Light-harvesting complex

A light-harvesting complex is one or more polypeptide chains containing photosynthetic pigments, which surrounds a photosynthetic reaction centre and focuses light inward toward its core. Many such proteins have an alpha solenoid tertiary structure.

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Light-harvesting complexes in bacteria

Main article: Bacterial antenna complex

Light-harvesting complexes in plants

Main article: Light-harvesting complexes of green plants

Chlorophylls and carotenoids are important in light-harvesting complexes present in plants. Chlorophyll b is almost identical to chlorophyll a except it has a formyl group in place of a methyl group. This small difference makes chlorophyll b absorb light with wavelengths between 400 and 500 nm more efficiently. Carotenoids are long linear organic molecules which have alternating single and double bonds along their length. Such molecules are called polyenes. Two examples of carotenoids are lycopene and β-carotene. These molecules also absorb light most efficiently in the 400 – 500 nm range. Due to their absorption region, carotenoids appear red and yellow and provide most of the red and yellow colours present in fruits and flowers.

The carotenoid molecules also serve a safeguarding function. Carotenoid molecules suppress damaging photochemical reactions, particularly those including oxygen, which exposure to sunlight can cause. Plants that lack carotenoid molecules quickly die upon exposure to oxygen and light.

The chlorophylls and carotenoids present in the light-harvesting complexes are referred to as accessory pigments. These accessory pigments are held inside the light harvesting proteins in a highly uniform fashion. The light-harvesting complexes are cylindrical in form and come in two sizes in alga: LH-1 which are large and completely surround the reaction centre and LH-2 which are smaller and are arranged in a ring structure around the LH-1 complex. The relative size and positioning of the proteins is shown in the image above.

In green plants the story is a little different, as the light-harvesting complexes do not form nice simple geometric shapes with respect to one another. Green plants contain chloroplasts which house the photosynthetic apparatus. Chloroplasts contain the thylakoid membrane, which holds the pigment protein complexes, known as LHC, light-harvesting complexes, I and II. The tightly spaced regions in the thylakoid membrane, known as grana, hold predominantly LHCII, which is the most abundant pigment-protein complex in green plants.

Phycobilisome

Little blue or red light reaches algae which reside at a depth of 1 metre or more in seawater, as this light is absorbed by seawater and fluorescent pigments of photosynthetic organisms above. A phycobilisome is a light-harvesting protein complex present in cyanobacteria, glaucocystophyta, and red algae. Fluorescent pigments, which are linked to the peptide chain absorb green light or red light. Other pigments which are present in the bacterial photosynthetic reaction centers, like bacteriochlorophyll and bacteriopheophytin do not absorb light in these regions. The fluorescent pigments which are present in the phycobilisome, such as phycocyanobilin and phycoerythrobilin re-emit the green light in regions which the other photosynthetic pigments can absorb.

The geometrical arrangement of a phycobilisome is very elegant and results in 95% efficiency of energy transfer. There is a central core of allophycocyanin which sits above the photosynthetic reaction center. There are phycocyanin and phycoerythrin subunits which radiate out from this center like thin tubes. This increases the surface area of the absorbing section and helps focus and concentrate light energy down into the reaction center. The energy transfer from exited electrons absorbed by pigments in the phycoercythrin subunits at the periphery of these antennas appears at the reaction centre in less than 100 ps.

A sculpture depicting LH-2

German-American artist Julian Voss-Andreae has created a sculpture called "Light-Harvesting Complex" (2003) based on the light-harvesting complex LH-2 found in Rhodopseudomonas acidophila^[1][2].

References

1. ^ Zardecki, Christine (February 2007). "Julian Voss-Andreae, Protein Sculptor". RCSB Protein Data Bank Newsletter 32: 9–11.
2. ^ Hanna, Raven (February 2006). "Gallery: Julian Voss-Andreae". Symmetry Magazine 3 (1): 26–29.