Chlorophyll Metabolism

Abstract

The two pathways of chlorophyll biosynthesis and chlorophyll catabolism in plants are located in distinct cellular compartments. Although the entire biosynthesis of chlorophyll from the formation of the first committed metabolic precursor, 5‐aminolevulinic acid, occurs in plastids, chlorophyll breakdown starts in plastids and ends with the storage of nonfluorescent breakdown products in vacuoles. Photosynthetic organisms developed a complex control over chlorophyll metabolism to adapt the need for chlorophyll to continuously changing environmental conditions and to avoid damage caused by intermediates accumulation. The balance between the most efficient way of harvesting the light energy available to the organism and damage or death caused by excess light energy or accumulating chlorophyll intermediates due to deregulation of the tetrapyrrole biosynthetic pathway, seems to employ one of the most sophisticated regulatory mechanisms seen in nature. Chlorophyll metabolism in the broad range of photosynthetic organisms including photosynthetic bacteria, algae and higher plants is outlined in the following report.

Key Concepts:

  • Chlorophyll (Chl) a is the key pigment involved in the primary reactions of oxygenic photosynthesis.

  • Photosynthesis requires chlorophyll for light absorption, excitation energy transfer and photooxidative charge separation to ultimately provide primary biomass and energy for almost all living beings and, additionally, supply oxygen for respiration.

  • Chlorophyll is always bound to chlorophyll‐binding proteins of the photosynthetic core complexes of photosystem I and II and their antenna complexes.

  • Chlorophyll biosynthesis always ensures the appropriate supply of the pigment, and chlorophyll catabolism prevents the accumulation of free chlorophyll during breakdown of photosynthetic complexes.

  • Control of chlorophyll metabolism avoids accumulation of photoreactive metabolic and catabolic intermediates and free chlorophyll.

Keywords: 5‐aminolevulinic acid; porphyrin; chloroplast; photosynthesis; Mg‐chelatase; tetrapyrroles; chlorophyll catabolite; chlorophyll cycle; bacteriochlorophyll

Figure 1.

Structure of chlorophyll a and chlorophyll b (left side) and bacteriochlorophyll a and bacteriochlorophyll b (right side). The major esterifying alcohol (5R) is phytol in chlorophyll a and b, geranylgeraniol in bacteriochlorophyll a and phytadienol in bacteriochlorophyll b.

Figure 2.

Flow chart of the metabolic pathway of tetrapyrrole biosynthesis towards the end products, chlorophyll and protohaem. Potential feedback control mechanisms in the metabolic pathway of tetrapyrroles are indicated. The synthesis of 5‐aminolaevulinate from glutamate is the rate‐limiting step of tetrapyrrole biosynthesis. Feedback regulation on the enzymes of the 5‐aminolevulinic acid pathway can start out from various sites: at the level of haem formation, at the level of protochlorophyllide reduction and at the beginning of the Mg‐porphyrin branch.

Figure 3.

Compartmentalisation of the tetrapyrrole biosynthetic pathway and chlorophyll catabolic pathway. The second site of haem synthesis in plants, proposed to be taking place in mitochondria, is indicated by the question mark.

Figure 4.

The pathway of chlorophyll catabolism. Chlorophyll a is the central molecule that connects the biosynthetic and the catabolic pathway as well as the ‘chlorophyll cycle’. Abbreviations: ALA, 5‐aminolaevulinic acid; CAO, chlorophyll(ide) a oxygenase; CBR, chlorophyll(ide) b reductase; Chl, chlorophyll; Chlase, chlorophyllase; Chlide, chlorophyllide; CS, chlorophyll synthase; HCAR, 7‐hydroxymethyl chlorophyll(ide) a reductase; PAO, pheophorbide a oxygenase; pFCC, primary fluorescent chlorophyll catabolite; Pheide a, pheophorbide a; Pchlide, protochlorophyllide; RCC, red chlorophyll catabolite; RCCR, RCC reductase.

Figure 5.

Structure and absorption spectra of three selected chlorophyll catabolites: (a) pheophorbide a, (b) primary fluorescent chlorophyll catabolite and (c) nonfluorescent chlorophyll catabolite.

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Further Reading

Blankenship RE (2002) Molecular Mechanisms of Photosynthesis. Oxford: Blackwell Science.

Czarnecki O, Peter E and Grimm B (2011) Methods for analysis of photosynthetic pigments and steady‐state levels of intermediates of tetrapyrrole biosynthesis. Methods in Molecular Biology 775: 357–385.

Tanaka R, Kobayashi K and Masuda T (2011) Tetrapyrrole metabolism in Arabidopsis thaliana. Arabidopsis Book 2011: e0145.

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Brzezowski, Pawel, and Grimm, Bernhard(Apr 2013) Chlorophyll Metabolism. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020084.pub2]