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Fig. 16.3 Schematics of a linear model of a PSU in the unfolded state (Reproduced from von
Wettstein et al.
1995
)
As mentioned in Chap.
15
, we have recently examined three possible models for that
scenario, which have been referred to as: (a) the single-branched Chl biosynthetic
pathway (SBP)-single location model, (b) the SBP-multilocation model, and (c) the
multi-branched Chl biosynthetic pathway (MBP)-sublocation model. The models
take into account the dimension of the PSU (Bassi et al.
1990
), the biochemical
heterogeneity of the Chl biosynthetic pathway (Rebeiz et al.
1994
,
2003a
) and the
biosynthetic and structural complexity of the thylakoid and the Chl.
Assembly of Chl-Protein Complexes: The SBP-Single Location Model
The SBP-single location model is depicted schematically below, in Fig.
16.4
, which
has been reproduced from Fig.
15.1
. As mentioned previously, within the PSU, this
model accommodates only one Chl-apoprotein biosynthesis center and no
Chl-apoprotein biosynthesis subcenters. Within the Chl-apoprotein biosynthesis
center, Chl
a
and
b
are formed via a single-branched Chl biosynthetic pathway
(Fig.
16.2
) at a location accessible to all Chl-binding apoproteins. The latter will
have to access that location in the unfolded state, pick up a complement of MV
Chl
a
and/or MV Chl
b
, and undergo appropriate folding. Then the folded
Chl-apoprotein complex has to move from the central location to a specific PSI,
PSII, or Chl
a/b
LHC-protein location within the Chl-apoprotein biosynthesis
center over distances of up to 225
˚
(Kolossov et al.
2003
; Kopetz et al.
2004
).
In this model, it is unlikely to observe resonance energy transfer between metabolic
tetrapyrroles and some of the Chl-apoprotein complexes located at distances longer
than 100
˚
. This is because resonance excitation energy transfer takes place only
over distances shorter than 100
˚
(Calvert and Pitts
1967
).
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