The Phycobilisomes

Cyanobacteria contain large membrane attached but extrinsic antenna complexes, the phycobilisomes. An artistic representation of the phycobilisomes is shown in Figure 1.4. The structures of all major pigment-containing proteins of the phycobilisomes have been determined [20, 41, 42] and are described in Chapter 11. It should be noted that no reports have been published on the crystallization and structure determination of the intact phycobilisome complex including the linkers, nor has the site where the phycobilisomes are attached to the photosystems been identified. However, X-ray pictures of the pigmented proteins and the biochemical as well as the biophysical evidence in combination with electron microscopy have made it possible to assemble a picture of the structural organization of the phyco-bilisome antenna.

The phycobilisome is probably the most efficient antenna system in nature as it allows the organism to fill in the "green gap" and use the full spectrum of visible light for photosynthesis. Phycobilisomes achieve this goal by using the unique feature of the phycobilisome pigments, which absorb strongly in the region 550-660 nm, thereby complimenting the spectral region covered by chlorophyll a in the blue and the red regions. The phycobilisomes mainly serve as peripheral antenna for Photosystem II, but can also move to Photosystem I in a process of state transitions. These are very fast processes that take place within a few seconds, thereby balancing the light capturing capacity in cyanobacteria between the two photosystems.

Figure 1.4 Artistic picture of the potential structural organization of the phycobilisome antenna. This picture shows the potential organization of the phycobilisome antenna system on top of the cyanobacterial Photosystem II structure. Three trimers of allophycocyanin from Porphyra yezoensis (1KN1, [39]) are shown at the core of the complex. The rods are depicted by the structure of phycocyanin from Gracilaria chilensis (2BV8, [40]). The alpha chains of phycocyanin are depicted in cyan, the beta chains in blue. Please note that this is a schematic picture, where important elements, such as the linker proteins are missing and the overall organization of the intact phycobilisome is only known at low resolution from electron microscopy. This picture is not an X-ray structure but combines high resolution X-ray structures of parts of the complex with an artist's rendition of the complex.

Figure 1.4 Artistic picture of the potential structural organization of the phycobilisome antenna. This picture shows the potential organization of the phycobilisome antenna system on top of the cyanobacterial Photosystem II structure. Three trimers of allophycocyanin from Porphyra yezoensis (1KN1, [39]) are shown at the core of the complex. The rods are depicted by the structure of phycocyanin from Gracilaria chilensis (2BV8, [40]). The alpha chains of phycocyanin are depicted in cyan, the beta chains in blue. Please note that this is a schematic picture, where important elements, such as the linker proteins are missing and the overall organization of the intact phycobilisome is only known at low resolution from electron microscopy. This picture is not an X-ray structure but combines high resolution X-ray structures of parts of the complex with an artist's rendition of the complex.

The structure of the entire phycobilisome can be described as a set of rod like stacks of disks that radiate from a central core of close-packed disks. The number of disks in the core and the height of the stacks is species dependent. There exists experimental evidence that allophycocyanin (APC) might be preferentially located in the core, while phycocyanin (PC) forms the inner part of the rods and phycoery-thrin (PE) is located at the periphery. This arrangement leads to a funneling effect which allows energetically downhill excitation energy transfer from the highest energy pigments in PE at the periphery via PC to allophycocyanin in the central core of the phycobilisome. Figure 1.4 is an artistic overview of the proposed structural organization of the phycobilisome on top of the PSII dimer. All phycobilip-roteins have the same general structural features. They consist of a heterodimer composed of two homologous subunits, a and P- which form an (a/P) heterodimer. Each subunit forms a compact globular structure that consists of six a-helices. The cofactors are covalently bound to the protein.

In addition to the core a/P structure, all phycobiliproteins contain two additional a-helices that extend out from the core and serve as the assembly interface of the monomer. Most of the phycobiliproteins form larger oligomeric assemblies. The most abundant are the (aP) 3 trimers, which form coin type structures that can come together into even larger assemblies, which can finally lead to the formation of the central core and the large rods, with the help of the linker proteins.

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