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Fig. 5.13 Conversion of protoporphyrinogen IX to protoporphyrin IX
5.6 Biosynthesis of Protoporphyrin IX (Proto)
Proto is the immediate precursor of Mg-Proto, which is the first committed intermedi-
ate of the Chl biosynthetic pathway. The role of Proto as an intermediate in the Chl
biosynthetic pathways was based on the detection of Proto in X-ray Chlorella mutants
inhibited in their capacity to formChl (Granick
1948
). It was conjectured that since the
mutants had lost the ability to formChl but accumulated Proto, the latter was a logical
precursor of Chl. The unambiguous role of Proto as a precursor of all Mg-porphyrins
and phorbins including Chl was established (a) by conversion of exogenous Proto to
Mg-Protomonomethyl ester (Mpe) by Rhodopseudomonas spheroides in the presence
of ATP and Mg (Gorchein
1972
)and(b)byconversionofexogenous
14
C- and
unlabeled-Proto to Pchlide
a
[the immediate precursor of Chlide
a
]inorganello
(Mattheis and Rebeiz
1977
), using a cell-free system capable of the conversion of
14
C-ALA to
14
C- Pchlide
a
,
14
C-Pchlide ester
a
and
14
C-Chl
a
and
b
(Rebeiz and
Castelfranco
1971a
,
b
), and capable of the net conversion of exogenous ALA to
Mg-Protoporphyrins and Pchlide
a
(Rebeiz et al.
1975
)(Fig.
5.13
).
5.6.1 Biosynthesis of Protoporphyrin IX (Proto)
via Oxidation of Protogen IX
Protoporphyrinogen IX oxidase (Protox for short) catalyzes the conversion of
Protogen IX to Proto (Jacobs and Jacobs
1987
; Poulson and Polglase
1975
). The
oxidation involves the removal of 4 peripheral (meso) hydrogens and two inner
hydrogens from the pyrrole nitrogens. In aerobic organisms, oxygen is the oxidant.
Removal of the hydrogens appears to be stereospecific (Battersby et al.
1976
). The
enzyme has been purified to apparent homogeneity from bovine liver (Siepker
et al.
1987
). It appears to be a monomer with a molecular weight of approximately
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