Allophycocyanin

Blue-green algae such as Spirulina maximize their light harvesting ability by using phycobiliproteins to absorb light over a broader spectrum. One of these proteins, allophycocyanin, can be seen on the right. It contains a chromophore called .

Allophycocyanin is a primarily protein. It contains , which each have one phycocyanobilin. Allophycocyanin has a complex quaternary structure. First, form in a circular fashion, then on top of each other to form an antenna-like structure called the phycobilisome.

Interestingly, the λmax of the chromophore can be tuned depending upon the protein binding it. For allophycocyanin, the λmax is 650 nm; in another phycobiliprotein, phycocyanin, the λmax is 625 nm, even though it uses the same chromophore. Phycocyanobilin is a highly flexible, linear tetrapyrrole that is covalently attached to the protein by a to a cysteine in the protein. Allophycocyanin holds the in place through a number of intermolecular interactions. forms hydrogen bonds with an amine in the pigment, while interacts with a carboxylic acid that is exposed to the solvent. A key also holds the molecular planar. These interactions stabilize the pigment in the protein and hold the pigment planar. When the protein is unfolded, the pigment is no longer planar, and loses the ability to transfer red light to chlorophyll.^[1] For the most part, residues (shown in grey) tend to be buried in the middle of the protein or along interaction surfaces.