The light intensity dependence of protochlorophyllide photoconversion and its significance to the catalytic mechanism of protochlorophyllide reductase

Biosynthesis and biotechnological interventions for commercial production of microalgal pigments: A review

Microalgae are photosynthetic eukaryotes that serve as microbial cell factories for the production of useful biochemicals, including pigments. These pigments are eco-friendly alternatives to synthetic dyes and reduce environmental and health risks. They also exhibit excellent anti-oxidative properties, making them a useful commodity in the nutrition and pharmaceutical industries. Light-harvesting pigments such as chlorophylls and phycobilins, and photoprotective carotenoids are some of the most common microalgal pigments. The increasing demand for these pigments in industrial applications has prompted a need to improve their metabolic yield in microalgal cells. So far, expensive cultivation methods and sensitivity to microbial contamination remain the main obstacles to the large-scale production of these pigments. This review highlights current issues and future prospects related to the production of microalgal pigments. The review also emphasizes the use of engineering approaches such as genetic engineering, and optimization of media components and physical parameters to increase their commercial-scale production.

Presence and absence of light-independent chlorophyll biosynthesis among Chlamydomonas green algae in an ice-covered Antarctic lake

The cold, permanently ice-covered waters of Lake Bonney, Antarctica, may seem like an uninviting place for an alga, but they are home to a diversity of photosynthetic life, including Chlamydomonas sp. UWO241, a psychrophile residing in the deep photic zone. Recently, we found that UWO241 has lost the genes responsible for light-independent chlorophyll biosynthesis, which is surprising given that this green alga comes from a light-limited environment and experiences extended periods of darkness during the Antarctic winter. Why discard such a process? We argued that it might be linked to the very high dissolved oxygen concentration of Lake Bonney at the depth at which UWO241 is found. Oxygen is the Achilles’ heel of the key enzyme involved in light-independent chlorophyll biosynthesis: DPOR. If this hypothesis is true, then other algae in Lake Bonney should also be susceptible to losing DPOR, such as Chlamydomonas sp. ICE-MDV, which predominantly resides in the chemocline, a depth with an even higher oxygen concentration than that where UWO241 exists. Here, we report that, contrary to our earlier prediction, ICE-MDV has maintained the genes encoding DPOR. We briefly discuss the implications of this finding in relation to the loss of light-independent chlorophyll synthesis in UWO241.

Photoenzymes and Related Topics: An Update

Photoenzymes are enzymes that catalyze photochemical reactions. For a long time, it was believed that only two types of photoenzymes exist: light-dependent NADPH:protochlorophyllide oxidoreductase and photolyase. However, other photoenzymes have now been discovered, most recently fatty acid photodecarboxylase.

Spectral Dependence of Chlorophyll Biosynthesis Pathways in Plant Leaves

This review covers studies on the dependence of chlorophyll photobiosynthesis reactions from protochlorophyllide on the spectral composition of actinic light. A general scheme of the reaction sequence for the photochemical stage in chlorophyll biosynthesis for etiolated plant leaves is presented. Comparative analysis of the data shows that the use of light with varied wavelengths for etiolated plant illumination reveals parallel transformation pathways of different protochlorophyllide forms into chlorophyllide, including a pathway for early photosystem II reaction center P-680 pigment formation.

Mechanisms of phototransformation of protochlorophyllide into chlorophyllide

The purpose of this review is to summarize and discuss data obtained in studies on the mechanisms of the primary photophysical and photochemical reactions of protochlorophyllide photoreduction in plant materials (etiolated leaves and leaf homogenates) and in model systems. Based on the results of numerous studies, it can be stated that the reduction of active forms of the chlorophyll precursor is a multistep process comprising two or three short-lived intermediates characterized by a singlet ESR signal. The first intermediate is probably a complex with charge transfer between protochlorophyllide and the hydride ion donor NADPH. The conserved tyrosine residue Tyr193 of protochlorophyllide oxidoreductase is the donor of the second proton.

Extensive horizontal gene transfer, duplication, and loss of chlorophyll synthesis genes in the algae

Background

Two non-homologous, isofunctional enzymes catalyze the penultimate step of chlorophyll a synthesis in oxygenic photosynthetic organisms such as cyanobacteria, eukaryotic algae and land plants: the light-independent (LIPOR) and light-dependent (POR) protochlorophyllide oxidoreductases. Whereas the distribution of these enzymes in cyanobacteria and land plants is well understood, the presence, loss, duplication, and replacement of these genes have not been surveyed in the polyphyletic and remarkably diverse eukaryotic algal lineages.

