Biology Reference
In-Depth Information
Chapter 6
The Iron and Magnesium Branches
of the Porphyrin Biosynthetic Pathway
In 1844 Verdiel, suggested a relationship between chlorophyll
and heme upon chemical conversion of chlorophyll to a red
pigment.
6.1 The Iron Branch of the Porphyrin Biosynthetic
Pathway: Biosynthesis of Heme
Protoporphyrin IX is a branching point for heme and Chl biosynthesis. Insertion of
ferrous iron into Proto leads to the formation of protoheme (Fig.
6.1
), while
insertion of Mg into the Proto macrocycle, leads to the formation of Mg-proto
which is the precursor of all Mg-porphyrins and Chls in nature (see below).
The terminal step in heme biosynthesis involves insertion of ferrous iron into
Proto by ferrochelatase to yield protoheme (Goldberg et al.
1956
). In animal cells,
conversion of Proto to protoheme takes place in the mitochondria. In
Euglena
,
It has been reported that protoheme is formed in the mitochondria from Proto
formed from ALA which is formed via the glycine-succinate pathway, and in the
plastid from Proto formed from ALA which is formed via the C5-pathway
(Weinstein and Beale
1983
). In higher plants ferrochelatase is found in the
mitochondria and the plastids, which strongly suggest that protoheme biosynthesis
takes place in both organelles (Little and Jones
1976
).
Ferrochelatase was first purified from rat liver (Taketani and Tokunaga
1981
).
Insertion of Fe
++
into Proto is accompanied by the release of two protons from the
pyrrole nitrogens (Fig.
6.2
). Mammalian ferrochelatase has a reported molecular
weight of about 40,000. Specificity of the enzyme for Proto is not absolute, as the
enzyme is able to handle a variety of porphyrin IX isomers, with substituents at the
2 and 4 positions of rings A and B that are smaller than hydroxyethyl in size and are
uncharged. In is not clear whether the same situation prevails in higher plants.
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