The Chlorophyll Biosynthetic Heterogeneity and Chloroplast Bioengineering - Chlorophyll Biosynthesis and Technological Applications - page 343

calls for Chl a and b to be formed via a single-branched Chl biosynthetic pathway 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 ˚
in the linear continuous array model, or over larger distances, in the laterally
heterogeneous model, to become part of PSI, PSII or LHCII. If this were the
case, then no resonance excitation energy transfer would be observed between
anabolic tetrapyrroles and the various Chl-protein complexes, and the distances
separating the anabolic tetrapyrroles from the various Chl-protein complexes would
be much larger than the values reported in Table 15.2 of Chap. 15 .
Incompatibility of the SBP-MLM Model with Experimental Data
The shorter distances separating anabolic tetrapyrroles from Chl-protein complexes
reported in Table 6.1 of Chap. 6 are compatible with the SBP-multilocation and
MBP-sublocation models. However, overwhelming experimental evidence argues
against the operation of a single-branched Chl biosynthetic pathway in plants
(Rebeiz et al. 2003a ).
Compatibility of the MBP-SUBLM Model with Experimental Data
The various considerations discussed above leaves the MBP-sublocation model
(MBPSUBLM) as a viable working hypothesis. In this model the unified multi-
branched Chl a/b biosynthetic pathway, is visualized as the template of a Chl-protein
biosynthesis center where the assembly of PSI, PSII and LHC takes place (Rebeiz
et al. 1999 , 2004 ). The multiple Chl biosynthetic routes are visualized, individually
or in groups of one or several adjacent routes, as Chl-apoprotein biosynthesis
subcenters earmarked for the coordinated assembly of individual Chl-apoprotein
complexes. Apoproteins destined to some of the subcenters may possess specific
polypeptide signals for specific Chl biosynthetic enzymes peculiar to that subcenter,
such as 4-vinyl reductases, formyl synthetases or Chl a and Chl b synthetases.
Once an apoprotein formed in the cytoplasm or in the plastid reaches its subcenter
destination and its signal is split off, it binds nascent Chl formed via one or more
biosynthetic routes, as well as carotenoids. During pigment binding, the apoprotein
folds properly and acts at that location, while folding or after folding, as a template
for the assembly of other pigment-proteins. This model is compatible with the lateral
heterogeneity of the PSU and can account for the observed resonance excitation
energy transfers Table 6.1 of Chap. 6 , and the short distances separating anabolic
tetrapyrroles from Chl-protein complexes in the distinct PSI, PSII and shuttling
LHCII entities that compose the PSU.