount of heme in E. coli, demonstrating this compound's ability to mimic real heme and inhibit the heme acquisition mechanisms.

Circadian clock adjustment to plant iron status depends on chloroplast and phytochrome function

Plant chloroplasts are not only the main cellular location for storage of elemental iron (Fe), but also the main site for Fe, which is incorporated into chlorophyll, haem and the photosynthetic machinery. How plants measure internal Fe levels is unknown. We describe here a new Fe-dependent response, a change in the period of the circadian clock. In Arabidopsis, the period lengthens when Fe becomes limiting, and gradually shortens as external Fe levels increase. Etiolated seedlings or light-grown plants treated with plastid translation inhibitors do not respond to changes in Fe supply, pointing to developed chloroplasts as central hubs for circadian Fe sensing. Phytochrome-deficient mutants maintain a short period even under Fe deficiency, stressing the role of early light signalling in coupling the clock to Fe responses. Further mutant and pharmacological analyses suggest that known players in plastid-to-nucleus signalling do not directly participate in Fe sensing. We propose that the sensor governing circadian Fe responses defines a new retrograde pathway that involves a plastid-encoded protein that depends on phytochromes and the functional state of chloroplasts.

The circadian clock of Arabidopsis is found to be hardwired to cellular iron levels, with chloroplasts playing a central role in iron sensing.

A BchD (magnesium chelatase) mutant of rhodobacter sphaeroides synthesizes zinc bacteriochlorophyll through novel zinc-containing intermediates

Heme and bacteriochlorophyll a (BChl) biosyntheses share the same pathway to protoporphyrin IX, which then branches as follows. Fe(2+) chelation into the macrocycle by ferrochelatase results in heme formation, and Mg(2+) addition by Mg-chelatase commits the porphyrin to BChl synthesis. It was recently discovered that a bchD (Mg-chelatase) mutant of Rhodobacter sphaeroides produces an alternative BChl in which Mg(2+) is substituted by Zn(2+). Zn-BChl has been found in only one other organism before, the acidophilic Acidiphilium rubrum. Our objectives in this work on the bchD mutant were to 1) elucidate the Zn-BChl biosynthetic pathway in this organism and 2) understand causes for the low amounts of Zn-BChl produced. The bchD mutant was found to contain a Zn-protoporphyrin IX pool, analogous to the Mg-protoporphyrin IX pool found in the wild type strain. Inhibition of ferrochelatase with N-methylprotoporphyrin IX caused Zn-protoporphyrin IX and Zn-BChl levels to decline by 80-90% in the bchD mutant, whereas in the wild type strain, Mg-protoporphyrin IX and Mg-BChl levels increased by 170-240%. Two early metabolites of the Zn-BChl pathway were isolated from the bchD mutant and identified as Zn-protoporphyrin IX monomethyl ester and divinyl-Zn-protochlorophyllide. Our data support a model in which ferrochelatase synthesizes Zn-protoporphyrin IX, and this metabolite is acted on by enzymes of the BChl pathway to produce Zn-BChl. Finally, the low amounts of Zn-BChl in the bchD mutant may be due, at least in part, to a bottleneck upstream of the step where divinyl-Zn-protochlorophyllide is converted to monovinyl-Zn-protochlorophyllide.

Methods for analysis of photosynthetic pigments and steady-state levels of intermediates of tetrapyrrole biosynthesis

Tetrapyrroles and carotenoids are required for many indispensable functions in photosynthesis. Tetrapyrroles are essential metabolites for photosynthesis, redox reaction, and detoxification of reactive oxygen species and xenobiotics, while carotenoids function as accessory pigments, in photoprotection and in attraction to animals. Their branched metabolic pathways of synthesis and degradation are tightly controlled to provide adequate amounts of each metabolite (carotenoids/tetrapyrroles) and to prevent accumulation of photoreactive intermediates (tetrapyrroles). Many Arabidopsis mutants and transgenic plants have been reported to show variations in steady-state levels of tetrapyrrole intermediates and contents of different carotenoid species. It is a challenging task to determine the minute amounts of these metabolites to assess the metabolic flow and the activities of both pigment-synthesising and degrading pathways, to unravel limiting enzymatic steps of these biosynthetic pathways, and to characterise mutants with accumulating intermediates. In this chapter, we present a series of methods to qualify and quantify anabolic and catabolic intermediates of Arabidopsis tetrapyrrole metabolism, and describe a common method for quantification of different plant carotenoid species. Additionally, we introduce two methods for quantification of non-covalently bound haem. The approach of analysing steady-state levels of tetrapyrrole intermediates in plants, when applied in combination with analyses of transcripts, proteins, and enzyme activities, enables the biochemical and genetic elucidation of the tetrapyrrole pathway in wild-type plants, varieties, and mutants. Steady-state levels of tetrapyrrole intermediates are only up to 1/1,000 of the amounts of the accumulating end-products, chlorophyll, and haem. Although present in very low amounts, the accumulation and availability of tetrapyrrole intermediates have major consequences on the physiology and activity of chloroplasts due to their additional photoreactive and possible signalling functions. Although adjusted for Arabidopsis tetrapyrrole metabolites, the presented methods can also be applied for analysis of cyanobacterial and other plant tetrapyrroles.

