The Chlorophyll Biosynthetic Heterogeneity and Chloroplast Bioengineering - Chlorophyll Biosynthesis and Technological Applications - page 341

Because of the shorter distances separating the accumulated tetrapyrroles from
Chl-protein complexes, within each subcenter, resonance excitation energy transfer
between various metabolic tetrapyrroles and Chl is readily observed. In this model,
both MV and DV Mp(e) may be present in some pigment-protein complexes, in
particular if more than one Chl biosynthetic route is involved in the Chl formation
of a particular Chl-protein complex.

16.5.2.2 Which Chl-Thylakoid Apoprotein Assembly Model
Is Favored by Experimental Evidence?

We tested the compatibility of the three aforementioned models by resonance
excitation energy transfer between anabolic tetrapyrrole intermediates of the Chl
biosynthetic pathway and various thylakoid Chl-protein complexes, in order to
determine which Chl-thylakoid apoprotein assembly model is likely to be func-
tional during thylakoid membrane formation.
Resonance excitation energy transfer from three tetrapyrrole donors to the Chl
a of various Chl-protein complexes were monitored, namely: from Proto, Mp
(e) and MV and DV Pchlide a . DV Proto is a common precursor of heme and
Chl. It is the immediate precursor of DV Mg-Proto. As such, it is an early interme-
diate along the Chl biosynthetic chain. Biosynthetically, it is several steps removed
from the Chl end product (Rebeiz et al. 2003b ). Mg-Proto is a mixed MV-DV,
dicarboxylic tetrapyrrole pool, consisting of DV and MV Mg-Proto (Rebeiz
et al. 2003b ). It is the precursor of DV and MV Pchlide a . The [(Pchl(ide)] of
higher plants consists of about 95 % Pchlide a and about 5 % Pchlide a ester . The
latter is esterified with long chain fatty alcohols at position 7 of the macrocycle.
While Pchlide a ester consists mainly of MV Pchlide a ester, Pchlide a consists of
DV and MV Pchlide a . The latter are the immediate precursors of DV and MV
Chlide a (Rebeiz et al. 2003b ).
Accumulation of the various tetrapyrrole donors was induced by incubation of
green tissues with

-aminolevulinic acid (ALA) and/or 2,2 0 -dipyridyl (Rebeiz
et al. 1988 ). The task of selecting appropriate Chl a -protein acceptors was facilitated
by the fluorescence properties of green plastids. At 77 K, emission spectra of isolated
chloroplasts exhibit maxima at 683-686 nm (~F685), 693-696 nm (~F695),
and 735-740 nm (~F735) (see Chap. 15 ) . Since these emission maxima are readily
observed in the fluorescence emission spectra of green tissues and are associated
with definite thylakoid Chl a -protein complexes, it was conjectured that they would
constitute a meaningful resource for monitoring excitation resonance energy
transfer between anabolic tetrapyrroles and representative Chl a -protein complexes.
To monitor the possible occurrence of resonance excitation energy transfer
between the accumulated anabolic tetrapyrroles and Chl a -protein complexes,
excitation spectra were recorded at 77 K at the respective emission maxima of the
selected Chl a acceptors, namely at ~685, ~695, and ~735 nm. It was conjectured
that if resonance excitation energy transfers were to be observed between the
tetrapyrrole donors and the selected Chl a acceptors, definite excitation maxima

δ