Reactions Between b-Aminolevulinic Acid and Protoporphyrin IX - Chlorophyll Biosynthesis and Technological Applications - page 175

5.5 Biosynthesis of Protoporphyrinogen IX (Protogen IX)
Protogen IX is the precursor of protoporphyrin IX (Proto). Conversion of Coprogen
III to Protogen IX involves oxidative decarboxylation of the two propionate side
chains on rings A and B and their conversion to vinyl groups (Sano and Granick
1961 ). The mammalian enzyme has an absolute requirement for oxygen, but
requires no reducing agent. Recent studies indicate that the mammalian enzyme
is a dimer of two 37,000 subunits (Kohno et al. 1983 ). The observation of
harderopoprphyrinogen accumulation (one vinyl at position 2 and one propionate
at position 4) (Sano and Granick 1961 ) and its subsequent isolation (Kennedy
et al. 1970 ) led to the proposal that the decarboxylation of ring A occurs before
that of ring B. The precise mechanism of oxidative decarboxylation is still uncer-
tain. In animal cells, cytoplasmic Coprogen III is transported to the mitochondria in
an ATP-dependent process where it is converted to Protogen IX (Rebeiz
et al. 1996 ). In plant cells, Coproporphyrinogen oxidase appears to be loosely
bound to the plastid membranes (Lee et al. 1991 ).
Beyond the possibility that in higher plants Protogen IX may contribute to the
formation of Proto in five different environments (Table 5.1 ), no specific efforts
have been made to document the nature and extent of Protogen IX biosynthetic
heterogeneity in plants (Fig. 5.12 ).
Fig. 5.11 Formation of coproporphyrinogen III
Fig. 5.12 Conversion of coproporphyrinogen III to protoporphyrinogen IX