Biology Reference
In-Depth Information
15.4
Incompatibility of the Single-Branched Pathway
(SBP)-Single Location Model with Resonance
Excitation Energy Transfer from Anabolic
Tetrapyrroles to Various Chl-Protein Complexes
Since resonance energy transfer is insignificant at distances larger than 100
˚
(Calvert and Pitts
1967
) the detection of pronounced resonance excitation energy
transfer from Proto, Mp(e), and Pchl(ide)
a
to Chl
a
~F685, ~F695, and ~F735
of 100
˚
or less of the Chl
a
acceptors. This in turn is incompatible with the
functionality of the SBP-single location Chl-thylakoid biogenesis model. Indeed, it
can be estimated from published data that the size of the PSU that includes the two
PS, LHC, as well as the CF0-CF1 ATP synthase is about 130
450
˚
(Bassi
et al.
1990
). Most PSU models depict a central Cyt b
6
complex flanked on one side
by PSI and coupling factor CF1, and on the other side by PSII and LHCPII. With
this configuration, the shortest distance between the single-branched Chl biosyn-
thetic pathway and PSI, PSII, and LHCII, in the SBP-single location model would
be achieved if the SBP occupied a central location within the PSU. In that case it
can be calculated from the PSU model proposed by Bassi et al. (
1990
) that the core
of PSII including CP29, would be located about 126
˚
away from the SBP. On the
other hand, LHCI-730 would be located about 159
˚
on the other side of the SBP.
The centers of the inner and outer halves of LHCII surrounding the PSII core would
be located about 156 (outer half) and 82 (inner half)
˚
from the SBP. The detection
of pronounced excitation resonance energy transfer from Proto, Mp(e) and Pchl
(ide)
a
to Chl
a
~F685, ~F695, and ~F735 indicates that these anabolic tetrapyrroles
are within distances of 100
˚
or less of the Chl
a
acceptors. In view of the above
considerations it was concluded that the detection of resonance excitation energy
transfer between anabolic tetrapyrroles and Chl
a
of the various thylakoid
Chl-protein complexes was not compatible with the functionality of
the
SBP-single location Chl-thylakoid biogenesis model.
15.5 Compatibility of the Multibranched Pathway
(MBP)-Multilocation Model with Resonance
Excitation Energy Transfer from Anabolic
Tetrapyrroles to Various Chl-Protein Complexes
Further calculations of resonance excitation energy transfer rates, and distances
separating tetrapyrrole donors from Chl
a
acceptors and other considerations
favored the operation of the MBP-sublocation Chl biosynthesis-thylakoid biogene-
sis model as described below.
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