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Light Harvesting
Green anoxygenic bacteria are composed of two major classes, Chlorobiaceae
and Chloroflexaceae. Of all of the photosynthetic bacteria, only these two
classes possess the unique feature of chlorosomes that are complexes approximately
200 nm in length attached to the cell membrane. Each chlorosome contains thousands
of bacteriochlorophylls that are organized to capture light energy and direct
it to the reaction center where energy conversion occurs. In green non-sulfur
bacteria, a bacteriochlorophyll a containing protein known as the Fenna-Matthews-Olson
(FMO) protein is located between the chlorosome and reaction center.
The bacteriochlorophyll protein, or FMO protein, from Chlorobium tepidum, that serves as a light harvesting complex that directs light energy from the chlorosomes attached to the cell membrane to the reaction center has been crystallized and the structure has been determined in collaboration with Robert Blankenship. The crystals belongs to a cubic space group and the structure has been refined to a resolution 2.2 Å with a R factor of 19.7%. The protein is a trimer with each subunit consisting of a series of strands that form two sheets enveloping seven bacteriochlorophylls in a "taco shell" arrangement. There are also 6 a helices with an average length of 10 amino acid residues that are located in the open end of the "taco shell".
Differences in the planarity of the tetrapyrroles, including the position of the magnesium atom relative to the macrocycle, can influence the electronic states of the bacteriochlorophylls. Comparison of the structure of Cb. tepidum and Pc. aestuarii reveals many small structural differences that probably collectively lead to the observed spectral differences. Bacteriochlorophyll 2, that is located near the interface of the protein subunits forming the trimer, is the only cofactor that is coordinated by a water molecule rather than the protein. The degree to which this structural features contributes to the redox properties of the protein will be tested by the determination of structure under both reducing and oxidizing conditions.
Related publications:
Y. Li, W. Zhou, R. E. Blankenship, and J. P. Allen (1997) "Crystal structure
of the bacteriochlorophyll a protein from Chlorobium tepidum" J. Mol.
Biol. 271, 456-471.
A. N. Melkozernov, J. M. Olson, Y.-F. Li, J. P. Allen, and R. E. Blankenship
(1998) “Orientation and excitonic interaction of the Fenna-Matthews-Olson
bacteriochlorophyll a protein in membranes of the green sulfur bacterium Chlorobium
tepidum” Photosynthesis Research, 56, 315-328.
A. Camara-Artigas, R. Blankenship, and J. P. Allen (2002) “The structure
of the FMO protein from Chlorobium tepidum at 2.2 Å resolution” Photosynthesis
Res., in press.
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Photosynthesis Center Arizona State University Box 871604 Room PSD 209 Tempe, AZ 85287-1604
13 February 2006 |
phone: (480) 965-1963 fax: (480) 965-2747 |