BILIPROTEINS
by Jameelah H. Aziz and Irene T. Lam
Photosynthesis is initiated by the absorption of photons by various pigments and the migration of these excitons to the photosynthetic reaction centers. The light-harvesting pigments are typically complexes of proteins and chromophores; a particular pigment the
biliprotein , phycoerythrin 545, has been selected to study the organization of its chromophores.
Phycoerythrin 545, used in this study, is found in the marine cryptomonad , Rhodomonas lens. It is isolated as an alpha_2 beta_2 protein, having two or more different alphapolypeptides distributed in the purified proteins. Each beta polypeptide has covalently-bonded phycoerythrobilins , and each alpha polypeptide has one covalently-bonded cryptobilin 562 chromophore. The structure of cryptobilin 562 has been obtained by mass spectrometry and NMR methods to be 15,16-dihydrobiliverdin. The beta polypeptide has only phycoerythrobilin, but the exact number of chromophores has not been determined. Excitons migrate very rapidly through biliproteins and, for a purified protein, most emission occurs from lowest energy chromophores.
The mechanism of energy transfer between pairs of chromophores is of interest. There are two general possibilities that can be approached, although there are many additional intricacies. It can be considered that excitons pass from one chromophore to another by Forster very weak dipole-dipole coupling, or certain pairs of chromophores are located close enough together so that their dipoles interact to produce exciton splitting. For exciton splitting, the pair of chromophores is considered to have the excitation energy delocalized between them. The spectrum of monomers is split into high- and low- energy bands for the pair, and the movement of excitation from high- to low- energy band may be designated as internal conversion. In very weak coupling, the donor and acceptor tend to retain the monomeric spectra.
Circular dichroism (CD) spectroscopy is uniquely effective in studies of exciton splitting. A pair of chromophores exhibiting exciton splitting will have a characteristic CD spectrum. This spectrum will be conservative having two bands of equal rotational strength, one positive and one negative. Therefore, CD together with absorption spectroscopy and various biochemical strategies will be used to investigate the types of chromophore interactions for phycoerythrin 545. Previous applications of CD methods to study of biliproteins have been reviewed. The number of chromophores located on the protein was also determined.
Why is this interesting?
The goal of this experiment is to determine the number of chromophores in the a and b polypeptides. The next step is to isolate the function of the chromophores and how it drives the photosynthetic mechanism in the biliproteins. Since Rhodomonas lens exists at such low depth withing the ocean floor, it must be extremely sensitive to the light in order to use photosynthesis. The human body has been found to have tetrapyrroles or chromophores which are vital in photosynthesis. The long term goal is to try to use the human's tetrapyrroles to produce photosynthetic behavior in the human body. Of course, this objective is still being research and the chance of trying to achieve it might be remote.