E. Gantt, PIGMENT-PROTEIN COMPLEXES AND THE CONCEPT OF THE PHOTOSYNTHETIC UNIT - CHLOROPHYLL COMPLEXES AND PHYCOBILISOMES, Photosynthesis research, 48(1-2), 1996, pp. 47-53
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.