A Question

Frantisek Vacha vacha at GENOM.UMBR.CAS.CZ
Fri Oct 3 10:49:49 EST 1997


Another wiev why plants are green

Two reasons:

First. We have to ask why plants use chlorophyll or generally porphyrins.
According to my opinion nature hadnlt much choices and plants used the
most convenient way to develop a useful pigment system. Well before
chlorophyll-like organisms there have been heterotrophic organisms with
Fe-porphyrins, hems. Hem is suitable for many enzymatic reactions but its
absorption properties are not good (main peak at about 400 nm and then
some nothing about 550 nm) and having a heavy metal Fe in the centre its
properties as a species for energy transfer, energy conservation (longer
excitation times) or even charge separation are bad. However, nature had
already developed path for synthesis of a potentially good pigment
(chlorophyll). Note that the synthesis pathway of hem and chlorophyll is
the same to the IX-protoporphyrin. Protoporphyrine and even
Mg-protoporphyrin have absorption mainly at about 400 nm and almost
nothing in the red region. The advantage of absorption in the red is made
by reduction of a 7-8 bond of protochlorophyll. The Mg atom in the centre
in not needed for such absorption profile as seen on pheophytin but it is
definitely needed for porphyrins to became pigments suitable for
photosynthesis.

However, there is also bacteriorhodopsin in Halobacterium and in
Holococcus. Is it photosynthesis? Synthesis of bacteriorhodopsin has
different pathways from chlorophyll. Here it is seen that nature had tried
more paths to evolve photoautotrophic organisms. And everything could have
been orange!


Second. Why isnlt the question aewhy are plants red-brown?" ? There are
green sulphur bacteria and purple bacteria. Green sulphur bacteria are
actually not very green (depends on the level of carotenoids) and their red
absorption maximum (Qy transition) is at 753 nm. So in the middle of the
evolution way we are still not green as we are now. Here I have to note
that bacteriochlorophyll absorbs far beyond 700 nm and the energy absorbed
by bchl is efficient to drive charge separation. Important is that
bacteriochlorophyll in its kation state is not able drive an electron from
water in any conditions which nature or evolution had tried. The limitation
of electron donors, the fact that there is enough water in environkment
lead to the evolution of system which started to use water as a donor of
electrons. This had to be probably initiated by changes of pigments to
chlorophyll a which has, under certain conditions in photosystem II, such
redox potential to drive electrons from water. And here the Qy transition
(red absorption peak) is moved to shorter wavelengths and the overall
colour of chlorophylls to the green.

I donlt know anything about evolution of photosynthetic pigments and some
people say that chl was before bchl but the key things are the similarity
of synthetic pathways of porphyrins hem and chlorophyll and the need of
chlorophyll a to drive electron from water.


Regards

F. Vacha


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Frantisek Vacha
Inst. Plant Molec. Biol.
Branisovska 31
370 05 Ceske Budejovice
tel. 00420-38-7775523
fax. 00420-38-41475







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