Chlorophyll‐Binding Proteins

Abstract

Chlorophylls and bacteriochlorophylls are the dominant pigments on Earth and serve – noncovalently bound to specific proteins – as principal light‐harvesting as well as energy‐transforming cofactors in photosynthetic organisms. The major groups of (bacterio)chlorophyll‐binding proteins are the photosynthetic reaction centres (RCs) and the associated light‐harvesting (antenna) complexes. Other proteins include water‐soluble chlorophyll‐binding proteins as well the chlorophyll biosynthetic and degrading enzymes. Although all RCs show structural homologies and appear to have evolved from a common ancestor, light‐harvesting antenna designs vary considerably. The enormous progress that has been achieved in the elucidation of structures and functions of chlorophyll‐binding proteins is reviewed. It has become increasingly clear that light‐harvesting complexes do not only serve to enlarge the absorption cross‐sections of the RCs but are important in the adaptation of the photosynthetic apparatus and regulation of the energy‐transforming processes in response to environmental and endogenous conditions.

Key Concepts:

  • Chlorophylls and bacteriochlorophylls serve – noncovalently bound to specific proteins – as principal energy‐transforming cofactors in photosynthesis.

  • Light‐harvesting antenna complexes ensure optimised light harvesting and transfer of excitation energy to the reaction centres of the photosystems.

  • Reaction centre proteins remained fairly conserved during evolution, whereas photosynthetic organisms developed many distinct forms of light‐harvesting complexes.

  • Oxygenic photosynthesis evolved to two different photosystems that have most likely emerged from a common ancestor.

  • The antenna system of plants and algae can ‘switch’ from effective light harvesting to photoprotective excess excitation dissipation to prevent photo‐oxidative damage.

Keywords: bacteriochlorophylls; chlorophylls; light‐harvesting complexes; photosynthesis; pigment–protein complexes; reaction centres; electron transport; photosystems; photoprotection; nonphotochemical quenching

Figure 1.

Structure of the PSII core‐antenna complex CP43 from the thermophilic cyanobacterium Thermosynecchococcus elongatus at 3.0 Å resolution (Loll et al., ). Side view with respect to the thylakoid membrane plane. The protein backbone comprising six transmembrane α‐helical domains is shown in grey. The 13 noncovalently bound chlorophylls a are shown in green, and the four β‐carotenes in orange.

Figure 2.

Schematic representation of the light‐harvesting antenna systems of PSII and PSI (Jansson, ).

Figure 3.

Structure of the trimeric main LHC II of plants at 2.72 Å resolution (Liu et al., ). Top view with respect to the thylakoid membrane plane. The protein backbone consisting of three transmembrane α‐helical domains per monomer is shown in grey. One monomeric subunit noncovalently binds eight chlorophylls a and six chlorophylls b (shown in cyan and green, respectively) as well as two luteins (yellow), one neoxanthin (orange) and one violaxanthin (or its de‐epoxidation products, antheraxanthin or zeaxanthin; in red).

Figure 4.

Evolution of the chlorophyll a/b‐binding light‐harvesting protein family by multiple gene duplication, fusion and, possibly, helix‐deletion events. Modified from Montane and Kloppstech, . One‐helix proteins (OHPs) have evolved from HLIPs, which are found in cyanobacteria. Subsequently, additional members of the protein family evolved, in plants: two‐helix proteins (SEP1, SEP2 and LIL3), the four‐helix PsbS protein, LHCs and ELIPS; in Chlamydomonas: LhcSR.

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Further Reading

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

Fromme P (ed.) (2008) Photosynthetic Protein Complexes: A Structural Approach. Weinheim: Wiley‐VCH.

Grimm B, Porra R, Rüdiger W and Scheer H (eds) (2006) Chlorophylls and Bacteriochlorophylls: Biochemistry, Biophysics, Functions and Applications (Advances in Photosynthesis and Respiration, vol. 25). Dordrecht: Springer.

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Lokstein, Heiko, and Grimm, Bernhard(May 2013) Chlorophyll‐Binding Proteins. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020085.pub2]