Chlorophylls

Abstract

Chlorophylls (Chls) are a group of cyclic tetrapyrrole pigments that are involved in the light reactions of photosynthesis. There are many Chls differing in chemical structures; Chls a, b, c, d and f and bacteriochlorophylls (BChls) a, b, c, d, e, f and g. All Chls are classified into three groups by their parental structure; porphyrins, chlorins and bacteriochlorins. Upon absorption of light the Chl molecule transitions into the first excited singlet state, and the captured energy is transferred to the neighbouring Chl molecules reaching the reaction centre Chl molecules to ignite the photosynthetic electron transfer. All Chls are synthesised from 5‐aminolevulinic acid via multiple enzymatic reactions. The first half of this biosynthetic pathway is shared with haem biosynthesis and the second half is specific for Chls, which is the so‐called magnesium (Mg) branch. In the Mg branch, the reactions from protoporphyrin IX to chlorophyllide a constitute a common pathway for all Chls.

Key Concepts

  • Chlorophylls have a central role in light harvesting and ignition of photosynthetic electron transfer.
  • Chlorophylls are functionally divided into antenna chlorophylls and reaction centre chlorophylls.
  • A variety of chemically different chlorophylls absorb different wavelengths of light, and photosynthetic organisms have specific sets of chlorophylls for utilising light in their natural habitats.
  • All chlorophylls are synthesised from 5‐aminolaevulinic acid, and the subsequent six reactions leading to protoporphyrin IX are common to haem biosynthesis.
  • Five reactions from protoporphyrin IX to chlorophyllide a are shared with the biosynthetic pathways of all chlorophylls.
  • The diversity of the biosynthetic pathways of different chlorophylls results from the evolution of photosynthetic organisms to adapt their light environments.
  • Chlorophyll biosynthesis is tightly regulated by multiple layers of regulatory mechanisms to avoid photooxidative damage caused by biosynthetic intermediates.
  • Elucidation of the reaction mechanisms of enzymes involved in the magnesium branch is one of the most important goals in plant physiology.

Keywords: photosynthesis; photosystems; light‐harvesting complex; reaction centre; tetrapyrrole; biosynthesis of chlorophylls

Figure 1. (a–c) Three oxidation states of the cyclic tetrapyrroles present in Chls and BChls: (a) porphyrin, with the IUPAC nomenclatures; (b) chlorin (17,18‐dihydroporphyrin); (c) bacteriochlorin (7,8,17,18‐tetrahydroporphyrin). The reduced bonds of C17–C18 and C7–C8 are highlighted with yellow boxes. (d–g) Structures of Chl a and its derivatives lacking the magnesium ion and/or phytol: (d) Chl a; (e) chlorophyllide a; (f) pheophytin a; (g) pheophorbide a.
Figure 2. Structure of various Chls (a) and BChls (b). (a) Chls a, b, c, d and f. Differences in the side‐chains of Chls b, c, d and f from Chl a are highlighted with yellow boxes. (b) BChls a, b, c, d, e, f and Zn‐BChl a. Differences in the side‐chains, ring oxidation state and the central metal ion of BChls b, c, d, e, g, f and Zn‐BChl a from BChl a are highlighted with yellow boxes. See Table for R1 (C7) and R2 (C8) in Chls c, and R1 (C8), R2 (C12) and R3 (C20) in BChls c, d, e and f.
Figure 3. Biosynthetic route of Chls and BChls. Early steps of the Chl biosynthesis from the ALA formation to protoporphyrin IX. These steps are common to haem biosynthesis. The numbers of the individual reactions correspond to those in Table.
Figure 4. Biosynthetic route of Chls and BChls, Mg branch. Late steps from protoporphyrin IX specific for Chls and BChls are shown. Modifications of the intermediates at various stages are highlighted with a yellow box. The numbers of individual reactions correspond to those in Table. The common pathway from protoporphyrin IX to chlorophyllide a is shown with thick arrows, and other reactions are shown with thin arrows.
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Further Reading

Burnap RL and Vermaas WFJ (2012) Functional Genomics and Evolution of Photosynthetic Systems. Dordrecht: Springer.

Blankenship RE (2014) Molecular Mechanisms of Photosynthesis, 2nd edn. Wiley Blackwell: Science.

Grimm B, Porra RJ, Rüdiger W and Scheer H (eds) (2006) Chlorophylls and Bacteriochlorophylls: Biochemistry, Biophysics, Functions and Applications. Dordrecht: Springer.

Kadish KM, Smith KM and Guilard R (eds) (2003a) The Porphyrin Handbook. Vol. 12 The Iron and Cobalt Pigments: Biosynthesis, Synthesis, and Degradation. Oxford: Academic Press.

Kadish KM, Smith KM and Guilard R (eds) (2003b) The Porphyrin Handbook. Vol. 13 Chlorophylls and Bilins: Biosynthesis, Synthesis, and Degradation. Oxford: Academic Press.

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

Warren MJ and Smith AG (2009) Tetrapyrroles: Birth, Life and Death. Austin, Texas: Landes Biosciences Springer Science + Business Media.

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Fujita, Yuichi(May 2015) Chlorophylls. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000661.pub3]