Biology Reference
In-Depth Information
Chapter 7
The Chl a Carboxylic Biosynthetic Routes:
Reactions Between Mg-Protoporphyrin IX
and Protochlorophyllide a
A man has to resolve either to put out nothing new or to become
a slave to defend it.
Faraday
7.1 The Mg-Protoporphyrin IX (Mg-Proto) Pool
Mg-protoporphyrin (Mg-Proto) (Fig.
7.1
) is the immediate precursor of Mg-proto
monomethyl ester (Mpe). The proposed role of Mg-Proto as an intermediate in
the Chl biosynthetic pathway was based on the detection of Mg-Proto in X-ray
Chlorella mutants inhibited in their capacity to form Chl (Granick
1948
). It was
conjectured that since the mutants had lost the ability to form Chl but accumulated
Mg-Proto, the latter was a logical precursor of Chl. On the basis of absorbance
spectroscopic determinations the accumulated Mg-Proto was assigned by Granick a
divinyl (DV) Chemical structure (Fig.
7.1
, I), with vinyl groups at positions 2 and
4 of the tetrapyrrole macrocycle.
Mg-Proto as a precursor of other Mg-porphyrins and of Pchlide
a
was
demonstrated by conversion of
3
H-Mg-Proto to
3
H-Pchlide
a,
the immediate pre-
cursor of chlorophyllide (Chlide)
a
, by crude homogenates of etiolated wheat
(Ellsworth and Hervish
1975
).
7.1.1 Heterogeneity of the Mg-Proto Pools
When more powerful fluorescence spectroscopic techniques were used to
reinvestigate the chemical structure of the Mg-Proto pool of plants it was discov-
ered that it was chemically heterogeneous and consisted of DV and monovinyl
(MV) components (Fig.
7.1
) (Belanger and Rebeiz
1982
). The chemical structure of
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