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Fig. 11.5 The Pchlide
a
E pool
11.1.2.1 Biosynthesis of Pchlide a Ester (Pchlide a E)
Because of the structural similarity between Pchlide
a
and Pchlide
a
E it was
convenient to propose that Pchlide
a
was the immediate precursor of Pchlide
a
E
(Granick
1950
). However, as early as 1970, precursor-product relationship studies
in vivo, between the biosynthesis of
14
C-Pchlide
a
and
14
C-Pchlide
a
E failed to
establish a precursor-product relationship between these two tetrapyrroles. Instead,
the results indicated that Pchlide
a
and Pchlide
a
E were most probably formed in
parallel from a common precursor (Rebeiz et al.
1970
). These studies were con-
firmed by in vitro investigations which also failed to establish precursor product
relationships between Pchlide
a
and Pchlide
a
E (Ellsworth and Nowak
1973
;
Mattheis and Rebeiz
1977
). Later on, more rigorous precursor-product relationship
studies between Pchlide
a
and Pchlide
a
E were carried out (McCarthy et al.
1982
).
Comparison of the ratio of
14
C-ALA and various
14
C-tetrapyrrole substrates
incorporation into
14
C-Pchlide
a
and
14
C-Pchlide
a
E in vitro, allowed the determi-
nation of which exogenous
14
C-tetrapyrrole substrate was the most likely common
precursor of Pchlide
a
and Pchlide
a
E. On the basis of these studies, it was proposed
that Pchlide
a
was formed via an acidic (monocarboxylic) biosynthetic route while
Pchlide
a
E was formed via a fully esterified route. It was also proposed that the two
routes are weakly linked at the level of Mg-Proto, Mpe and Pchlide
a
by porphyrin
ester synthetases (McCarthy et al.
1982
).
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