Chlorosomes baseplate

Steady-state and time-resolved spectroscopy has been used to follow the flow of excitations from the BChl c,dor e in the interior of the chlorosome, through the baseplate and into the membrane, where the energy is trapped by the reaction center (7-11). The results are consistent with a sequential energy transfer pathway, from the BChl c/d ore in the chlorosome interior to the BChl a 795 pigment in the chlorosome baseplate, to membrane-bound BChl a antenna complexes and finally to the reaction center (Fig. 1). The green sulfur bacteria (Fig. IB) also contain an additional antenna species, the trimeric water-soluble BChl a protein the three dimensional structure of which was determined by Fenna and Matthews (12). 

Absorption and fluorescence excitation spectra of Cb, vibrioforme cells under anaerobic conditions were nearly superimposable (Fig. 2), indicating that excitation transfer from BChl d in the chlorosomes to BChl a (the chromophore fluorescing at 835 nm) was essentially 100% efficient. Aerobic conditions did not affect the absorption spectrum, but caused a 90% decrease in the relative height of the peak at 730 nm (where BChl d absorbs) in the fluorescence excitation spectrum (Fig. 2). Thus aerobic conditions caused the efficiency of excitation transfer from BChl d to BChl a to decrease to about 10%. Similar effects were observed in isolated chlorosomes, in which 835 nm fluorescence comes from BChl a in the chlorosome baseplate (data not shown). 

Figure 23-28 (A) Model of a light-harvesting chlorosome from green photosynthetic sulfur bacteria such as Chlorobium tepidum and species of Prosthecochloris. The chlorosome is attached to the cytoplasmic membrane via a baseplate, which contains the additional antenna bacteriochlorophylls (795 BChl a) and is adjacent to the trimeric BChl protein shown in (B) and near the reaction center. After Li et al.302 and Remigy et a/.304 (B) Alpha carbon diagram of the polypeptide backbone and seven bound BChl a molecules in one subunit of the trimeric protein from the green photosynthetic bacterium Prosthecochloris. For clarity, the magnesium atoms, the chlorophyll ring substituents, and the phytyl chains, except for the first bond, are omitted. The direction of view is from the three-fold axis, which is horizontal, toward the exterior of the molecule. From Fenna and Matthews.305 See also Li et al.302...

Green photosynthetic bacteria intramembrane antenna complexes, baseplate systems and the accessory antenna systems (chlorosomes)... 

Chlorosome BChl a-protein trimers ("FMO" protein) Baseplate... 

Typically, each chlorosome contains as many as 10,000 BChl-c molecules, some carotenoids and several hundred BChl-a molecules serving as the sub-antenna in the baseplate and as core antennae in the cytoplasm. Chlorosomes and the BChl a-protein, or the FMO protein, are discussed in Chapter 8. 

The baseplate is about 5 nm thick and contains a particular BChl a-protein complex, namely B795 (see Fig. 4 below). Here B stand for the word bulk, as in Chapter 3 where reference is made to lightharvesting BChl-proteins, and the number refers to the wavelength of the major absorption bands of the complex in the far-red. The B795-complex serves as an intermediate for transfer of electronic excitation energy from the chlorosome to components in the cytoplasmic membrane. The core complex in the cytoplasmic membrane consists of small core antenna complexes, B806-866, and the reaction-center complex (P865 in Chloroflexaceae sp.). 

The transition dipole moment of BChl c-744 is nearly parallel to the axis of the rod element, while that of BChl c-727 is more random. The presence of the longer-wave length BChl c-complex, BChl c-766, has been suggested by the results of deconvolution of the linear-dichroism spectra as well as by more recent measurements of time-resolved fluorescence spectra of oriented chlorosomes. The orientation of the transition moment of BChl c-766, determined from its fluorescence maximum at 778 mn, is intermediate between that of BChl c-744 and that of the baseplate BChl c-protein complex, B795. 

Time-resolved fluorescence spectra clearly indicate a linear cascade of excitation-energy transfer from BChl c in the chlorosome rods, through the BChl a-containing baseplate, to the core-antenna BChl a in the membrane, and finally to the reaction center. Overall excitation transfer efficiency from BChl c to the core antenna (B806-866) in Cf. aurantiacus cells has been reported to be 69 13% at 50 °C, but only -15% at 4 K. Excitation transfer from B806 to B866 within the core antenna is 100%. When the reaction center is closed, the 883-nm fluorescence decays less rapidly in -250 ps. 

The fitting parameters for Fig. 4 are listed in Table I along with data from isolated Chlorobium chlorosomes and whole cells. Note that in anaerobic Chlorobium samples, the major decay times of the 760 nm fluorescence, 68 and 44 ps for cells and chlorosomes respectively, were closely matched by corresponding rise components (negative amplitudes) of 60 and 46 ps in the 820 nm emission. This is direct evidence for sequential energy transfer from BChl d to BChl a of the baseplate. Such rise components were not observ in the data for aerobic Chlorobium samples it is likely that because these lifetimes were much shorter, the rise components were below the level of detection of the instrument. 

Most isolation procedures produce chlorosomes with part of the baseplate, and consequently some Bchl a, attached. 

The light-harvesting system of green bacteria consists of BChl c containing chlorosomes attached to the cell membrane by a baseplate containing BChl a. The cell membrane contains the B 808-866 antenna complex and the reaction center. We report here a comparison of the properties of chlorosome preparations obtained by different treatments. Energy transfer in these preparations was studied by picosecond absorbance recovery and steady-state fluorescence measurements. The results are used to propose a new model for the structure of chlorosomes. 

On excitation of the main chlorosome absorption band at 750 nm (Fig. 2) a very fast (11 2 ps) absorbance recovery is observed (see Table 1). This transient comprises about 80% of the total intensity. The second component (15%-20%) has a decay time of about 30 ps and less than 5% of the signal is due to more slowly decaying components. With excitation of the BChl a in the baseplate at 800 nm, two components of similar amplitude are observed. The fast decay has a lifetime of ca. 30 ps and the lifetime of the slow component is about 200-300 ps (Table 1). 

On excitation of BChl c at 690 nm, we observe fluorescence emission maxima at 764 and 803 nm in chlorosome preparations prepared with both thiocyanate and Miranol (Fig. 3). It seems likely that these arise from the rods and baseplate, respectively. This result shows that there must be some energy equilibration between the two antenna systems. The fluorescence anisotropy around 750 nm is 0.18, close to the mean value of the absorbance isotropy at t = 40 ps. This probably represents the... 

Chlorosomes are flattened discoid structures attached to the outer surface of the plasma membrane. Within their outer envelope of proteins are the light absorbing units, 120 rods carrying about 10,000 non-covalently bound bacteriochlorophyll c (BChl c) molecules and some of y-carotene. Each rod is composed of 12 identical proteins, each of which has 7 bound BChl c molecules. The rods lie on a baseplate composed of water-soluble, globular proteins to which BChl a molecules are non-covalently bound. 

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