The cryptophytes are a group of photosynthetic organisms that have derived their chloroplasts from an endosymbiosed red alga. The photosynthetic antenna of the red alga, the phycobilisome, had subsequently been ripped apart in the cryptophytes. The red alga phycobilisome derived protein subunit (beta subunit) then complexed with peptides derived from the endosymbiont (alpha subunits) and was shuttled into the thylakoid up to a high density. These new quaternary structures assemble into a heterodimer of dimers referred to as the phycobiliproteins—each dimer composed of a globin-folded beta subunit core with one of the alpha subunits completing the structure.
The phycobiliproteins are a group of light harvesting antennae proteins, varied both into their solar spectral coverage and in their energy transfer rates. Each contains eight, covalently-linked linear tetrapyroles as their light harvesting chromophores. Furthermore, phycobiliproteins fall into two classes: closed—where the two dimers and the chromophores along the dimeric seam are in van der Waals contact; and open—where the two dimer halves are separate by a central, water-filled hole moving the chromophores out of van der Waals contact. The latter is formed by a single residue insertion in the alpha subunit and produces a reduced energy transfer rate.
Differences in spectral coverage between phycobiliproteins can largely be explained by mixing and matching chromophores, however, there are examples of differences in absorption spectra between proteins with the same chromophore complement and arrangement. The protein does not directly harvest sunlight, instead it tunes the absorption properties (excitation energy) of the chromophores setting chromophore conformations and their local electronic environment (electric field). Merely by mixing and matching alpha chains, energy transfer can be tuned. Broadly, with such a large complement of antennae, how is energy funnelled in space to the reaction centre? Furthermore, why keep proteins with a decreased transfer rate?