Comparative Biochemistry of Photosynthetic Light-Harvesting Systems

Biochemical composition and organization of higher plant photosystem II light-harvesting pigment-proteins.

Light guides. Directional energy transfer in a photosynthetic antenna.

Phycobilisome a macromolecular complex optimized for light energy transfer

Phycobilisomes serve as the primary light-harvesting antennae for Photosystem II in cyanobacteria and red algae. These supramolecular complexes are primarily composed of phycobiliproteins, a brilliantly colored family of water-soluble proteins bearing covalently attached, open-chain tetrapyrroles known as phycobilins. In addition, phycobilisomes also contain smaller amounts ‘linker polypeptides,’ most of which do not bear chromophores. These components are absolutely required for proper assembly and functional organization of the structure. Phycobilisomes are constructed from two main structural elements: a core substructure and peripheral rods that are arranged in a hemidiscoidal fashion around that core. The core of most hemidiscoidal phycobilisomes is composed of three cylindrical subassemblies. The peripheral rods radiate from the lateral surfaces of the core substructure which are not in contact with the thylakoid membrane. Absorbed light energy is transferred by very rapid, radiation-less downhill energy transfer from phycoerythrin or phycoerythrocyanin (if present) to C-phycocyanin and then to allophycocyanin species that act as the final energy transmitters from the phycobilisome to the Photosystem II or (partially) Photosystem I reaction centers.

Phycobilisome and Phycobiliprotein Structures

Dating from the Pre-Cambrian era, cyanobacteria have a long history of adaptation to the Earth's environment. By evolving oxygen via photosynthetic reactions similar to those of plants and green algae, these prokaryotes were essential to the evolution of the present biosphere. They continue to make a large contribution to the equilibrium of the Earth's atmosphere by production oxygen and removing carbon dioxide. To survive in extreme or variable environments, cyanobacteria have developed specific regulatory systems, in addition to more general mechanisms equivalent to those of other prokaryotes or photosynthesis eukaryotes. Specific regulatory systems control the differentiation of specialized nitrogen-fixing cells and of cell types facilitating the dispersion of species. In the past decade, considerable progress has been made towards understanding the expression of the cyanobacterial genome in response to variations in the intensity and spectral quality of incident light and in response to nutritional conditions, especially carbon, nitrogen and sulphur sources. These studies have provided insights into the relationships between carbon and nitrogen intermediary metabolism, and a start towards understanding of the interconnected pathways which lead from the perception of environmental signals to the regulation of enzyme activities and gene expression. Cyanobacterial regulatory mechanisms share common features with those of other prokaryotes, but are unique since these essentially photo-autotrophic organisms must maintain a proper cellular C/N balance, in spite of dailty variations in incident light. Thus an appropriate coordination between photosynthesis and other metabolic processes must be achieved through control of the catalytic activity of key enzymes by reducing equivalents and ATP produced by photosynthetic or respiratory electron transport. Recently discovered kinases/phosphatases act by post-translational modification of specific proteins which probably act as signal transducers or modulators of gene expression in a manner similar to the well-known two-component regulatory systems described in other bacteria. In this overview, we present our current knowledge on the molecular aspects of the biology of cyanobacteria, as well as on their mechanisms of resistance to metal ions and their responses to metabolic stress.

Comparative Biochemistry of Photosynthetic Light-Harvesting Systems

Citations

Biochemical composition and organization of higher plant photosystem II light-harvesting pigment-proteins.

Light guides. Directional energy transfer in a photosynthetic antenna.

Phycobilisome a macromolecular complex optimized for light energy transfer

Phycobilisome and Phycobiliprotein Structures

Adaptation of cyanobacteria to environmental stimuli: new steps towards molecular mechanisms

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