trans

134 ENERGY METABOLISM – ANAEROBIC PROTOZOA
[reaction 11], phosphoenolpyruvate carboxytransferase
[reaction 13] and PP i -acetate kinase
[not in the Figures]. The complement of PP i -
linked enzymes differs in the four species studied.
Only PP i -PFK is present in all four. Pyruvate
dikinase is known from the two Type I organisms.
E. histolytica is the only eukaryote known
to contain phosphoenolpyruvate carboxytransferase
and PP i -acetate kinase. Thus the statement
that PP i -linked reactions are a major
characteristic of amitochondriate parasites is
essentially incorrect.
Nor is the presence of PP i -linked enzymes a
character that distinguishes amitochondriate
and mitochondriate eukaryotes. PP i -PFK is
present in plants and in mitochondriate protists
(Naegleria, Toxoplasma, Eimeria, etc.), while
pyruvate dikinase is present in many plants. At
the same time, ATP- and PP i -linked enzymes
catalyzing the same overall reaction coexist in
Type I organisms. G. intestinalis contains both
pyruvate kinase and pyruvate dikinase, and
E. histolytica harbors ATP-phosphofructokinase
in addition to the PP i -specific one.
EFFECTS OF OXYGEN
Although mitochondrial-type, cytochromemediated
electron transport and a cytochrome
oxidase as terminal oxidase are absent, amitochondriate
protists avidly take up O 2 if it is present.
In fact, much of the early biochemical
exploration of these organisms was performed
with respirometry. The rate of O 2 uptake is of
the same order as the overall carbon flow measured
under anaerobic conditions. Metabolism
remains fermentative leading to the same endproducts,
the ratio of which, however, shifts in
favor of the more oxidized compounds (acetate,
succinate) and away from more reduced ones
(ethanol, alanine).
Respiration exhibits a high O 2 affinity.
G. intestinalis and Tr. foetus have K m values for
O 2 in the low μM range, similar to those
observed for cytochrome oxidase-mediated
respiration. Respiration is insensitive to most
inhibitors and uncouplers of mitochondrial
metabolism. In fact, the insensitivity of respiration
to cyanide, indicating the absence of
cytochrome oxidase, was noted in amitochondriates
before mitochondria were identified as
respiratory organelles in other eukaryotes.
Increased respiration in the presence of
exogenous carbohydrates shows that most of
the reducing equivalents are derived from
carbohydrate catabolism. However, the link
between extended glycolysis and O 2 uptake
remains conjectural. Cytochromes are absent
and only low levels of quinones have been
detected. Cytosolic NADH and NADPH oxidases
are present and probably contribute
significantly to O 2 uptake. These enzymes are
similar to H 2 O-forming pyridine nucleotide
oxidases found in cytochrome-deficient Grampositive
bacteria.
Isolated trichomonad hydrogenosomes act
as respiratory organelles when pyruvate and
ADP are added to the medium, a process probably
fueled by auto-oxidation of the iron–sulfur
centers in the components of the electron
pathway from pyruvate to H 2 .
REGULATION OF ENZYME
LEVELS
Only few studies have been devoted to the regulation
of enzyme levels and their experimental
modification at the level of transcription
and translation in T. vaginalis. Chemostat cultures
in which growth rate and the level of an
exogenous carbohydrate source can be controlled
showed a complex pattern of changes
in the activities of glycolytic enzymes. Iron
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA