Relationship of Chlorophyll Biosynthesis to the Assembly of Chlorophyll-Protein Complexes - Chlorophyll Biosynthesis and Technological Applications - page 319

discussion of Chl biosynthetic heterogeneity in the context of a photosynthetic unit
made up of PSI, PSII and light harvesting Chl-protein complexes (LHCs) (see
below).
15.2.1 Chlorophyll Biosynthesis-Thylakoid Membrane
Biogenesis Working Models
In 1999, Rebeiz and coworkers proposed that the unified multibranched Chl a/b
biosynthetic pathway might be visualized as a template of Chl-protein biosynthesis
centers, upon which the assembly of PSI, PSII and light harvesting Chl-protein
complexes (LHC) takes place (Rebeiz et al. 1999 ). Then in an extension of the
working hypothesis described above, we have recently proposed three Chl-thylakoid
apoprotein biosynthesis models (Kolossov et al. 2003 ; Rebeiz et al. 2003b , 2004 ).
These models took into account the structure and dimensions of the photosynthetic
unit (PSU) (Allen and Forsberg 2001 ; Anderson 2002 ; Bassi et al. 1990 ; Staehelin
2003 ), the biochemical heterogeneity of the Chl biosynthetic pathway (Rebeiz
et al. 1994 ), and the biosynthetic and structural complexity of thylakoid membranes
(Sundqvist and Ryberg 1993 ). Within a PSU, the three putative Chl-apoprotein
thylakoid biosynthesis models are referred to as: (a) the single branched biosynthetic
pathway (SBP)-single location model, (b) the SBP-multilocation model and (c) the
multibranched biosynthetic pathway (MBP)-sublocation model.
15.2.1.1 Single-Branched Pathway (SBP)-Single Location Model
Within the PSU, the SBP-single location model Fig. 15.1 was considered to accom-
modate only one Chl-apoprotein thylakoid biosynthesis center and no Chl-apoprotein
thylakoid biosynthesis sub-centers. Within the Chl-apoprotein thylakoid biosynthesis
center, Chl a and b were considered to be formed via the conventional single-
branched Chl biosynthetic pathway Described below in Fig. 15.2 , at a single location
accessible to all Chl-binding apoproteins. An apoprotein moved to that location in the
unfolded state, picked up a complement of MV Chl a and/or MV Chl b ,and
underwent appropriate folding. Then the folded Chl-apoprotein complex moved at
random from the Chl biosynthesis location to a specific PSI, PSII, or LHC location
within the Chl-apoprotein biosynthesis center.
15.2.1.2 Single-Branched Pathway (SBP)-Multilocation Model
In the SBP-multilocation model Fig. 15.3 , every location within the photosynthetic
unit is considered to be a Chl-apoprotein thylakoid biosynthesis subcenter. In every
Chl-apoprotein biosynthesis subcenter, a complete single-branched Chl a/b biosyn-
thetic pathway is active. Association of Chl a and/or Chl b with specific PSI, PSII,
or LHC apoproteins at any location is random. In every Chl-apoprotein biosynthesis