In cyanobacteria, phycobilisomes (PBS) act as antenna of the photosynthetic pigment apparatus. They contain brightly colored phycobiliproteins (PBP) and form giant supramolecular complexes (up to 3000-7000 kDa) containing 200 to 500 phycobilin chromophores covalently bound to the proteins. There are over ten various PBP known, which falls into one of three groups: phycoerythrins, phycocyanins, and allo phycocyanins. Hollow disks of PBP trimers and hexamers are arranged into cylinders by colorless linker pro teins; the cylinders are then assembled into PBS. Typical semidiscoidal PBS consists of a central core formed by three allophycocyanin cylinders and of six lateral cylinders consisting of other PBP and attached as a fan to the nucleus. The PBS number, size, and pigment composition in cyanobacteria depend on light conditions and other ambient factors. While PBSs have certain advantages compared to other antennae, these pigmentprotein complexes require more energy for their biosynthesis than the chlorophyll a/b and chlorophyll a/c proteins of oxygenic photosynthetic organisms.

Using computational modeling and known 3D structure of proteins, we arrived at a rational spatial model of the orange carotenoid protein (OCP) and phycobilisome (PBS) interaction in the non-photochemical fluorescence quenching. The site of interaction is formed by the central cavity of the OCP monomer in the capacity of a keyhole to the characteristic external tip of the phycobilin-containing domain (PB) and folded loop of the core-membrane linker LCM within the PBS core. The same central protein cavity was shown to be also the site of the OCP and fluorescence recovery protein (FRP) interaction. The revealed geometry of the OCP to the PBLCM attachment is believed to be the most advantageous one as the LCM, being the major terminal PBS fluorescence emitter, gathers, before quenching by OCP, the energy from most other phycobilin chromophores of the PBS. The distance between centers of mass of the OCP carotenoid 3'-hydroxyechinenone (hECN) and the adjacent phycobilin chromophore of the PBLCM was determined to be 24.7 Å. Under the dipole-dipole approximation, from the point of view of the determined mutual orientation and the values of the transition dipole moments and spectral characteristics of interacting chromophores, the time of the direct energy transfer from the phycobilin of PBLCM to the S1 excited state of hECN was semiempirically calculated to be 36 ps, which corresponds to the known experimental data and implies the OCP is a very efficient energy quencher. The complete scheme of OCP and PBS interaction that includes participation of the FRP is proposed.

Phycobilisome (PBS) is a giant water-soluble photosynthetic antenna transferring the energy of absorbed light mainly to the photosystem II (PSII) in cyanobacteria. Under the low light conditions, PBSs and PSII dimers form coupled rows where each PBS is attached to the cytoplasmic surface of PSII dimer, and PBSs come into contact with their face surfaces (state 1). The model structure of the PBS core that we have developed earlier by comparison and combination of different fine allophycocyanin crystals, as reported in Zlenko et al. (Photosynth Res 130(1):347-356, 2016b), provides a natural way of the PBS core face-to-face stacking. According to our model, the structure of the protein-protein contact between the neighboring PBS cores in the rows is the same as the contact between the APC hexamers inside the PBS core. As a result, the rates of energy transfer between the cores can occur, and the row of PBS cores acts as an integral PBS "supercore" providing energy transfer between the individual PBS cores. The PBS cores row pitch in our elaborated model (12.4 nm) is very close to the PSII dimers row pitch obtained by the electron microscopy (12.2 nm) that allowed to unite a model of the PBS cores row with a model of the PSII dimers row. Analyzing the resulting model, we have determined the most probable locations of ApcD and ApcE terminal emitter subunits inside the bottom PBS core cylinders and also revealed the chlorophyll molecules of PSII gathering energy from the PBS.

The phycobilisome (PBS) is a major light-harvesting complex in cyanobacteria and red algae. To obtain the detailed structure of the hemidiscoidal PBS core composed of allophycocyanin (APC) and minor polypeptide components, we analyzed all nine available 3D structures of APCs from different photosynthetic species and found several variants of crystal packing that potentially correspond to PBS core organization. Combination of face-to-face APC trimer crystal packing with back-to-back APC hexamer packing suggests two variants of the tricylindrical PBS core. To choose one of these structures, a computational model of the PBS core complex and photosystem II (PSII) dimer with minimized distance between the terminal PBS emitters and neighboring antenna chlorophylls was built. In the selected model, the distance between two types of pigments does not exceed 37 Å corresponding to the Förster mechanism of energy transfer. We also propose a model of PBS and photosystem I (PSI) monomer interaction showing a possibility of supercomplex formation and direct energy transfer from the PBS to PSI.

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