Structure, function and applications of phycoerythrin: A unique light harvesting protein in algae

Photosynthesis is an ancient and important chemical reaction process. Under light conditions, plants, algae, and certain bacteria convert solar energy into chemical energy. And then, chemical energy is directly used by living cells. A special light-harvesting pigment protein, phycobiliprotein (PBP), exists in red algae, cyanobacteria (blue-green algae), and some cryptophytes to capture light energy in the water environment. Since the first discovered of PBP, scientists have conducted in-depth research on the structure, function and application of PBP. PBP is a water-soluble protein. It is formed by the covalent connection of apoprotein and phycobilin. According to the spectral characteristics, PBP can be divided into four types: Phycoerythrin (PE), phycocyanin (PC), allophycocyanin (APC) and phycoerythrocyanin (PEC). Through the coordination of four PBPs, more than 95% of the energy can be efficiently transmitted. PE is the beginning of the energy transfer of the PBP system. It is composed of three subunits alpha, beta and gamma, and the molecular weight is about 240 kD. Among them, the gamma subunit has the function of connecting and stabilizing, so that PE can exist in a stable (alphabeta) 6gamma form. PE can covalently connect chromophores such as phycoerythrobilin (PEB) and phycourobilin (PUB), and has a strong absorption efficiency for short-wavelength blue and green light. It enables red algae and cyanobacteria to efficiently capture and transfer light energy in the deep blue and green light environment. PE is a bright red fluorescent protein. When excited by a specific wavelength, PE will emit strong fluorescence. Foodgrade PE can be used as a natural colorant in food, cosmetics and other fields. High-purified PE is combined with proteins such as biotin and monoclonal antibodies to make fluorescent probes for immunodetection and other technologies; PE can also be used as a new photosensitizer for photodynamic therapy of deep tissues. In addition, PE has also been proven to have anti-oxidant and anti-inflammatory biological activities, and has a certain therapeutic effect on Alzheimer's disease, cancer, diabetes, liver and kidney toxicity and other diseases. But there is no doubt that PE has great potential in the field of biotechnology. The application of PE is closely related to its purity. China's abundant red algae resources provide a large amount of raw materials for large-scale preparation of PE. However, there are many obstacles in the preparation of highpurified PE. The most commonly used column chromatography has the problem of expansion, while other purification processes are not mature. In view of the above problems, this review highlights an overview on the structure, function and purification process of PE. Then the applications of PE in optical and biological activities are reviewed and prospected. This article provides a reference for the high value processing and application of marine algae