Chlorophyll: Structure and Function

Abstract

Chlorophyll is the dominant pigment on Earth and serves as the light‐trapping and energy transferring chromophore in photosynthetic organisms. In recent years research has contributed enormously to a better understanding of the metabolic pathway of chlorophyll synthesis and its biochemical, biophysical and structural properties in close association with the pigment binding proteins of the reaction centre and the antenna complexes of the photosynthetic units.

Keywords: tetrapyrrole; chloroplast; photosynthesis; haem light harvesting; metabolism; gene expression; photodynamic damage

Figure 1.

Structure of (a) chlorophyll a, (b) chlorophyll b, (c) bacteriochlorophyll a and (d) bacteriochlorophyll b. The major esterifying alcohol (= R) is phytol in chlorophyll a and b, geranylgeraniol in bacteriochlorophyll a, and phytadienol in bacteriochlorophyll b.

Figure 2.

Potential feedback control mechanisms in the metabolic pathway of tetrapyrroles. The synthesis of 5‐aminolaevulinate (ALA) from glutamate is the rate‐limiting step of tetrapyrrole biosynthesis. Feedback regulation on the enzymes of the ALA pathway, glutamyl‐tRNA reductase or glutamate 1‐semialdehyde aminotransferase, can start out from three possible sites: at haem formation, at the reduction of protochlorophyllide, and at the level of Mg‐protoporphyrin IX and Mg‐protoporphyrin IX monomethylester (Mg‐Proto MME) formation.

Figure 3.

The possible hazardous reactions of excited porphyrins after formation of their long‐lived excited triplet state. Free radicals can be generated by electron or hydrogen transfer (type I reaction) or energy is transferred to oxygen giving rise to the formation of toxic singlet oxygen (type II reaction). Additionally, the quench mechanism of excited porphyrins and singlet oxygen by carotenoids (Car) are illustrated (green arrows). Zeaxanthin is assumed to dissipate efficiently excitation energy.

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References

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Further Reading

Chadwick DJ and Ackrill K (1994) The Biosynthesis of the Tetrapyrrole Pigments. Ciba Foundation symposium 180. Chichester: Wiley.

Dailey HA (1990) Biosynthesis of Heme and Chlorophylls. New York: McGraw Hill.

Henningsen KW, Boynton JE and von Wettstein D (1993) Mutants at xantha and albina loci in relation to chloroplast biogenesis in barley (Hordeum vulgare L.). Biologiske Skrifter 42: Munksgaard, Copenhagen. [Whole issue.]

Ort DR and Yocum CF (1996) Oxygenic Photosynthesis: The Light Reaction. Dordrecht: Kluwer Academic.

Scheer H (ed.) (1991) Chlorophylls. Boca Raton, FL: CRC Press.

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Grimm, Bernhard(Apr 2001) Chlorophyll: Structure and Function. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001310]