trans

130 ENERGY METABOLISM – ANAEROBIC PROTOZOA
Conversion of triosephosphates into
phosphoenolpyruvate
Dihydroxyacetone phosphate and glyceraldehyde
3-phosphate are kept in equilibrium
through the action of triosephosphate
isomerase [reaction 5]. Glyceraldehyde
3-phosphate dehydrogenase [reaction 6], 3-
phosphoglycerate kinase [reaction 7],
3-phosphoglycerate mutase [reaction 8], and
enolase [reaction 9] convert glyceraldehyde
3-phosphate to phosphoenolpyruvate (PEP).
The triosephosphates represent the first
branch-point of fermentation, since an alternative
path leads from dihydroxyacetone phosphate
to glycerol, a metabolic end-product
arising through the successive actions of glycerolphosphate
dehydrogenate [reaction v] and
glycerolphosphate phosphatase [reaction w],
a process detected in T. vaginalis, but not in
E. histolytica.
Further conversions of
phosphoenolpyruvate
PEP is the central, key intermediate of carbohydrate
catabolism. PEP and its metabolites
can be processed in different directions; thus
several branch-points of metabolism are present
in the pathways downstream of PEP. While
Type I and Type II amitochondriates differ
significantly in the processes leading from
PEP to the dominant metabolic end-products
and in their subcellular location, there are several
common features as well (Figures 7.2 and
7.3). A main pathway leads to pyruvate by the
action of pyruvate kinase [reaction 10] and/or
pyruvate, orthophosphate dikinase (pyruvate
dikinase) [reaction 11]. Interestingly, the presence
of these enzymes is species dependent.
G. intestinalis contains both activities, E. histolytica
has only pyruvate dikinase, T. vaginalis
carries pyruvate kinase, and so far neither
enzyme has been detected in Tr. foetus. A large
part of the pyruvate is subsequently processed
to acetyl-CoA, as discussed below in the section
on the enzymatic differences between
amitochondriate and mitochondriate cells.
A putative alternative fate of PEP is carboxylation
to oxalacetate [reaction 12 or 13] (Figures
7.2 and 7.3). Oxalacetate can be reduced by
malate dehydrogenase [reaction 14] to malate,
which in turn can be oxidatively decarboxylated
to pyruvate by malate dehydrogenase
(decarboxylating) or ‘malic enzyme’ [reaction
15]. In essence, this sequence of reactions
represents a possible bypass to the pyruvate
kinase/pyruvate dikinase reaction. Oxalacetate
probably enters other pathways that remain to
be identified. In Tr. foetus, cytosolic fumarate
hydratase [reaction x) and fumarate reductase
(reaction y) reduce oxalacetate to succinate
(Figure 7.3), which is released as a metabolic
end-product.
The most significant differences between
Type I and Type II amitochondriates are in the
metabolic steps beyond pyruvate (Figures 7.2
and 7.3). In Type I organisms all steps take
place in the cytosol (Figure 7.2). Pyruvate can
be transaminated to alanine [reaction 16] or
oxidatively decarboxylated [reaction 17]. Acetyl-
CoA formed in the latter reaction is converted
either to acetate or ethanol. Acetate formation
is catalyzed by a single enzyme, acetyl-CoA
synthetase (ADP-forming) accompanied by
substrate-level phosphorylation [reaction 18].
Ethanol is produced by the action of a fused
NAD-specific aldehyde/alcohol dehydrogenase
[reaction 19]. In eukaryotes, these two enzymes
have been found so far only in Type I amitochondriates.
Reducing equivalents produced in
pyruvate oxidation are transferred to ferredoxin.
The mechanism of the reoxidation of ferredoxin
remains unknown, though it is likely that
the reducing equivalents are used in ethanol
production. Linking ferredoxin to ethanol
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA