Phycocyanin is a pigment-protein complex from the light-harvesting phycobiliprotein family, along with allophycocyanin and phycoerythrin. It is an accessory pigment to chlorophyll, found in cyanobacteria (also called blue-green algae). Because all phycobiliproteins are water-soluble, they cannot exist within the phospholipid cell membranes like carotenoids can. Instead, phycobiliproteins aggregate to form clusters that adhere to the membrane, called phycobilisomes. The term "phycocyanin" come from the Greek phyco meaning "algae" and cyanin is from the English word "cyan", which conventionally means a shade of blue-green (close to aqua) and is also derived from the Greek "kyanos", which means a somewhat different color of dark color. Phycocyanin has a characteristic light blue color, absorbing orange and red light, particularly the 620 nm wavelength (depending on which specific type it is), and emits fluorescence at about 650 nm (also depending on which type it is). The related allophycocyanin absorbs and emits at longer wavelengths than phycocyanin C or phycocyanin R. C-phycocyanin is often found in cyanobacteria that thrive around hot springs, as it can be stable up to around 70 °C (158 °F), with identical spectroscopic (light absorbing) behaviours at 20 °C (68 °F) and 70 °C. Thermophiles contain slightly different amino acid sequences making it stable under these higher conditions. The phycobiliproteins are made of two subunits (alpha and beta) having a protein backbone to which 1–2 linear tetrapyrrole chromophores are covalently bound, with a molecular weight around 30,000 Da. Stability of this protein in vitro at these temperatures has been shown to be substantially lower. Photo-spectral analysis of the protein after 1 min exposure to 65 °C conditions in a purified state demonstrated a 50% loss of tertiary structure. Phycobiliproteins have fluorescent properties that are used in immunoassay kits. The phycocyanin produced by Aphanizomenon flos-aquae and Spirulina, for example, is used in the food and beverage industry as the natural coloring agent 'Lina Blue' or 'EXBERRY Shade Blue' and is found in sweets and ice cream. In addition, fluorescence detection of phycocyanin pigments in water samples is a useful method to monitor cyanobacteria biomass.
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)3, which have rotational symmetry and a central channel. Trimers aggregate in pairs to form hexamers (αβ)6, 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 21, 2023, there are 310 crystal structures of phycocyanin deposited in the Protein Data Bank.
Spectral characteristics C-phycocyanin has a single absorption peak at ~621 nm, varying slightly depending on the organism and conditions such as temperature, pH, and protein concentration in vitro. Its emission maximum is ~642 nm. This means that the pigment absorbs orange light, and emits reddish light. R-phycocyanin has an absorption maxima at 533 and 544 nm. The fluorescence emission maximum of R-phycocyanin is 646 nm.
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