Phycocyanin Chemical Structure
Phycocyanin shares a common structural theme with all phycobiliproteins.The structure begins with the assembly of phycobiliprotein monomers, which are heterodimers composed of α and β subunits, and their respective chromophores linked via thioether bond.
Each subunit is typically composed of eight α-helices. Monomers spontaneously aggregate to form ring-shaped trimers (αβ), which have rotational symmetry and a central channel. Trimers aggregate in pairs to form hexamers (αβ), sometimes assisted with additional linker proteins. Each phycobilisome rod generally has two or more phycocyanin hexamers. Despite the overall similarity in structure and assembly of phycobiliproteins, there is a large diversity in hexamer and rod conformations, even when only considering phycocyanins. On a larger scale phycocyanins also vary in crystal structure, although the biological relevance of this is debatable.
As an example, the structure of C-phycocyanin from Synechococcus vulcanus has been refined to 1.6 Angstrom resolution. The (αβ) monomer consists of 332 amino acids and 3 thio-linked phycocyanobilin (PCB) cofactor molecules. Both the α- and β-subunits have a PCB at amino acid 84, but the β-subunit has an additional PCB at position 155 as well. This additional PCB faces the exterior of the trimeric ring and is therefore implicated in inter-rod energy transfer in the phycobilisome complex. In addition to cofactors, there are many predictable non-covalent interactions with the surrounding solvent (water) that are hypothesized to contribute to structural stability.
R-phycocyanin II (R-PC II) is found in some Synechococcus species. R-PC II is said to be the first PEB containing phycocyanin that originates in cyanobacteria. Its purified protein is composed of alpha and beta subunits in equal quantities. R-PC II has PCB at beta-84 and the phycoerythrobillin (PEB) at alpha-84 and beta-155.
As of March 7, 2018, there are 44 crystal structures of phycocyanin deposited in the Protein Data Bank.
Phycocyanin can have a potent potential as a drug in a wide range of clinical applications. Phycocyanin shows a wide range of pharmacological effects, with anti-oxidation, anti-cancer, anti-inflammatory activity, photo-induced cytotoxicity and stimulating the immune system.
Phycocyanin plays an antioxidant role in inhibiting hepatic lipid peroxidation and being helpful to liver protection. Phycocyanin also scavenges free radicals from damaged nerve cells, which could avoid DNA oxidative damage cause from free radicals and prevent neuronal cell apoptosis.
Phycocyanin in plants
Pigments are chemical compounds which reflect only certain wavelengths of visible light. This makes them appear "colorful". Flowers, corals, and even animal skin contain pigments which give them their colors. More important than their reflection of light is the ability of pigments to absorb certain wavelengths.
Because they interact with light to absorb only certain wavelengths, pigments are useful to plants and other autotrophs --organisms which make their own food using photosynthesis. In plants, algae, and cyanobacteria, pigments are the means by which the energy of sunlight is captured for photosynthesis. However, since each pigment reacts with only a narrow range of the spectrum, there is usually a need to produce several kinds of pigments, each of a different color, to capture more of the sun's energy.
Phycocyanin molecular weight
Sedimentation and immunodiffusion experiments indicate that the molecular weight for the minimum molecular unit of C-phycocyanin is 30,000. This result agrees with an analysis of available data on amino acid content for C-phycocyanins from several different algae.
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