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the last two steps catalyzed by coproporphyrinogen oxidase (CPOX) and
ferrochelatase (FeCH) invariably happen back in the mitochondrion
(
Fig. 8.16
; Oborn´k and Green, 2005). It was proposed that such an arrange-
ment of the pathway reflects a major need of tetrapyrrole products in this
organelle of primary heterotrophs (
Koˇen´ et al., 2011; Koˇenˇ et al.,
2013
). All other organisms, such as bacteria except
a
-proteobacteria and
eukaryotic phototrophs, synthesize ALA by C5 pathway via NADPH-
dependent reduction of glutamyl-tRNA
Glu
catalyzed by glutamyl-tRNA
reductase. Resulting labile glutamate-1-semialdehyde is converted by
glutamyl-1,2-semialdehyde aminomutase in a pyroxidal-5-phosphate-
dependent reaction into ALA (
Beale, 1999
).
In eukaryotic phototrophs, the entire tetrapyrrole pathway is located
within the plastid, which corresponds to the main requirement for the tet-
rapyrrole products in this compartment of phototrophic eukaryotes
(
Fig. 8.16
;
Koˇen´ et al., 2011
; Oborn´k and Green, 2005). It is well known
that the apicomplexans synthesize heme via a noncanonical pathway, which
is actually an unprecedented combination of both above-mentioned routes.
Hence, ALA is synthesized by the C4 pathway in the mitochondrion and
subsequently exported into the apicoplast where the next steps occur.
The last two steps, which are localized in the mitochondrion and catalyzed
by CPOX and FeCH, terminate the metabolic route (
Nagaraj et al., 2009,
2010; Ralph et al., 2004; Sato and Wilson, 2003
).
It has been supposed that mitochondrial location of the initial and termi-
nal steps of the heme biosynthesis in the Apicomplexa is a consequence of
their heterotrophic, particularly, parasitic lifestyle. However, the tetrapyr-
role route in their closest phototrophic relative
C. velia
starts with the syn-
thesis of ALA by the heterotrophic C4 pathway. ALA is then exported to the
plastid, where the rest of the pathway takes place. Several of its enzymes are
present in multiple versions of apparently different origins, likely the rem-
nants of passed endosymbioses. Therefore, the
C. velia
cell contains three
nuclear-encoded plastid-targeted copies of uroporphyrinogen decarboxyl-
ase (UROD), each of which originates either from a cyanobacterium,
nucleus of the exosymbiont (
¼
secondary host), or nucleus of the endosym-
biont (
primary host). CPOX is also present in two copies, one coming
from the exosymbiont nucleus while the origin of the other CPOX cannot
be specified with certainty. Last but not least, one of the two ferrochelatases
is of cyanobacterial origin, while the second homologue was probably
acquired by lateral gene transfer from a proteobacterium (
Koˇen´ et al.,
2011
). Interestingly, while the cyanobacterial ferrochelatase was apparently
¼
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