- Phycobilisome
Phycobilisomes are light harvesting antennae of
photosystem II incyanobacteria ,red algae andglaucophytes .General structure
Phycobilisomes are protein complexes (up to 600
polypeptides ) anchored tothylakoid membranes. They are made of stacks of chromophorylated proteins, thephycobiliprotein s, and their associated linker polypeptides. Each phycobilisome consists of a core made ofallophycocyanin , from which several outwardly oriented rods made of stacked disks ofphycocyanin and (if present)phycoerythrin (s) orphycoerythrocyanin . The spectral property of phycobiliproteins are mainly dictated by theirprosthetic group s, which are lineartetrapyrrole s known asphycobilin s includingphycocyanobilin ,phycoerythrobilin ,phycourobilin andphycobiliviolin . The spectral properties of a given phycobilin is influenced by its protein environment.Function
Each phycobiliprotein has a specific absorption and fluorescence emission maximum in the visible range of light. Therefore, their presence and the particular arrangement within the phycobilisomes allow absorption and unidirectional transfer of light energy to
chlorophyll "a" of the photosystem II. In this way, the cells take advantage of the available wavelengths of light (in the 500-650 nm range), which are inaccessible to chlorophyll, and utilize their energy for photosynthesis. This is particularly advantageous deeper in thewater column , where light with longer wavelengths is less transmitted and therefore less available directly to chlorophyll.The geometrical arrangement of a phycobilisome is very elegant and results in 95% efficiency of energy transfer.
Evolution and diversity
There are many variations to the general phycobilisomes structure. Their shape can be hemidiscoidal (in cyanobacteria) or hemiellipsoidal (in red algae).
The phycobiliproteins themselves show little sequence evolution due to their highly constrained function (absorption and transfer of specific wavelengths). In some species of cyanobacteria, when both phycocyanin and phycoerythrin is present, the phycobilisome can undergo significant restructuring as response to light color. In green light the distal portions of the rods are made of red colored phycoerythrin, which absorbs green light better. In red light, this is replaced by blue colored phycocyanin, which absorbs red light better. This reversible process is known as complementary chromatic adaptation.
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