Allophycocyanins

Allophycocyanins are a group of phycobiliproteins, which are light-harvesting pigments found in cyanobacteria and red algae. They are responsible for the blue-green color of these organisms. Allophycocyanins are composed of a protein moiety and a chromophore, which is a tetrapyrrole molecule derived from biliverdin. The chromophore is attached to the protein via a thioether linkage to a cysteine residue.

Allophycocyanins are part of the phycobilisome, a large protein complex that functions as a light-harvesting antenna in these organisms. They are responsible for absorbing light in the blue and green regions of the spectrum and transferring the energy to the reaction center, where it is used to drive photosynthesis.

Allophycocyanins are also involved in the regulation of photosynthesis. They can undergo a reversible conformational change in response to changes in light intensity, which allows them to regulate the amount of light that is absorbed by the phycobilisome. This helps to protect the organism from photoinhibition, which is damage to the photosynthetic apparatus caused by excessive light.

Allophycocyanins are also found in some eukaryotic algae, such as diatoms and brown algae. However, their function in these organisms is not as well understood as it is in cyanobacteria and red algae.