Antenna molecules, chlorophyll

Light excites an antenna molecule (chlorophyll or iccessory pigment), raising an electron to a higher energy level. 

FIGURE 19-46 Exciton and electron transfer. This generalized scheme shows conversion of the energy of an absorbed photon into separation of charges at the reaction center. The steps are further described in the text. Note that step (T) may repeat between successive antenna molecules until the exciton reaches a reaction-center chlorophyll. The asterisk ( ) represents the excited state of an antenna molecule. 

The two photochemical reactions are performed by two photosystems. Each photosystem consists of a so-called reaction centre, where the primary energy conversion takes place, associated with a few hundred pigment molecules (chlorophylls and carotenoids see Fig. 2) serving as light-harvesting antennas, which transfer the absorbed energy as electronic excitation energy to the reaction centres. 

The excited antenna molecule passes energy to a neighboring chlorophyll molecule (resonance energy transfer), exciting it. 

It is now known that in the bacterial systems the donor is a pair of chlorophyll molecules, the special pair", P, and the initial acceptors is either another chlorophyll molecule, an auxiliary chlorophyll B, or a bacteriopheophytin, H, a chlorophyll molecule in which the central magnesium ion has been replaced by a pair of protons. Although many pigments are present acting as antenna molecules to gather the light, additional chlorophylls are most common. How can the same molecule have such different roles In spite of the fact that the antenna and the RC pair molecules are the same, light is transferred from the antenna to the RC with unit efficiency. 

The reaction center is a part of the photosynthetic apparatus in chloroplasts where the photochemical process occurs. Reaction centers are a specific pair of chlorophyll molecules in a photosystem that collect light energy absorbed by other chlorophyll molecules in a photosystem and pass it along to an electron acceptor, such as an electron transport chain. The remaining chlorophyll molecules are referred to as antenna molecules of the light-harvesting complexes because they absorb the photons and pass the energy by resonance transfer to the reaction centers. The process occurs on the order of lO seconds. 

The first part of the process occurs in light-harvesting complexes. Each multisubunit protein complex contains multiple antenna pigment molecules, chlorophylls and some accessory pigments, and two chlorophyll molecules that act as the reaction center. The reaction center traps energy quanta excited by the absorption of light. 

Resonance transfer is important for the way chlorophylls function in cells. Only about 1 02 molecule is produced for every 2500 chlorophylls. Instead, most of the chlorophyll molecules act as antenna molecules of the light-harvesting complexes. Antenna molecules absorb photons and the energy is passed by resonance transfer to specific chlorophyll molecules in a relatively few reaction centers. The path the energy takes to arrive at the energy center is random (Figure 17.11). 

Schematic diagram of the surface of a photosystem in the thylakoid membrane. It contains a patch-like mosaic of several hundred chlorophyll and carotenoid antenna molecules oriented in the membrane. An exciton absorbed by an antenna molecule quickly migrates via the pigment molecules to the reaction centre, P700 its path is shown by the coloured arrows. Although all the antenna molecules can absorb light, only the reaction centre molecule can convert the excitation energy into electron flow.

The conditions for efficient energy transfer by inductive resonance are that the phycobiliprotein chromophore (the donor) and the chlorophyll antenna molecule (the acceptor) be in close proximity, usually about 5 nm apart. The "resonance" part is how the frequency of the flurorescence emis-... 

The PSI-A and PSI-B polypeptides with molecular masses around 82 kDa (3,4,5) are homologous as demonstrated from nucleotide sequencing of their chloroplast genes psaA and psaB in a number of plant species (8-13). A heterodimer of the polypeptides are thought to bind the reaction center P700, the primary acceptors Ao, Ai and X, and about 60 chlorophyll a antennae molecules (4,5,6,29). This complex is often referred to as... 

The heterodimer of PSI-A and PSI-B binds a number of chlorophyll a antennae molecules. A comparison of the sequence information from different species demonstrates that the PSI-A and PSI-B polypeptides contain 38 and 32 conserved histidine residues, respectively. The major part of these histidines (52 residues) are positioned in the hydrophobic a-helices. Histidine residues have been implicated in the coordination of chlorophyll in light-harvesting antennae complexes (36). The number of conserved histidines in the heterodimer corresponds well to the experimental value of 60 chlorophyll a/P700 in the barley core complex (4). 

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

The light-harvesting complex LHl is directly associated with the reaction center in purple bacteria and is therefore referred to as the core or inner antenna, whereas LH2 is known as the peripheral antenna. Both are huilt up from hydrophohic a and p polypeptides of similar size and with low hut significant sequence similarity. The two histidines that hind to chlorophyll with absorption maxima at 850 nm in the periplasmic ring of LH2 are also present in LHl, but the sequence involved in binding the third chlorophyll in LH2 is quite different in LHl. Not surprisingly, the chlorophyll molecules of the periplasmic ring are present in LHl but the chlorophyll molecules with the 800 nm absorption maximum are absent. 

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