Abstract
Photosystem I of higher plants is characterized by red-shifted spectral forms deriving from chlorophyll chromophores. Each of the four Lhca1 to -4 subunits exhibits a specific fluorescence emission spectrum, peaking at 688, 701, 725, and 733 nm, respectively. Recent analysis revealed the role of chlorophyll-chlorophyll interactions of the red forms in Lhca3 and Lhca4, whereas the basis for the fluorescence emission at 701 nm in Lhca2 is not yet clear. We report a detailed characterization of the Lhca2 subunit using molecular biology, biochemistry, and spectroscopy and show that the 701-nm emission form originates from a broad absorption band at 690 nm. Spectroscopy on recombinant mutant proteins assesses that this band represents the low energy form of an excitonic interaction involving two chlorophyll a molecules bound to sites A5 and B5, the same protein domains previously identified for Lhca3 and Lhca4. The resulting emission is, however, substantially shifted to higher energies. These results are discussed on the basis of the structural information that recently became available from x-ray crystallography (Ben Shem, A., Frolow, F., and Nelson, N. (2003) Nature 426, 630-635). We suggest that, within the Lhca subfamily, spectroscopic properties of chromophores are modulated by the strength of the excitonic coupling between the chromophores A5 and B5, thus yielding fluorescence emission spanning a large wavelength interval. It is concluded that the interchromophore distance rather than the transition energy of the individual chromophores or the orientation of transition vectors represents the critical factor in determining the excitonic coupling in Lhca pigment-protein complexes.
Highlights
Photosystems I and II have a common general organization including a core complex moiety, binding Chl1 a and -caro
It should be noticed that Lhca bears His as a ligand for Chl A5, yet its fluorescence emission is red-shifted by 20 nm with respect to the highly homologous subunit Lhcb6 (CP24) and other Lhcb proteins, opening the question of whether a different mechanism of modulation for the physico-chemical properties of chromophores is at work in Lhca2
We proceeded with the analysis of the chromophore organization in Lhca2 and identified the absorption band responsible for the 701-nm emission band, with the aim of understanding the mechanism that allows modulation of the physicochemical properties of chlorophyll ligands
Summary
Chlorophyll; FWHM, full-width half-maximum; LD, linear dichroism; LHCI and LHCII, light-harvesting complex I and II, respectively; PSI and PSII, photosystem I and II, respectively; WT, wild type. LHCI, the outer antenna of PSI, is the most red form-enriched compartment and is composed of four different complexes, the products of the genes lhca to -4 [2]. By performing a detailed analysis by biochemical and spectroscopic methods on WT and mutant Lhca recombinant proteins, we conclude that the fluorescence emission band at 701 nm derives from excitonic coupling between chromophores A5 and B5, despite the presence of a His ligand for Chl A5. These results are discussed on the basis of the structural information that re-. We suggest that in green plant LHCI, differences in interchromophore distances rather than in transition energies of the individual chromophores of the interacting pair or in the orientation of their transition vectors represent the critical factor in determining excitonic coupling
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