Organization and role of the long-wave chlorophylls in the photosystem I of the Cyanobacterium spirulina.
The data on the organization and function of the photosystem I pigment-protein complexes of the cyanobacterium Spirulina and the characteristics of pigment antenna of the photosystem I monomeric and trimeric core complexes are presented and discussed. We proved that the photosystem I complexes in the cyanobacterial membrane pre-exist mainly as trimers, though both types of complexes contribute to the photosynthetic electron transport. In contrast to monomers, the antenna of the photosystem I trimeric complexes of Spirulina contains the extreme long-wave chlorophyll form absorbing at 735 nm and emitting at 760 nm (77 K). The intensity of fluorescence at 760 nm depends strongly on the P700 redox state: it is maximum with the reduced P700 and strongly decreased with the oxidized P700 which is the most efficient quencher of fluorescence at 760 nm. The energy absorbed by the extreme long-wave chlorophyll form is active in the photooxidation of P700 in the trimeric complex. The data obtained indicate that the long-wave form of chlorophyll originates from interaction of the chlorophyll molecules localized on monomeric subunits forming the photosystem I trimer. Kinetic analysis of the P700 photooxidation and light-induced quenching of fluorescence at 760 nm (77 K) allows the suggestion that the excess energy absorbed by the antenna monomeric subunits within the trimer migrates via the extreme long-wave chlorophyll to the P700 cation radical and is quenched, which prevents the photodestruction of the pigment-protein complex.
Karapetyan
N V
NV
Bakh Institute of Biochemistry, Russian Academy of Sciences, Moscow. inbio@glas.acp.org
D002734:Chlorophyll / Q000032:analysis* / Q000378:metabolism*; D000458:Cyanobacteria / Q000378:metabolism*; D046911:Macromolecular Substances; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism*
D046911:Macromolecular Substances; D045322:Photosynthetic Reaction Center Complex Proteins; D002734:Chlorophyll