PIGMENT-PROTEIN COMPLEXES AND THE CONCEPT OF THE PHOTOSYNTHETIC UNIT - CHLOROPHYLL COMPLEXES AND PHYCOBILISOMES

The photosynthetic unit includes the reaction centers (RC 1 and RC 2) and the light-harvesting complexes which contribute to evolution of on e O-2 molecule. The light-harvesting complexes, that greatly expand th e absorptance capacity of the reactions, have evolved along three prin cipal lines. First, in green plants distinct chlorophyll (Chl) a/b-bin ding intrinsic membrane complexes are associated with RC 1 and RC 2. T he Chl a/b-binding complexes may add about 200 additional chromophores to RC 2. Second, cyanobacteria and red algae have a significant type of antenna (with RC 2) in the form of phycobilisomes. A phycobilisome, depending on the size and phycobiliprotein composition adds from 700 to 2300 light-absorbing chromophores. Red algae also have a sizable Ch l a-binding complex associated with RC 1, contributing an additional 7 0 chromophores. Third, in chromophytes a variety of carotenoid-Chl-com plexes are found. Some are found associated with RC 1 where they may g reatly enhance the absorptance capacity. Association of complexes with RC 2 has been more difficult to ascertain, but is also expected in ch romophytes. The apoprotein framework of the complexes provides specifi c chromophore attachment sites, which assures a directional energy tra nsfer within complexes and between complexes and reaction centers. The major Chl-binding antenna proteins generally have a size of 16-28 kDa , whether of chlorophytes, chromophytes, or rhodophytes. High sequence homology observed in two of three transmembrane regions, and in putat ive chlorophyll-binding residues, suggests that the complexes are rela ted and probably did not evolve from widely divergent polyphyletic lin es.