Induction of isoforms of tetrapyrrole biosynthetic enzymes, AtHEMA2 and AtFC1, under stress conditions and their physiological functions in Arabidopsis

In the tetrapyrrole biosynthetic pathway, isoforms of glutamyl-tRNA reductase (HEMA2) and ferrochelatase1 (FC1) are mainly expressed in nonphotosynthetic tissues. Here, using promoter-beta-glucuronidase constructs, we showed that the expressions of Arabidopsis (Arabidopsis thaliana) HEMA2 (AtHEMA2) and FC1 (AtFC1) were induced in photosynthetic tissues by oxidative stresses such as wounding. Transcript levels and beta-glucronidase activity were rapidly induced within 30 min, specifically in the wound area in a jasmonate-independent manner. Transcriptome analysis of wound-specific early inducible genes showed that AtHEMA2 and AtFC1 were coinduced with hemoproteins outside plastids, which are related to defense responses. Ozone fumigation or reagents generating reactive oxygen species induced the expression of both genes in photosynthetic tissues, suggesting that reactive oxygen species is involved in the induction. Since cycloheximide or puromycin induced the expression of both genes, inhibition of cytosolic protein synthesis is involved in the induction of these genes in photosynthetic tissues. The physiological functions of AtHEMA2 and AtFC1 were investigated using insertional knockout mutants of each gene. Heme contents of the roots of both mutants were about half of that of the respective wild types. In wild-type plants, heme contents were increased by ozone exposure. In both mutants, reduction of the ozone-induced increase in heme content was observed. These results suggest the existence of the tetrapyrrole biosynthetic pathway controlled by AtHEMA2 and AtFC1, which normally functions for heme biosynthesis in nonphotosynthetic tissues, but is induced in photosynthetic tissues under oxidative conditions to supply heme for defensive hemoproteins outside plastids.

Physicochemical Properties of Heme Iron Products in the Korean Market

Since absorption efficacy of heme iron (HI) is critically dependent on its solubility in aqueous solution, we investigated the physicochemical properties of two HI products available in the Korean market. The two HI products did not differ in ingredients and color. However, HI polypeptide (HIP), produced in Korea, was fairly soluble over a wide pH range in water-based solutions, whereas HI imported from Japan was insoluble except in strong acid and base solutions. Analysis using an ultraviolet-visible spectrophotometer showed that the chromophore of HIP was shifted to the red compared with that of HI. Fourier transform-infrared analysis revealed that HIP contained mainly amide (NH) groups, while HI largely contained amine (NH(2)) groups. With regard to constituents, between HIP and HI, their major components were different from each other according to their ratio of fronts obtained by thin-layer chromatography. These results suggest that determination of solubility should be included in the quality control process of HI products.

Photosystem II Assembly in CP47 Mutant of Synechocystis sp. PCC 6803 Is Dependent on the Level of Chlorophyll Precursors Regulated by Ferrochelatase

Accumulation of chlorophyll and expression of the chlorophyll (Chl)-binding CP47 protein that serves as the core antenna of photosystem II are indispensable for the assembly of a functional photosystem II. We have characterized the CP47 mutant with an impaired photosystem II assembly and its two spontaneous pseudorevertants with their much improved photoautotrophic growth. The complementing mutations in these pseudorevertants were previously mapped to the ferrochelatase gene (1). We demonstrated that complementing mutations dramatically decrease ferrochelatase activity in pseudorevertants and that this decrease is responsible for their improved photoautotrophic growth. Photoautotrophic growth of the CP47 mutant was also restored by in vivo inhibition of ferrochelatase by a specific inhibitor. The decrease in ferrochelatase activity in pseudorevertants was followed by increased steady-state levels of Chl precursors and Chl, leading to CP47 accumulation and photosystem II assembly. Similarly, supplementation of the CP47 mutant with the Chl precursor Mg-protoporphyrin IX increased the number of active photosystem-II centers, suggesting that synthesis of the mutated CP47 protein is enhanced by an increased Chl availability in the cell. The probable role of ferrochelatase in the regulation of Chl biosynthesis is discussed.

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.

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.

Antisense HEMA1 RNA expression inhibits heme and chlorophyll biosynthesis in arabidopsis

5-aminolevulinic acid (ALA) is a precursor in the biosynthesis of tetrapyrroles including chlorophylls and heme. The formation of ALA involves two enzymatic steps which take place in the chloroplast in plants. The first enzyme, glutamyl-tRNA reductase, and the second enzyme, glutamate-1-semialdehyde-2,1-aminomutase, are encoded by the nuclear HEMA and GSA genes, respectively. To assess the significance of the HEMA gene for chlorophyll and heme synthesis, transgenic Arabidopsis plants that expressed antisense HEMA1 mRNA from the constitutive cauliflower mosaic virus 35S promoter were generated. These plants exhibited varying degrees of chlorophyll deficiency, ranging from patchy yellow to total yellow. Analysis indicated that these plants had decreased levels of chlorophyll, non-covalently bound hemes, and ALA; their levels were proportional to the level of glutamyl-tRNA reductase expression and were inversely related to the levels of antisense HEMA transcripts. Plants that lacked chlorophyll failed to survive under normal growth conditions, indicating that HEMA gene expression is essential for growth.