Results

A phylogenetic reconstruction of the history of the POR enzyme (encoded by the por gene in nuclei) in eukaryotic algae reveals replacement and supplementation of ancestral por genes in several taxa with horizontally transferred por genes from other eukaryotic algae. For example, stramenopiles and haptophytes share por gene duplicates of prasinophytic origin, although their plastid ancestry predicts a rhodophytic por signal. Phylogenetically, stramenopile pors appear ancestral to those found in haptophytes, suggesting transfer from stramenopiles to haptophytes by either horizontal or endosymbiotic gene transfer. In dinoflagellates whose plastids have been replaced by those of a haptophyte or diatom, the ancestral por genes seem to have been lost whereas those of the new symbiotic partner are present. Furthermore, many chlorarachniophytes and peridinin-containing dinoflagellates possess por gene duplicates.

In contrast to the retention, gain, and frequent duplication of algal por genes, the LIPOR gene complement (chloroplast-encoded chlL, chlN, and chlB genes) is often absent. LIPOR genes have been lost from haptophytes and potentially from the euglenid and chlorarachniophyte lineages. Within the chlorophytes, rhodophytes, cryptophytes, heterokonts, and chromerids, some taxa possess both POR and LIPOR genes while others lack LIPOR. The gradual process of LIPOR gene loss is evidenced in taxa possessing pseudogenes or partial LIPOR gene compliments. No horizontal transfer of LIPOR genes was detected.

Conclusions

We document a pattern of por gene acquisition and expansion as well as loss of LIPOR genes from many algal taxa, paralleling the presence of multiple por genes and lack of LIPOR genes in the angiosperms. These studies present an opportunity to compare the regulation and function of por gene families that have been acquired and expanded in patterns unique to each of various algal taxa.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0286-4) contains supplementary material, which is available to authorized users.

With or without light: comparing the reaction mechanism of dark-operative protochlorophyllide oxidoreductase with the energetic requirements of the light-dependent protochlorophyllide oxidoreductase

The addition of two electrons and two protons to the C₁₇=C₁₈ bond in protochlorophyllide is catalyzed by a light-dependent enzyme relying on NADPH as electron donor, and by a light-independent enzyme bearing a (Cys)₃Asp-ligated [4Fe–4S] cluster which is reduced by cytoplasmic electron donors in an ATP-dependent manner and then functions as electron donor to protochlorophyllide. The precise sequence of events occurring at the C₁₇=C₁₈ bond has not, however, been determined experimentally in the dark-operating enzyme. In this paper, we present the computational investigation of the reaction mechanism of this enzyme at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level of theory. The reaction mechanism begins with single-electron reduction of the substrate by the (Cys)₃Asp-ligated [4Fe–4S], yielding a negatively-charged intermediate. Depending on the rate of Fe–S cluster re-reduction, the reaction either proceeds through double protonation of the single-electron-reduced substrate, or by alternating proton/electron transfer. The computed reaction barriers suggest that Fe–S cluster re-reduction should be the rate-limiting stage of the process. Poisson–Boltzmann computations on the full enzyme–substrate complex, followed by Monte Carlo simulations of redox and protonation titrations revealed a hitherto unsuspected pH-dependence of the reaction potential of the Fe–S cluster. Furthermore, the computed distributions of protonation states of the His, Asp and Glu residues were used in conjuntion with single-point ONIOM computations to obtain, for the first time, the influence of all protonation states of an enzyme on the reaction it catalyzes. Despite exaggerating the ease of reduction of the substrate, these computations confirmed the broad features of the reaction mechanism obtained with the medium-sized models, and afforded valuable insights on the influence of the titratable amino acids on each reaction step. Additional comparisons of the energetic features of the reaction intermediates with those of common biochemical redox intermediates suggest a surprisingly simple explanation for the mechanistic differences between the dark-catalyzed and light-dependent enzyme reaction mechanisms.

Carbon-carbon double-bond reductases in nature

Reduction of C = C bonds by reductases, found in a variety of microorganisms (e.g. yeasts, bacteria, and lower fungi), animals, and plants has applications in the production of metabolites that include pharmacologically active drugs and other chemicals. Therefore, the reductase enzymes that mediate this transformation have become important therapeutic targets and biotechnological tools. These reductases are broad-spectrum, in that, they can act on isolation/conjugation C = C-bond compounds, α,β-unsaturated carbonyl compounds, carboxylic acids, acid derivatives, and nitro compounds. In addition, several mutations in the reductase gene have been identified, some associated with diseases. Several of these reductases have been cloned and/or purified, and studies to further characterize them and determine their structure in order to identify potential industrial biocatalysts are still in progress. In this study, crucial reductases for bioreduction of C = C bonds have been reviewed with emphasis on their principal substrates and effective inhibitors, their distribution, genetic polymorphisms, and implications in human disease and treatment.

Dominance of a 675 nm chlorophyll(ide) form upon selective 632.8 or 654 nm laser illumination after partial protochlorophyllide phototransformation

The phototransformation pathways of protochlorophyllide forms were studied in 8-14-day-old leaves of dark-germinated wheat (Triticum aestivum L.) using white, 632.8 nm He-Ne laser and 654 nm laser diode light. The photon flux density (PFD) values (0.75-360 μmol photons m(-2) s(-1)), the illumination periods (20 ms-10 s) and the temperature of the leaves (between -60 °C and room temperature) were varied. The 77 K fluorescence spectra of partially phototransformed leaves showed gradual accumulation or even the dominance of the 675 nm emitting chlorophyllide or chlorophyll form at room temperature with 632.8 nm of PFD less than 200 μmol photons m(-2) s(-1) or with 654 nm of low PFD (7.5 μmol photons m(-2) s(-1)) up to 1 s. Longer wavelength (685 or 690 nm) emitting chlorophyllide forms appeared at illuminations under -25 °C with both laser lights or at room temperature when the PFD values were higher or the illumination period was longer than above. We concluded that the formation of the 675 nm emitting chlorophyllide form does not indicate the direct photoactivity of the 633 nm emitting protochlorophyllide form; it can derive from 644 and 657 nm forms via instantaneous disaggregation of the newly-produced chlorophyllide complexes. The disaggregation is strongly influenced by the molecular environment and the localization of the complex.

Metabolic network reconstruction of Chlamydomonas offers insight into light-driven algal metabolism

A comprehensive genome-scale metabolic network of Chlamydomonas reinhardtii, including a detailed account of light-driven metabolism, is reconstructed and validated. The model provides a new resource for research of C. reinhardtii metabolism and in algal biotechnology.

The genome-scale metabolic network of Chlamydomonas reinhardtii (iRC1080) was reconstructed, accounting for >32% of the estimated metabolic genes encoded in the genome, and including extensive details of lipid metabolic pathways.

This is the first metabolic network to explicitly account for stoichiometry and wavelengths of metabolic photon usage, providing a new resource for research of C. reinhardtii metabolism and developments in algal biotechnology.

Metabolic functional annotation and the largest transcript verification of a metabolic network to date was performed, at least partially verifying >90% of the transcripts accounted for in iRC1080. Analysis of the network supports hypotheses concerning the evolution of latent lipid pathways in C. reinhardtii, including very long-chain polyunsaturated fatty acid and ceramide synthesis pathways.

A novel approach for modeling light-driven metabolism was developed that accounts for both light source intensity and spectral quality of emitted light. The constructs resulting from this approach, termed prism reactions, were shown to significantly improve the accuracy of model predictions, and their use was demonstrated for evaluation of light source efficiency and design.

Algae have garnered significant interest in recent years, especially for their potential application in biofuel production. The hallmark, model eukaryotic microalgae Chlamydomonas reinhardtii has been widely used to study photosynthesis, cell motility and phototaxis, cell wall biogenesis, and other fundamental cellular processes (Harris, 2001). Characterizing algal metabolism is key to engineering production strains and understanding photobiological phenomena. Based on extensive literature on C. reinhardtii metabolism, its genome sequence (Merchant et al, 2007), and gene functional annotation, we have reconstructed and experimentally validated the genome-scale metabolic network for this alga, iRC1080, the first network to account for detailed photon absorption permitting growth simulations under different light sources. iRC1080 accounts for 1080 genes, associated with 2190 reactions and 1068 unique metabolites and encompasses 83 subsystems distributed across 10 cellular compartments (Figure 1A). Its >32% coverage of estimated metabolic genes is a tremendous expansion over previous algal reconstructions (Boyle and Morgan, 2009; Manichaikul et al, 2009). The lipid metabolic pathways of iRC1080 are considerably expanded relative to existing networks, and chemical properties of all metabolites in these pathways are accounted for explicitly, providing sufficient detail to completely specify all individual molecular species: backbone molecule and stereochemical numbering of acyl-chain positions; acyl-chain length; and number, position, and cis–trans stereoisomerism of carbon–carbon double bonds. Such detail in lipid metabolism will be critical for model-driven metabolic engineering efforts.

We experimentally verified transcripts accounted for in the network under permissive growth conditions, detecting >90% of tested transcript models (Figure 1B) and providing validating evidence for the contents of iRC1080. We also analyzed the extent of transcript verification by specific metabolic subsystems. Some subsystems stood out as more poorly verified, including chloroplast and mitochondrial transport systems and sphingolipid metabolism, all of which exhibited <80% of transcripts detected, reflecting incomplete characterization of compartmental transporters and supporting a hypothesis of latent pathway evolution for ceramide synthesis in C. reinhardtii. Additional lines of evidence from the reconstruction effort similarly support this hypothesis including lack of ceramide synthetase and other annotation gaps downstream in sphingolipid metabolism. A similar hypothesis of latent pathway evolution was established for very long-chain fatty acids (VLCFAs) and their polyunsaturated analogs (VLCPUFAs) (Figure 1C), owing to the absence of this class of lipids in previous experimental measurements, lack of a candidate VLCFA elongase in the functional annotation, and additional downstream annotation gaps in arachidonic acid metabolism.

The network provides a detailed account of metabolic photon absorption by light-driven reactions, including photosystems I and II, light-dependent protochlorophyllide oxidoreductase, provitamin D₃ photoconversion to vitamin D₃, and rhodopsin photoisomerase; this network accounting permits the precise modeling of light-dependent metabolism. iRC1080 accounts for effective light spectral ranges through analysis of biochemical activity spectra (Figure 3A), either reaction activity or absorbance at varying light wavelengths. Defining effective spectral ranges associated with each photon-utilizing reaction enabled our network to model growth under different light sources via stoichiometric representation of the spectral composition of emitted light, termed prism reactions. Coefficients for different photon wavelengths in a prism reaction correspond to the ratios of photon flux in the defined effective spectral ranges to the total emitted photon flux from a given light source (Figure 3B). This approach distinguishes the amount of emitted photons that drive different metabolic reactions. We created prism reactions for most light sources that have been used in published studies for algal and plant growth including solar light, various light bulbs, and LEDs. We also included regulatory effects, resulting from lighting conditions insofar as published studies enabled. Light and dark conditions have been shown to affect metabolic enzyme activity in C. reinhardtii on multiple levels: transcriptional regulation, chloroplast RNA degradation, translational regulation, and thioredoxin-mediated enzyme regulation. Through application of our light model and prism reactions, we were able to closely recapitulate experimental growth measurements under solar, incandescent, and red LED lights. Through unbiased sampling, we were able to establish the tremendous statistical significance of the accuracy of growth predictions achievable through implementation of prism reactions. Finally, application of the photosynthetic model was demonstrated prospectively to evaluate light utilization efficiency under different light sources. The results suggest that, of the existing light sources, red LEDs provide the greatest efficiency, about three times as efficient as sunlight. Extending this analysis, the model was applied to design a maximally efficient LED spectrum for algal growth. The result was a 677-nm peak LED spectrum with a total incident photon flux of 360 μE/m²/s, suggesting that for the simple objective of maximizing growth efficiency, LED technology has already reached an effective theoretical optimum.

In summary, the C. reinhardtii metabolic network iRC1080 that we have reconstructed offers insight into the basic biology of this species and may be employed prospectively for genetic engineering design and light source design relevant to algal biotechnology. iRC1080 was used to analyze lipid metabolism and generate novel hypotheses about the evolution of latent pathways. The predictive capacity of metabolic models developed from iRC1080 was demonstrated in simulating mutant phenotypes and in evaluation of light source efficiency. Our network provides a broad knowledgebase of the biochemistry and genomics underlying global metabolism of a photoautotroph, and our modeling approach for light-driven metabolism exemplifies how integration of largely unvisited data types, such as physicochemical environmental parameters, can expand the diversity of applications of metabolic networks.

