Antenna Systems and Energy Transfer in Cyanophyta and Rhodophyta

Abstract

The molecular architecture and energy transfer processes in the phycobilin-chlorophyll antenna systems of Cyanophyta, Glaucophyta, Rhodophyta and Cryptophyta are discussed with an emphasis on the molecular structures of the individual phycobiliprotein building blocks. The assembly of phycobilisomes from phycobiliproteins is explained, from the binding of chromophores to the individual apoproteins to the binding of phycobilisomes to thylakoid membranes. Structure-function relationships in phycobiliproteins are discussed in the light of the crystal structures and a normal-mode analysis. The normal-mode analysis helps to clarify aspects of the protein structure that are critical for determining the optical properties of phycobiliproteins. A gradient of energy levels in phycobilisomes is realized by binding of specific linker polypeptides to common building blocks. Energy-transfer processes are clearly shown by the time-resolved fluorescence spectra of intact cells and isolated phycobilisomes. The rate of energy transfer between weakly interacting chromophorcs can be described by the Forster mechanism, and the predictions agree well with the experimental results. Evolution of phycobiliproteins seems to have increased both the number of chromophorcs per unit monomer and the range of energy levels in the complex, so as to capture an increasing fraction of the available solar energy. Further analyses on the basis of crystal structures should lead to an improved understanding ofhow interactions of the chromophores with the protein maximize the efficiency of light capture.