trans

132 ENERGY METABOLISM – ANAEROBIC PROTOZOA
CO 2 GDP GTP
P-enolpyruvate Oxalacetate Fumarate Succinate
ADP
[13] [x] [y]
[22]
[10]
2H
H 2
Fd Fd
ATP
[25]
[17]
[20]
Lactate Lactate Pyruvate Pyruvate
Acetyl-CoA Acetate
[16]
Aminoacid
CO 2 Succinate Succinyl-CoA
[23]
Alanine Alanine -Ketoacid
CoA
P i
CO 2
[21]
ATP ADP
Ethanol Ethanol Acetaldehyde
[24]
HYDROGENOSOME
Succinate
CELL
MEMBRANE
H 2
Acetate
FIGURE 7.3 Map of the extensions of glycolysis beyond phosphoenolpyruvate in Type II amitochondriate parasites,
T. vaginalis and Tr. foetus. Hydrogenosomal malate metabolism omitted. Solid line shows the hydrogenosomal
membrane. Fate of reducing equivalents omitted. Metabolic end-products boxed. [10] pyruvate kinase, [13] phosphoenolpyruvate
carboxykinase (GTP-dependent), [16] pyruvate aminotransferase, [17] pyruvate:ferredoxin oxidoreductase,
[20] acetate/succinate CoA transferase, [21] succinyl-CoA synthetase, [22] hydrogenase, [23] pyruvate
decarboxylase, [24] alcohol dehydrogenase, [25] lactate dehydrogenase (T. vaginalis only), [x] fumarate hydratase
(Tr. foetus only), [y] fumarate reductase (Tr. foetus only), [Fd] ferredoxin.
succinate [reactions x and y], depending on the
species) with the exception of acetate. Acetate
is the only known major end-product of the
hydrogenosomal metabolism. The intermediates
entering the organelle are pyruvate (or
malate that is converted to pyruvate by a
hydrogenosomal malic enzyme, not shown).
Pyruvate is oxidatively decarboxylated to
acetyl-CoA by pyruvate:ferredoxin oxidoreductase
[reaction 17] . The mechanism of conversion
of acetyl-CoA to acetate, in contrast to
the single step process seen in Type I amitochondriates,
comprises two reactions. First the
CoA moiety is transferred to succinate by
acetate:succinate CoA transferase with the liberation
of acetate [reaction 20]. In the second
step, succinyl-CoA serves as a substrate for
succinyl-CoA synthetase [reaction 21]. In this
reaction succinate and free CoA-SH are produced,
accompanied by substrate level phosphorylation
of ADP to ATP.
The electrons generated by the oxidation
of pyruvate are transferred to ferredoxin
which, under anaerobic conditions, is reoxidized
by a [Fe]hydrogenase with the production
of H 2 [reaction 22]. This process gave the
name ‘hydrogenosome’ to the organelle. If
O 2 is present, H 2 evolution is inhibited and
the hydrogenosome acts as a respiratory
organelle. The identity of the terminal oxidase
is not known.
Mechanisms of transport of substrates
and products through the hydrogenosomal
membrane have not been studied. ADP is
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA