Modification of cysteine residues in the ChlI and ChlH subunits of magnesium chelatase results in enzyme inactivation

Structural and functional consequences of removing the N-terminal domain from the magnesium chelatase ChlH subunit of Thermosynechococcus elongatus

Magnesium chelatase (MgCH) initiates chlorophyll biosynthesis by catalysing the ATP-dependent insertion of Mg²⁺ into protoporphyrin. This large enzyme complex comprises ChlH, I and D subunits, with I and D involved in ATP hydrolysis, and H the protein that handles the substrate and product. The 148 kDa ChlH subunit has a globular N-terminal domain attached by a narrow linker to a hollow cage-like structure. Following deletion of this ~18 kDa domain from the Thermosynechoccus elongatus ChlH, we used single particle reconstruction to show that the apo- and porphyrin-bound forms of the mutant subunit consist of a hollow globular protein with three connected lobes; superposition of the mutant and native ChlH structures shows that, despite the clear absence of the N-terminal ‘head’ region, the rest of the protein appears to be correctly folded. Analyses of dissociation constants shows that the ΔN159ChlH mutant retains the ability to bind protoporphyrin and the Gun4 enhancer protein, although the addition of I and D subunits yields an extremely impaired active enzyme complex. Addition of the Gun4 enhancer protein, which stimulates MgCH activity significantly especially at low Mg²⁺ concentrations, partially reactivates the ΔN159ChlH–I–D mutant enzyme complex, suggesting that the binding site or sites for Gun4 on H do not wholly depend on the N-terminal domain.

The N-terminal domain of the 148 kDa ChlH is essential for normal activity of the ChlH–I–D magnesium chelatase complex. Deleting this 18 kDa domain retains the hollow cage-like structure and porphyrin binding. Chelatase activity is partially restored by the Gun4 protein.

Chlorophyll modifications and their spectral extension in oxygenic photosynthesis

Chlorophylls are magnesium-tetrapyrrole molecules that play essential roles in photosynthesis. All chlorophylls have similar five-membered ring structures, with variations in the side chains and/or reduction states. Formyl group substitutions on the side chains of chlorophyll a result in the different absorption properties of chlorophyll b, chlorophyll d, and chlorophyll f. These formyl substitution derivatives exhibit different spectral shifts according to the formyl substitution position. Not only does the presence of various types of chlorophylls allow the photosynthetic organism to harvest sunlight at different wavelengths to enhance light energy input, but the pigment composition of oxygenic photosynthetic organisms also reflects the spectral properties on the surface of the Earth. Two major environmental influencing factors are light and oxygen levels, which may play central roles in the regulatory pathways leading to the different chlorophylls. I review the biochemical processes of chlorophyll biosynthesis and their regulatory mechanisms.

Thiol-based redox control of enzymes involved in the tetrapyrrole biosynthesis pathway in plants

The last decades of research brought substantial insights into tetrapyrrole biosynthetic pathway in photosynthetic organisms. Almost all genes have been identified and roles of seemingly all essential proteins, leading to the synthesis of heme, siroheme, phytochromobilin, and chlorophyll (Chl), have been characterized. Detailed studies revealed the existence of a complex network of transcriptional and post-translational control mechanisms for maintaining a well-adjusted tetrapyrrole biosynthesis during plant development and adequate responses to environmental changes. Among others one of the known post-translational modifications is regulation of enzyme activities by redox modulators. Thioredoxins and NADPH-dependent thioredoxin reductase C (NTRC) adjust the activity of tetrapyrrole synthesis to the redox status of plastids. Excessive excitation energy of Chls in both photosystems and accumulation of light-absorbing unbound tetrapyrrole intermediates generate reactive oxygen species, which interfere with the plastid redox poise. Recent reports highlight ferredoxin-thioredoxin and NTRC-dependent control of key steps in tetrapyrrole biosynthesis in plants. In this review we introduce the regulatory impact of these reductants on the stability and activity of enzymes involved in 5-aminolevulinic acid synthesis as well as in the Mg-branch of the tetrapyrrole biosynthetic pathway and we propose molecular mechanisms behind this redox control.

Post-translational control of tetrapyrrole biosynthesis in plants, algae, and cyanobacteria

The tetrapyrrole biosynthetic pathway provides the vital cofactors and pigments for photoautotrophic growth (chlorophyll), several essential redox reactions in electron transport chains (haem), N- and S-assimilation (sirohaem), and photomorphogenic processes (phytochromobilin). While the biochemistry of the pathway is well understood and almost all genes encoding enzymes of tetrapyrrole biosynthesis have been identified in plants, the post-translational control and organization of the pathway remains to be clarified. Post-translational mechanisms controlling metabolic activities are of particular interest since tetrapyrrole biosynthesis needs adaptation to environmental challenges. This review surveys post-translational mechanisms that have been reported to modulate metabolic activities and organization of the tetrapyrrole biosynthesis pathway.

Tetrapyrrole Metabolism in Arabidopsis thaliana

Higher plants produce four classes of tetrapyrroles, namely, chlorophyll (Chl), heme, siroheme, and phytochromobilin. In plants, tetrapyrroles play essential roles in a wide range of biological activities including photosynthesis, respiration and the assimilation of nitrogen/sulfur. All four classes of tetrapyrroles are derived from a common biosynthetic pathway that resides in the plastid. In this article, we present an overview of tetrapyrrole metabolism in Arabidopsis and other higher plants, and we describe all identified enzymatic steps involved in this metabolism. We also summarize recent findings on Chl biosynthesis and Chl breakdown. Recent advances in this field, in particular those on the genetic and biochemical analyses of novel enzymes, prompted us to redraw the tetrapyrrole metabolic pathways. In addition, we also summarize our current understanding on the regulatory mechanisms governing tetrapyrrole metabolism. The interactions of tetrapyrrole biosynthesis and other cellular processes including the plastid-to-nucleus signal transduction are discussed.

Redox regulation of chlorophyll biosynthesis

Chlorophyll captures and redirects light-energy and is thus essential for photosynthetic organisms. The demand for chlorophyll differs throughout the day and night and in response to changing light conditions. Moreover, the chlorophyll biosynthesis pathway is up to certain points shared between the different tetrapyrroles; chlorophyll, heme, siroheme and phytochromobilin, for which the cell has different requirements at different time points. Combined with the phototoxic properties of tetrapyrroles which, if not properly protected, can lead to formation of reactive oxygen species (ROS), the need for a strict regulation of the chlorophyll biosynthetic pathway is obvious. Here we describe the current knowledge on regulation of chlorophyll biosynthesis in plants by the chloroplast redox state with emphasis on the Mg-chelatase situated at the branch point between the heme and the chlorophyll pathway. We discuss the proposed role of the Mg-chelatase as a key regulator of the tetrapyrrole pathway by its effect on enzymes both up- and downstream in the pathway and we specifically describe how redox state might regulate the Mg-branch. Finally, we propose that a recently identified NADPH-dependent thioredoxin reductase (NTRC) could be involved in redox regulation or protection of chlorophyll biosynthetic enzymes and describe the possible modes of action by this enzyme.

Disulphide proteomes and interactions with thioredoxin on the track towards understanding redox regulation in chloroplasts and cyanobacteria

Light-dependent disulphide/dithiol exchange catalysed by thioredoxin is a classical example of redox regulation of chloroplast enzymes. Recent proteome studies have mapped thioredoxin target proteins in all chloroplast compartments ranging from the envelope to the thylakoid lumen. Progress in the methodologies has made it possible to identify which cysteine residues interact with thioredoxin and to tackle membrane-bound thioredoxin targets. To date, more than hundred targets of thioredoxin and glutaredoxin have been found in plastids from Arabidopsis, spinach, poplar and Chlamydomonas reinhardtii. Thioredoxin-mediated redox control appears to be a feature of the central pathways for assimilation and storage of carbon, sulphur and nitrogen, as well as for translation and protein folding. Cyanobacteria are oxygenic photosynthetic prokaryotes, which presumably share a common ancestor with higher plant plastids. As in chloroplasts, cyanobacterial thioredoxins receive electrons from the photosynthetic electron transport, and thioredoxin-targeted proteins are therefore highly interesting in the context of acclimation of these organisms to their environment. Studies of the unicellular model cyanobacterium Synechocystis sp. PCC 6803 revealed 77 thioredoxin target proteins. Notably, the functions of all these thioredoxin targets highlight essentially the same processes as those described in chloroplasts suggesting that thioredoxin-mediated redox signalling is equally significant in oxygenic photosynthetic prokaryotes and eukaryotes.

Chlorophyll Biosynthesis in Bacteria: The Origins of Structural and Functional Diversity

The use of photochemical reaction centers to convert light energy into chemical energy, chlorophototrophy, occurs in organisms belonging to only five eubacterial phyla: Cyanobacteria, Proteobacteria, Chlorobi, Chloroflexi, and Firmicutes. All chlorophototrophs synthesize two types of pigments: (a) chlorophylls and bacteriochlorophylls, which function in both light harvesting and uniquely in photochemistry; and (b) carotenoids, which function primarily as photoprotective pigments but can also participate in light harvesting. Although hundreds of carotenoids have been identified, only 12 types of chlorophylls (Chl a, b, d; divinyl-Chl a and b; and 8(1)-hydroxy-Chl a) and bacteriochlorophylls (BChl a, b, c, d, e, and g) are currently known to occur in bacteria. This review summarizes recent progress in the identification of genes and enzymes in the biosynthetic pathways leading to Chls and BChls, the essential tetrapyrrole cofactors of photosynthesis, and addresses the mechanisms for generating functional diversity for solar energy capture and conversion in chlorophototrophs.

Tetrapyrrole biosynthesis in higher plants

Tetrapyrroles play vital roles in various biological processes, including photosynthesis and respiration. Higher plants contain four classes of tetrapyrroles, namely, chlorophyll, heme, siroheme, and phytochromobilin. All of the tetrapyrroles are derived from a common biosynthetic pathway. Here we review recent progress in the research of tetrapyrrole biosynthesis from a cellular biological view. The progress consists of biochemical, structural, and genetic analyses, which contribute to our understanding of how the flow and the synthesis of tetrapyrrole molecules are regulated and how the potentially toxic intermediates of tetrapyrrole synthesis are maintained at low levels. We also describe interactions of tetrapyrrole biosynthesis and other cellular processes including the stay-green events, the cell-death program, and the plastid-to-nucleus signal transduction. Finally, we present several reports on attempts for agricultural and horticultural applications in which the tetrapyrrole biosynthesis pathway was genetically modified.

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.