trans

160 ENERGY METABOLISM – APICOMPLEXA
dehydrogenase activity in malaria parasites.
Unlike other organisms, the activity of Plasmodium
lactate dehydrogenase is insensitive to
pyruvate or lactate, ensuring NAD regeneration
and the continued flux of glucose regardless
of substrate or product concentration.
Plasmodium lactate dehydrogenase is also
distinguished by the ability of 3-acetylpyridine
adenine dinucleotide (APAD, an NAD analog)
to enhance activity in vitro. These unique
kinetics and cofactor specificities have been
exploited to develop diagnostic tests for parasitemia,
and to assess drug efficacy. The crystal
structure of P. falciparum LDH reveals significant
differences from the host enzyme, presumably
explaining these differing biochemical
properties. A five-amino acid insertion (KSDKE)
near the pyruvate contact region creates a
unique active-site architecture and has generated
considerable interest as a possible chemotherapeutic
target.
The large quantities of lactic acid produced
in the parasite cytosol might be expected to
greatly reduce intracellular pH. While malarial
glycolytic enzymes are resistant to acidic conditions,
excessively acidic conditions could
threaten the osmotic stability of the cell. Studies
on P. falciparum have demonstrated the
presence of a monocarboxylate transporter on
the parasite surface that couples the efflux of
both lactate and protons into the extracellular
space. These moieties are thought to freely
diffuse out of the parasitophorous vacuole into
the red cell, where they may be excreted into the
serum via erythrocyte transporters and/or the
new permeation pathways (NPP) established
by the parasite on the host cell surface.
In summary, the complete pathway of
apicomplexan glycolytic catabolism produces
either two or three molecules of ATP per mole
of hexose (depending on whether PFK utilizes
ATP or pyrophosphate). This energy production
pathway is easily demonstrated in the cultured
stages of Plasmodium parasites through the
rapid and efficient conversion of radiolabeled
glucose to lactate (up to 85% in P. falciparum),
and by the rapid depletion of intracellular ATP
upon glucose withdrawal. Inhibitors of mitochondrial
electron transport or ATP synthetase
induce only a minor, transient drop in parasite
ATP pools, but the mitochondrion may well be
essential for energy production in other phases
of parasite growth, as discussed below.
Branching pathways
Although flux through the anaerobic Embden–
Meyerhoff–Parnas pathway predominates
during malarial intraerythrocytic growth of
Plasmodium parasites, it is clear that not all
glycolytic intermediates are funneled to lactate.
The hexose monophosphate (or pentose
phosphate) shunt utilizes glucose 6-phosphate
to generate both reductive energy in the form
of NADPH, and to supply ribose 5-phosphate,
a precursor in the synthesis of nucleic acids
(Chapter 9). In P. falciparum, the first steps
in this shunt are performed by a single protein,
glucose-6-phosphate dehydrogenase-6-
phosphogluconolactonase, a novel bifunctional
enzyme presently known only in Plasmodium
species. Both this enzyme and the subsequent
6-phosphogluconate dehydrogenase step provide
NADPH for glutathione reductase, an
enzyme crucial for oxidative damage protection.
As noted above, reduced glutathione
generated within the parasite is exported into
the erythrocyte, where it may serve to reduce
oxidative stress within the host cell.
Plasmodium and other apicomplexan parasites
require CO 2 for survival, and are able to fix
carbon dioxide, presumably for the production
of amino acids and citric acid-cycle intermediates.
Two enzyme activities capable of reversibly
fixing CO 2 to phosphoenolpyruvate (PEP) to
yield oxaloacetate have been detected in
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA