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Carotenoids in Light Harvesting Complexes

Higher plants capture light for photosynthesis with the help of light-harvesting complexes (LHCs). These are proteins associated with three carotenoids in very specific proportions: neoxanthin, violaxanthin, and lutein. Recently, it has been shown that in the photosynthetic (but parasitic) plant, Cuscuta reflexa, neoxanthin is stoichiometrically replaced by lutein-5,6-epoxide [Bungard, R.A. et.al.;Proc. Natl. Acad. Sci. USA, 96, 1135 (1999)]. Neoxanthin is thought to have two key roles: (1.) as part of LHC's and (2.) as a precursor to the plant growth hormone abscisic acid (ABA). Its replacement by lutein-5,6-epoxide in C. reflexa without compromising the plant's light-harvesting capacity suggests that its role in ABA formation is more important. Loss of neoxanthin could be an early step in the evolution of parasitism in plants. Lutein-5,6-epoxide may also have an additional role in C. reflexa's LHC. It appears to be involved in a light-driven cycle that mimics but does not replace a similar cycle involving the carotenoid violaxanthin. The violaxanthin cycle protects plants from damage by excessive light and occurs in all higher plants.


Abscisic Acid (Plant Growth Regulator - an abscission accelerating plant hormone)


The earth's biosphere derives its energy from sunlight through photosynthesis. Plants, algae and photosynthetic bacteria, have developed efficient systems to harvest the light of the sun and use this energy to drive the metabolic reactions required for life, such as the reduction of carbon dioxide to sugar. The ubiquitous green color of plants, due to chlorophyllic pigments, is a key molecular participant in the light harvesting of plants. While the chlorophylls are efficient in absorbing the red and blue portions of the light spectrum, they do not efficiently absorb other parts of the sunlight spectrum. More hidden in growing plants is a second participating molecular class, carotenoids, which can absorb other portions of the strectrum. In green leaves the color of the carotenoids is masked by the much more abundant chlorophylls while in red ripe tomatoes or petals of yellow or orange flowers, the carotenoids predominate. Chlorophyll molecules exist in two different chemical structures in various photosynthetic organisms, as chlorophyll a or b in plants or algae, and as bacteriochlorophyll a or b in photosynthetic bacteria. Molecules such as the chlorophylls and carotenoids that absorb light and impart color to living matter and other materials are the biological pigments. In general, biological pigments are non-covalently bound to proteins, forming the so-called pigment-protein complexes. The pigment-protein complexes are organized as the photosynthetic unit (PSU). The bacterial PSU consists of two types of pigment-protein complexes: the photosynthetic reaction centers and the light-harvesting complexes. The main function of the light-harvesting complexes is to gather light energy and to transfer this energy to the reaction centers for the photo-induced redox processes.

Much of the knowledge about the Carotenoid LHC's was originally derived from studies on purple photosynthetic bacteria that reside at great depth below the waters surface. [see Cogdell R.J., Isaacs N.W., Freer A.A., Arrelano J., Howard T.D., Papiz M.Z., Hawthorn-thwaite-Lawless A.M. & Prince S.M. (1997) The Structure and function of the LH2 (B800-850; complex from the purple bacteria Rhodopseudomonas acidophila strain 10050. Prog. Biophys. molec. Biol. Vol 68 No. 1. pp 1-27; see also NATURE (6th.April 1995 Vol.374, 517).].

The following links provide a more in depth review of LHC's:

LIGHT HARVESTING COMPLEX II -stunning photographs from NATURE of the molecular structure

Light Harvesting in Bacterial Photosynthesis - an excellent overview


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