Metabolic network reconstruction encompasses existing knowledge about an organism's metabolism and genome annotation, providing a platform for omics data analysis and phenotype prediction. The model alga Chlamydomonas reinhardtii is employed to study diverse biological processes from photosynthesis to phototaxis. Recent heightened interest in this species results from an international movement to develop algal biofuels. Integrating biological and optical data, we reconstructed a genome-scale metabolic network for this alga and devised a novel light-modeling approach that enables quantitative growth prediction for a given light source, resolving wavelength and photon flux. We experimentally verified transcripts accounted for in the network and physiologically validated model function through simulation and generation of new experimental growth data, providing high confidence in network contents and predictive applications. The network offers insight into algal metabolism and potential for genetic engineering and efficient light source design, a pioneering resource for studying light-driven metabolism and quantitative systems biology.

Rapid dark repression of 5-aminolevulinic acid synthesis in green barley leaves

In photosynthetic organisms chlorophyll and heme biosynthesis is tightly regulated at various levels in response to environmental adaptation and plant development. The formation of 5-aminolevulinic acid (ALA) is the key regulatory step and provides adequate amounts of the common precursor molecule for the Mg and Fe branches of tetrapyrrole biosynthesis. Pathway control prevents accumulation of metabolic intermediates and avoids photo-oxidative damage. In angiosperms reduction of protochlorophyllide (Pchlide) to chlorophyllide is catalyzed by the light-dependent NADPH:Pchlide oxidoreductase (POR). Although a correlation between down-regulated ALA synthesis and accumulation of Pchlide in the dark was proposed a long time ago, the time-resolved mutual dependency has never been analyzed. Taking advantage of the high metabolic activity of young barley (Hordeum vulgare L.) seedlings, in planta ALA synthesis could be determined with high time-resolution. ALA formation declined immediately after transition from light to dark and correlated with an immediate accumulation of POR-bound Pchlide within the first 60 min in darkness. The flu homologous barley mutant tigrina d(12) uncouples ALA synthesis from dark-suppression and continued to form ALA in darkness without a significant change in synthesis rate in this time interval. Similarly, inhibition of protoporphyrinogen IX oxidase by acifluorfen resulted in a delayed accumulation of Pchlide during the entire dark period and a weak repression of ALA synthesis in darkness. Moreover, it is demonstrated that dark repression of ALA formation relies rather on rapid post-translational regulation in response to accumulating Pchlide than on changes in nuclear gene expression.

Enzymes of the last steps of chlorophyll biosynthesis: modification of the substrate structure helps to understand the topology of the active centers

Enzymes catalyzing two of the late steps of chlorophyll biosynthesis are NADPH:protochlorophyllide oxidoreductase (POR), responsible for the light-dependent reduction of protochlorophyllide to chlorophyllide, and chlorophyll synthase that catalyses the esterification of chlorophyllide to chlorophyll. Inhibitors of these enzymes are of interest as potential herbicides. Both enzymes presumably form a complex, and the question arose whether chlorophyll synthase can react with chlorophyllide while it is still bound to POR. Here, we describe the chemical modification of protochlorophyllides and chlorophyllides with space-filling substituents at rings A, B, and E of the tetrapyrrole macrocycle and the reactivity of the modified substrates. Both enzymes tolerate the large and flexible phenylamino substituent at ring B, indicating that ring B points toward the enzyme surface while the substrate is bound. On the basis of the standard compound zinc protopheophorbide a (100% activity), the 7(1)-phenylamino derivative shows a comparable activity (83%) with POR that is higher than that of the parent formyl derivative zinc protopheophorbide b (58% activity). In contrast, the 3(1)-phenylamino derivative is less active (12%) than the parent formyl compound zinc protopheophorbide d (49% activity), indicating that the binding pocket leaves less space around ring A than around ring B. Almost no space must be left around ring E because substitution of the 13(2)-carboxymethyl ester (100% activity) by the 13(2)-carboxyethyl ester reduces the activity to 0.2%. Chlorophyll synthase leaves somewhat more space around ring E on the A side of the tetrapyrrole in the binding pocket; substitution of the 13(2)-proton (100% activity) by a methoxy group (53% activity) and an ethoxy group (11% activity) is tolerated to a certain extent, while the carbomethoxy group in this position is not accepted. Opening of ring E to a chlorin e6 dimethylester is tolerated (39% activity), while the large benzylamide residue at this site leads to the loss of activity. We conclude that the tetrapyrroles bind to both enzymes in the same direction: rings C, D, and E are oriented to the interior of the binding cleft, and rings A and B are oriented to the surface of the enzyme; this excludes simultaneous binding to both enzymes.

