Energy Transfer within the Phycobilisome

The existence of crystal structures has revealed many of the structural requirements for the assembly of an efficient energy transfer antenna complex. Relatively high quality images of entire antenna complexes have been obtained from non-oxygenic bacteria [106-109], PSI [93, 110] and PSII [33, 111]. This has enabled the formulation of more complete models of light harvesting [2] that propose the positions of chromophores are determined by universal physical constraints such as photon flux, time scales of photon absorption, energy transfer, and competing processes. The design of antennas has been optimized through evolution to provide maximum energy transfer.

The mechanism of energy transfer in the phycobilisome has been extensively studied by a variety of spectroscopic techniques on the entire complex, but more often on isolated subunits [ 112] - However, since the crystal structures that exist contain, at most, one (aP)6 hexameric unit, the actual method of directed energy transfer down to the reaction center can only be conjectured. Species-specific differences not-withstanding, energy transfer rates are extremely fast [113] and overall quantum yields are high - about 95% [21, 41, 114]. The phycobilisome differs from the chlorophyll-based antennas by having rather large distances between cofactors. In the chlorophyll-based antennas, distances between adjacent cofactors are typically close enough to envision closely interacting absorption rings or aggregates. In the bacterial LH2 complex, the two rings of bacteriochlorophylls have spacing of ~9A and ~20A , center to center, respectively; the distance between rings is also ~20A. The addition ofcarotenoid molecules makes this antenna quite chromo-phore dense. In the antenna beds of both PSI and PSII, there are many chlorophyll molecules, positioned with very complex arrangements, which can roughly be separated into two groups, on either face of the membrane -however typical distances are on the order of 10A . The densest of all chlorophyll-based systems is the chlorosome found in green sulfur (Chlorobiaceae) and green filamentous bacteria (Chloroflexaceae). In these, an entire organelle encloses thousands of pigment molecules stacked in close proximity, without intervening protein subunits [115-117].

Within each minimal PC unit, the distances between the a84 and both the P84 / / and P155 bilins is ~50A , while the P84 and P155 bilins are separated by -40 A (Figure

11.1a). Association of the PC monomers into trimers and hexamers (Figure 11.1b) results in a slightly shorter distance between certain bilins, but none are closer than -20A - 29, 118] - In assembled PE hexamers, which contain five bilins per monomer, the density is slightly higher, with the nearest approach between two bilins on the order of 15 A [ 52]. Thus, it can be assumed that efficient energy transfer does not particularly require the dense packing of cofactors, rather, that dense packing maximizes the potential for light absorption with the lowest expenditure on protein synthesis. Since this is not the case for the phycobilisome, it could be assumed that the secondary role of the phycobilisome, serving as a reservoir of nutrients for starved cells, must be of great enough importance to have preserved this form of antenna system.

The central mechanism for energy transfer proposed by Förster [ 119] requires the existence of weak coupling between electronic energy levels of nearby chro-mophores. Due to the relatively large distances between cofactors in the phycobilisome, this coupling is weak; structure-based theoretical energy transfer rate calculations utilizing the Förster mechanism match experimentally obtained values well. Debreczeny and coworkers measured energy transfer rates in isolated PC and obtained values between 50-500 psec for different pathways within the (aß) monomer [120]. Formation of (aß)3 trimers allowed for much faster energy transfer in the 0.5-1 psec range [36], showing the importance of the level of complex formation when dealing with the functional characteristics of the phyco-bilisome. Ultra fast, two-color pump-probe spectroscopic measurements on different organizational states of APC have revealed energy-transfer at less than 100 fsec, which has been suggested to occur due the formation of dimer-exciton states [37, 38]. These states can only exist between the PCBs on adjacent monomers; the very fast decay component is probably outside of the time-frame accessible to Förster resonance energy transfer. It is still unclear what the effect of further aggregation to full cylinders, cores, or phycobilisomes will have on the energy transfer rates, especially with the presence of linker proteins. However, use of isolated subunit also allows for extremely high quality descriptions of the cofactor/protein/solvent environments during absorption and energy transfer [39, 121][ Detailed discussions on energy transfer within isolated PBP can be found in a number of excellent reviews [21, 41, 113, 122].

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