trans

MITOCHONDRIAL METABOLISM 161
Plasmodium extracts. PEP carboxykinase transfers
a phosphate to ADP, while PEP carboxylase
releases free phosphate. Expression studies indicate
that maximal PEP carboxykinase expression
takes place during the sexual stages of
parasite development in the mosquito, probably
correlating with mitochondrial activation
(see below). Oxaloacetate can then be converted
into malate by the cytosolic malate dehydrogenase,
followed by transport into the mitochondria
and entry into the citric acid cycle.
MITOCHONDRIAL
METABOLISM
While the cytoplasm of a generic eukaryotic
cell (e.g. a mammalian fibroblast) may contain
several dozen ATP-generating mitochondria,
most apicomplexan parasites appear to harbor
only a single one of these endosymbiotic
organelles. The 6 kb linear organellar genome
of Plasmodium spp. is the smallest known
mitochondrial DNA, encoding only three proteins:
cytochrome b, and cytochrome oxidase
subunits I and III (Chapter 12). Apicomplexan
parasites possess at least a partial electron
transport system and are capable of oxygen
consumption. Mitochondrial proteins missing
from the organellar genome are encoded in
the nucleus, translated in the cytoplasm, and
imported post-translationally. The mitochondrion
occupies approximately the same fraction
of subcellular volume as typically observed in
mammalian cells; the presence of a only single
organelle per cell probably reflects architectural
constraints on organellar distribution during
schizogony rather than reduced functional
capacity.
Microscopic analyses of C. parvum in various
intracellular life-cycle stages initially failed
to recognize a mitochondrion, and attempts to
detect mitochondrial enzyme activities have
proven fruitless. Genomic sequence data reveal
dozens of genes encoding probable mitochondrial
proteins, however. These open reading
frames, such as the mitochondrial chaperone
CPN60 and several iron–sulfur cluster proteins,
suggest that C. parvum may harbor a mitochondrion
after all. Riordan et al. have identified
a ribosome-studded organelle enclosed by
a double membrane, posterior to the nucleus.
Although acristate, this organelle accumulates
electron potential-sensitive dyes, and is sensitive
to cyanide and other electron-transport
inhibitors.
Biochemical evaluation from various malarial
species has produced conflicting evidence
concerning the presence of oxidative phosphorylation
or a complete citric acid (TCA)
cycle during the blood stages of growth. Avian
parasites (e.g. Plasmodium gallinaceum) are
able to stimulate oxygen uptake in the presence
of lactate, pyruvate and TCA intermediates,
and can generate radioactive carbon dioxide
from radiolabeled glucose. In contrast, the
blood stages of mammalian malarial species
appear devoid of such activities. Consistent with
these data, morphological studies on avian
malaria species show numerous cristae, the
membranous folds that organize cytochromes
to form a functional electron transport chain.
Mammalian malaria species exhibit relatively
few cristae during the blood-cell stages, suggesting
limited activity.
Electron transport
The classical electron transport pathway consists
of NADH dehydrogenase (complex I), succinate
dehydrogenase (complex II), ubiquinone
(coenzyme Q), cytochrome bc 1 oxidoreductase
(complex III), cytochrome c, and cytochrome
oxidase (complex IV) (Figure 7.7). Proton
translocation occurs at complexes I, III, and
IV, providing a gradient to drive ATP synthesis.
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA