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DV Mg-Proto. As such, it is an early intermediate along the Chl biosynthetic chain.
Biosynthetically, it is several steps removed from the Chl end product. Mg-Proto is a
mixed MV-DV, dicarboxylic tetrapyrrole pool, consisting of DV and MV Mg-Proto.
It is the precursor of DV and MV Pchlide
a
. The protochlorophyll(ide) [(Pchl(ide)] of
higher plants consists of about 95 % protochlorophyllide (Pchlide)
a
andabout5%
Pchlide
a
ester (Pchlide
a
E). The latter is esterified with long chain fatty alcohols
(LCFAs) 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 chlorophyllide (Chlide)
a
. Accumulation of the
various tetrapyrrole donors was induced by incubation of green tissues with
δ
-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). It is believed that the fluorescence emitted at ~F685 nm arises
from the Chl
a
of LHCII, the major thylakoid LHC antenna, and LHCI-680, one of the
LHC antennae of PSI (Bassi et al.
1990
). That emitted at ~F695 nm is believed to
originate mainly from the Chl
a
of CP47 and CP29, two PSII antennae (Bassi
et al.
1990
). That emitted at ~F735 nm is believed to originate primarily from the Chl
a
of LHCI-730, a PSI antenna (Bassi et al.
1990
). 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 excitation resonance
energy transfers were to be observed between the tetrapyrrole donors and the selected
Chl
a
acceptors, definite excitation maxima would be observed. These excitation
maxima would correspond to absorbance maxima of the various tetrapyrrole donors,
and would represent the peaks of the excitation resonance energy transfer bands.
Pronounced excitation resonance energy transfer bands from Proto (Table
15.1
),
was unambiguous except for a few cases at the short wavelength and long wave-
length extremes of excitation bands. Contrary to previous believes, it was surprising
to observe a significant diversity in various intra-membrane environments of Proto,
Mp(e), and Pchl(ide)
a
. This diversity was manifested by a differential donation of
resonance excitation energy transfer to the different Chl
a
-apoprotein complexes
from multiple Proto, Mp(e) and Pchl(ide) a sites, and is highly compatible with
biosynthetic heterogeneity of the Chl biosynthetic pathway. Thus, the multi-
account for the existence of multiple Proto, Mp(e) and Pchl(ide)
a
donor sites by
depicting multiple Biosynthetic routes that originate in multiple ALA, Proto,
Mg-Proto and Pchlide
a
sites.
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