Making light work of enzyme catalysis: protochlorophyllide oxidoreductase

In the chlorophyll biosynthetic pathway, the enzyme protochlorophyllide oxidoreductase (POR) catalyses a key light-driven reaction that triggers a profound transformation in plant development. Because POR is activated by light, it can provide information on the way in which light energy can be harnessed to power enzyme reactions and it presents us with a unique opportunity to study catalysis at low temperatures and on ultrafast timescales that are not accessible for most analyses of enzyme function. Recent advances in our understanding of the catalytic mechanism of POR illustrate why it is an important generic model for studying enzyme catalysis and reaction dynamics.

Protochlorophyllide reduction: mechanisms and evolutions

Protochlorophyllide (Pchlide) reductases are key enzymes in the process of chlorophyll biosynthesis. In this review, current knowledge on the molecular organization, substrate specificity and assembly of the light-dependent reduced nicotinamide adenine dinucleotide phosphate:Pchlide oxidoreductases are discussed. Characteristics of light-independent enzymes are also described briefly, and the possible reasons for the selection of light-dependent enzymes during the course of evolution are discussed.

Early reactions of light-induced protochlorophyllide and chlorophyllide transformations analyzed in vivo at room temperature with a diode array spectrofluorometer

The steps of protochlorophyllide (Pchlide) photoreduction and subsequent chlorophyllide (Chlide) transformations which occur in the seconds to minutes time-scale were studied using a diode array spectrofluorometer in dark-grown barley leaves. The intensity of the excitation light was varied between 3 and 2,500 micromol m(-2) s(-1) and a series of fluorescence spectra were recorded at room temperature in the seconds and minutes time scales. In certain experiments, 77-K emission spectra were measured with the same equipment. The high quality of the spectra allowed us to run spectral resolution studies which proved the occurrence, at room temperature, of multiple Pchlide and Chlide forms found previously in 77-K spectra. The comparison of the 77-K and room-temperature spectra showed that the fluorescence yields of the nonphotoactive 633-nm Pchlide form and of the Chlide product emitting at 678 nm were temperature independent. The fluorescence intensity of aggregated NADPH-pigment-POR complexes (photoactive 656-nm Pchlide and 693-nm Chlide forms) were strongly increased at 77 K, while that of the NADP(+)-Chlide-POR (684-686-nm Chlide form) was much less affected by temperature. Information was obtained also about the dynamics of the transformation of pigment forms in the light at different photon densities. At low light intensities, the phototransformation of the 642-644-nm Pchlide form was faster than that of the 654-656-nm form. The relative amplitudes of Gaussian components related to different Chlide forms found after exposure to a constant amount of photons strongly depended on the light intensity used. Strong quenching of all Chlide components occurred upon prolonged exposure to high intensity light. These effects are discussed by considering the interconversion processes between different forms of the pigment-protein complexes, their relative fluorescence yields and energy migration processes.

Carotenoid dependence of the protochlorophyllide to chlorophyllide phototransformation in dark-grown wheat seedlings

