trans

MITOCHONDRIAL METABOLISM 163
sensitive to salicylhydroxamic acid (SHAM) and
propyl gallate, well-characterized inhibitors
of an alternative oxidase activity in plant (and
certain fungal) mitochondria. This enzyme
transfers electrons directly from ubiquinol to
oxygen, circumventing the cytochrome complexes.
SHAM and propyl gallate inhibit parasite
growth and exacerbate the activity of
cyanide and atovaquone, suggesting that both
electron-transport pathways are important for
mitochondrial function. Because the alternative
oxidase bypasses the proton pumping machinery,
this pathway does not contribute to energy
production. Instead, the alternative oxidase
pathway is thought to play a role in stress protection
by channeling electrons safely to oxygen
during conditions of excess or improper electron
flow within the cytochrome chain.
Inhibitor studies suggest that electron flow
from ubiquinone to oxygen is essential for
apicomplexan survival, but mitochondrial
contributions to the total parasite ATP pool
remain uncertain. The entire P. falciparum
electron transport system may simply serve to
dispose of electrons generated by the mitochondrial
flavoenzyme reactions noted above,
particularly for dihydroorotate dehydrogenase,
an enzyme critical for pyrimidine biosynthesis
(Chapter 9). The import of substrates
and proteins for other essential mitochondrial
processes (such as heme biosynthesis) requires
a proton motive force, and could explain the
continued maintenance of a membrane potential
in Plasmodium.
Oxidative phosphorylation
The presence or absence of the canonical
‘bookends’ of the electron transport system –
a proton-pumping NADH dehydrogenase
(complex I) and an ATP synthase complex –
has been controversial, but both biochemical
evidence and genomic sequence data challenge
the traditional notion that Plasmodium mitochondria
are less sophisticated than their
mammalian counterparts. Using low concentrations
of digitonin to selectively increase the
permeability of P. berghei plasma membranes
without affecting the functional integrity of
parasite mitochondria, Docampo et al. were
able to show that flavoprotein-linked substrates
entering the system at ubiquinone (succinate,
glycerol 3-phosphate, dihydroorotate) enhance
ADP phosphorylation. Further, a collapse of
membrane potential was observed in the presence
of rotenone (a complex I inhibitor), and
ATP production was sensitive to oligomycin,
a well known inhibitor of mitochondrial ATP
synthase. Assuming that these data are not
attributable to contaminating host-cell activity,
these results suggest that at least some
Plasmodium mitochondria contain both
NADH dehydrogenase and ATP synthase
activity, despite the failure of many groups to
detect rotenone sensitivity or ATP synthesis in
P. falciparum mitochondria.
Bolstering these biochemical data, components
of an F 1 F 0 ATP synthase have been
detected in the genomes of P. yoelii (a rodent
malaria species closely related to P. berghei)
as well as P. falciparum. The P. falciparum
genome does not encode a rotenone-sensitive,
proton-pumping complex I, but shows evidence
for a single-subunit ‘alternative’ NAD(P)H:
ubiquinone oxidoreductase (Figure 7.7). This
enzyme could potentially couple NAD(P)H
formed during the TCA cycle or other reactions
to the electron transport chain, but is predicted
to be rotenone-insensitive and would not translocate
protons. The presence of a mitochondrial
ATP synthase indicates that Plasmodium parasites
are likely to engage in oxidative ATP
production during at least part of the parasite
life cycle. Although there is no convincing
evidence for oxidative phosphorylation in
erythrocytic stage P. falciparum, the marked
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA