Allophycocyanin is a protein from the phycobiliprotein family, as well as phycocyanin, phycoerythrin, and phycoerythrin. It is a secondary pigment of chlorophyll. All phycobiliproteins are water-soluble, so they cannot exist in the membrane like carotenoids. Instead, they aggregate to form clusters on the membrane, called phycobilisomes. Phycocyanin absorbs and emits red light (maximum 650 and 660 nm, respectively) and is easily found in cyanobacteria (also known as blue-green algae) and red algae. Phycobilitin pigments have fluorescent properties for use in immunoassay kits. In flow cytometry, it is usually the abbreviation of APC. For effective use in applications such as FACS, high-throughput screening (HTS), and microscopy, APC requires chemical crosslinking. Phycocyanin can be isolated from a variety of red or blue-green algae, each of which produces slightly different molecular forms. It consists of two different subunits (α and β), each of which has a phycocyanin (PCB) chromophore. The subunit structure of APC has been identified as (αβ) 3. APC has a molecular weight of 105,000 Daltons.
Allophycocyanin, a type of phycobilirubin, is the same as phycocyanin in the cyanobacteria, red algae, and cryptophyta. Monomers consist of an alpha peptide chain and a beta peptide chain, with a molecular weight of approximately 30,000, and each peptide chain is associated with a molecule of phycocyanin. is a type of phycobilirubin. Like phycocyanin, it is a photosynthetic pigment commonly found in cyanobacteria, red algae, and cryptocrypts. Monomers consist of an alpha peptide chain and a beta peptide chain, with a molecular weight of approximately 30,000, and each peptide chain is associated with a molecule of phycocyanin. The absorption spectrum of the trimer has obvious absorption peaks at 650 nanometers, side absorption peaks at 620 nanometers, absorption bands at 360 and 306 nanometers in the near ultraviolet part, and protein absorption peaks at 278 nanometers. Both cyanobacteria and red algae are the same. Allophycocyanin is coordinately bound to the thylakoid side of the phycobiliprotein body, and has the function of transmitting the excitation energy of phycoerythrin and phycocyanin to the chlorophyll alpha of the thylakoid. As mentioned above, in order for APC to be useful in immunoassays, it must be chemically cross-linked to prevent APC from dissociating into its constituent subunits in common physiological buffers. The conventional method for achieving this is through a destructive process, in which the treated APC trimer is chemically destroyed in 8M urea and then reassociated in physiological buffer. Another method can be used that preserves the structural integrity of the APC trimer and provides a brighter, more stable end product.
Figure 1. Allophycocyanin dodekamer, Gloeobacter violaceus (PDB: 2vjt).
Many applications and instruments have been developed specifically for allophycocyanin. It is commonly used for immunoassays by time-resolved fluorescent fluorescence energy transfer (TR-FRET) analysis, such as FACS, flow cytometry, and high-throughput screening, as a Euro receptor.