trans

ENZYMATIC DIFFERENCES BETWEEN AMITOCHONDRIATE AND MITOCHONDRIATE CELLS 133
imported and ATP is exported by the organelle
by a transporter inhibited by atractyloside,
an inhibitor of the mitochondrial ADP/ATP
exchange transporter.
ENZYMATIC DIFFERENCES
BETWEEN
AMITOCHONDRIATE AND
MITOCHONDRIATE CELLS
In addition to the absence of mitochondrial
components, two characteristics are generally
quoted as defining the core metabolism of
amitochondriate protists. The first is the ‘dominant
role’ of iron–sulfur center-mediated electron
transfer, and the second is the privileged
role of inorganic pyrophosphate (PP i ) instead
of ATP in glycolysis. Since neither of these
cases is as clear cut as generally held, they need
to be considered in some detail.
The most significant divergence of amitochondriate
from mitochondriate organisms is
in the mechanism of oxidative decarboxylation
of pyruvate to acetyl-CoA, a central step of core
metabolism [reaction 17]. The enzyme responsible
for this step in amitochondriates is the
iron–sulfur protein, pyruvate:ferredoxin oxidoreductase
(PFOR) and not the pyruvate
dehydrogenase complex (PDH), found in mitochondria.
The two enzymes are not homologous
and differ in many properties (Table 7.1).
The closest homologs of this enzyme are found
in anaerobic and N 2 -fixing eubacteria. In addition
to a typical PFOR, G. intestinalis and
T. vaginalis contain closely related enzymes acting
on different -keto acids (e.g. oxoglutarate)
with as yet undefined roles in metabolism.
Small molecular mass iron–sulfur proteins,
ferredoxins, serve as electron acceptors for
PFOR. These have been isolated and characterized
and belong to different ferredoxin types
in the different species. In Type I organisms
TABLE 7.1 Properties of pyruvate:ferredoxin oxidoreductase
(PFOR) and pyruvate dehydrogenase complex
(PDH)
Property PFOR PDH
Molecular mass 240–280 kDa 10 6 kDa
Subunit composition homodimer heteromer of
at least three
different
subunits
(E 1 , E 2 and E 3 )
Intramolecular [FeS] center lipoamide
electron transfer
Electron acceptor ferredoxin NAD
Catalyzed reaction reversible irreversible
the main ferredoxin is of 2[4Fe4S] type, while
in Type II organisms it is of the [2Fe2S] type. It
is likely that additional minor types are also
present, as indicated for G. lamblia.
The PFOR-ferredoxin system occupies a
central position in core metabolism. In Type II
organisms it is linked to a further FeS protein,
hydrogenase. While these iron–sulfur proteins
indeed play a central role in the metabolism of
amitochondriates, they are restricted to a single,
but important, pathway. We deal only with
a single characteristic here, and not with an
all-encompassing dichotomy between amitochondriate
and mitochondriate organisms.
PP i indeed replaces ATP in some glycolytic
reactions of amitochondriate protists. The
assumed physiological significance of this substitution
is a decreased ATP input into the
process, corresponding to an increased overall
ATP yield. Clearly increased energy generation
could be important for amitochondriates,
which depend on glycolysis as the main source
of ATP. Several PP i -linked enzymes have been
detected in amitochondriates: PP i -linked phosphofructokinase
(PP i -PFK, or more correctly,
PPi-glucose-6-phosphate phosphotransferase)
[reaction 3], pyruvate, orthophosphate dikinase
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA