Functional assignment of chromophores and energy transfer in C phycocyanin isolated from the thermophilic cyanobacterium Mastigocladus laminosus

Abstract

The optical characteristics and pathway of energy transfer in the C phycocyanin trimer isolated from the thermophilic cyanobacterium Mastigocladus laminosus were investigated at steady state by absorption, circular dichroism, fluorescence and fluorescence polarization spectroscopy. Based on the comparison of optical data with the 3-dimensional structure of the C-phycocyanin trimer determined by X-ray analysis (Schirmer, T., Bode, W., Huber, R., Sidler, W. and Zuber, H. (1984) in Proceedings of the Symposium on Optical Properties and Structure of Tetrapyrroles, (Blauer, G. and Sund, M., eds.), pp. 445–449, Walter de Gruyter, Berlin, and (1985) J. Mol. Biol. 184, 257–277), the functional assignment of three types of chromophore was established. An α subunit has an s chromophore and the chromophores at the positions 84 and 155 in the amino acid sequence of the β subunit are assigned as f and s chromophores, respectively. In the C phycocyanin trimer energy transfer occurs from the α chromophore in one monomer to the β f chromophore in an adjacent monomer, and from the β s chromophore to the β f chromophore in the same monomer. The direction of energy flow is from the outside to the inside of the trimer, where the locus for the binding of a colourless polypeptide is postulated. In the phycobilisomes the energy concentrated at the β f chromophores might be transferred toward the allophycocyanin core mainly by the β f chromophores in the phycocyanin rods.