The influence of carotenoids on partial protochlorophyllide (Pchlide) photoreduction and the successive formation of long-wavelength chlorophyllide (Chlide) forms was studied by low-temperature fluorescence spectroscopy (77 K). Wheat leaves with a decreased content of carotenoids obtained from norflurazon-treated seedlings (10 and 100 micromol l(-1)) were compared with leaves containing normal amounts of these pigments. Partial photoreduction of Pchlide was achieved by irradiation of the leaves with one light flash in combination with a number of neutral gray and/or red Perspex filters. There were significant differences between the fluorescence emission spectra (the position and height of the peaks) of dark-grown normal and carotenoid-deficient leaves irradiated with non-saturating white light of increasing intensity. The long-wavelength Chlide forms appeared first in the leaves nearly devoid of carotenoids (treated with 100 micromol l(-1) norflurazon), then in the leaves with carotenoid deficiency (treated with 10 micromol l(-1) norflurazon), and finally in normal leaves. After irradiation with non-saturating light of the same intensity, the ratio Chlide/Pchlide(657) was always the highest in the leaves nearly deficient of carotenoids, medium in the leaves with carotenoid deficiency and lowest in the normal leaves. Similarly to white light, red light of low intensity induced faster formation of long-wavelength Chlide species in the leaves with carotenoid deficiency in comparison to the normal leaves. We propose that, in leaves with reduced carotenoid content, a greater number of Pchlide molecules transform to Chlide per light flash than in normal leaves. The results are discussed in relation to the involvement of carotenoids in competitive absorption and light screening, as well as to their influence on Pchlide-Chlide interactions.

Tyr275 and Lys279 stabilize NADPH within the catalytic site of NADPH:protochlorophyllide oxidoreductase and are involved in the formation of the enzyme photoactive state

Fluorescence spectroscopic and kinetic analysis of photochemical activity, cofactor and substrate binding, and enzyme denaturation studies were performed with highly purified, recombinant pea NADPH:protochlorophyllide oxidoreductase (POR) heterologously expressed in Escherichia coli. The results obtained with an individual stereoisomer of the substrate [C8-ethyl-C13(2)-(R)-protochlorophyllide] demonstrate that the enzyme photoactive state possesses a characteristic fluorescence maximum at 646 nm that is due to the presence of specific charged amino acids in the enzyme catalytic site. The photoactive state is converted directly into an intermediate having fluorescence at 685 nm in a reaction involving direct hydrogen transfer from the cofactor (NADPH). Site-directed mutagenesis of the highly conserved Tyr275 (Y275F) and Lys279 (K279I and K279R) residues in the enzyme catalytic pocket demonstrated that the presence of these two amino acids in the wild-type POR considerably increases the probability of photoactive state formation following cofactor and substrate binding by the enzyme. At the same time, the presence of these two amino acids destabilizes POR and increases the rate of enzyme denaturation. Neither Tyr275 nor Lys279 plays a crucial role in the binding of the substrate or cofactor by the enzyme. In addition, the presence of Tyr275 is absolutely necessary for the second step of the protochlorophyllide reduction reaction, "dark" conversion of the 685 nm fluorescence intermediate and the formation of the final product, chlorophyllide. We propose that Tyr275 and Lys279 participate in the proper coordination of NADPH and PChlide in the enzyme catalytic site and thereby control the efficiency of the formation of the POR photoactive state.

Protochlorophyllide oxidoreductase B-catalyzed protochlorophyllide photoreduction in vitro: insight into the mechanism of chlorophyll formation in light-adapted plants

The mechanism of the protochlorophyllide (PChlide) photoreduction reaction operating in light-adapted plants and catalyzed by NADPH:protochlorophyllide oxidoreductase B (PORb) has been analyzed by low-temperature fluorescence spectroscopy by using purified barley PORb overexpressed heterologously in Escherichia coli as a fusion protein with the maltose-binding protein. We show that the PORb-catalyzed PChlide reduction reaction consists of two steps, one photochemical and the other nonphotochemical. The initial photochemical reaction follows a single quantum mechanism and leads to the formation of an unstable intermediate with mixed pigment electronic structure and an EPR spectrum that suggests the presence of a free electron. The second step involves the spontaneous conversion of the unstable intermediate into chlorophyllide as defined by its spectroscopic characteristics and migration on an HPLC column. Both steps of the reaction can be performed at subzero temperatures in frozen samples, suggesting that they do not include major changes in enzyme conformation or pigment rearrangement within the active site. The rate of the reaction at room temperature depends linearly on enzyme and substrate (PChlide) concentration, and the kinetic parameters are consistent with one molecule of substrate bound per active monomer in solution. The PORb-catalyzed reaction in vitro is spectroscopically similar to that identified in leaves of light-adapted plants, suggesting that the same reaction sequence observed operates in planta.