trans

188 PROTEIN METABOLISM
a cMDH has been found. This has led to the
proposal that AHADH is derived from a cMDH
no longer present in the parasite, through
a limited number of point mutations; indeed,
directed mutagenesis experiments have shown
that replacement of only two amino acid
residues in AHADH results in the expression of
an enzyme with MDH activity as high as that
of AHADH, which is actually increased in the
double mutants. The enzymes responsible for
aromatic amino acid catabolism have not
been characterized in T. brucei, although this
parasite has AHADH activity. An enzyme with
similar specificity has been described in a
Phytomonas, again with high homology to the
Phytomonas glycosomal MDH and low homology
with the cMDHs. It seems, therefore, that
T. cruzi and Phytomonas, although belonging to
different subfamilies, have both used an MDH
as a scaffold to build a new specificity for aromatic
keto acids. L. donovani excretes indolyl
lactate and contains aminotransferase and
dehydrogenase. In L. mexicana, the aminotransferase
responsible for aromatic amino
acid catabolism is not a TAT, but a broadspecificity
aspartate aminotransferase; evidence
for an AHADH-like enzyme is lacking. It appears
that Trypanosomatids have a simple catabolic
pathway for aromatic amino acids, but seem
to have adapted different enzymes on the evolutionary
road leading to its development.
Biosynthesis of amino acids
Parasites synthesize few amino acids, taking
most of them from the environment, or, in
cases like Plasmodium spp. and Schistosoma
spp., from the intracellular or extracellular
digestion of host proteins. In general, the
amino acids synthesized are only those arising
from short pathways, starting from metabolic
intermediates, such as pyruvate in the case of
L-alanine, or some TCA cycle intermediates, in
the cases of aspartate and glutamate. Some
amino acids can be obtained by interconversion
of other amino acids, as in the case of serine
and cysteine, or from the catabolic pathway of
another amino acid, as cysteine from methionine.
Two special cases are the synthesis of
alanine and cysteine.
Alanine is an end-product of glucose catabolism
in some Trypanosomatids, G. lamblia and
T. vaginalis. Production and excretion of alanine
has been associated with the re-oxidation
of glycolytic NADH, through alanine aminotransferase
and the glutamate dehydrogenases.
In the case of T. cruzi, both the NADP-linked
and the NAD-linked glutamate dehydrogenases
have been proposed to participate in a
mechanism which additionally involves the
cytosolic malic enzyme isoform (Figure 8.6).
The amino group of alanine may come from
a number of different amino acids, transaminated
to -ketoglutarate to yield glutamate,
whereas the carbon skeleton is the end-product
of glycolysis. Alanine has also been reported as
a final product of anaerobic glycolysis in some
helminths.
Unlike its mammalian host, and also with
other parasitic protozoa, E. histolytica and
E. dispar have a cysteine biosynthetic pathway
similar to that present in bacteria and
plants, and are thus able to perform de novo
synthesis of this amino acid, which is
very important as an antioxidant for these
organisms, which lack glutathione. The two
enzymes in the pathway, serine acetyltransferase,
and cysteine synthase, which replaces
the O-acetyl group by a sulfhydryl arising from
sulfide, have been characterized, and the
genes encoding them cloned and sequenced.
Sulfide is obtained by reduction of the inorganic
sulfate incorporated in the reaction of
ATP sulfurylase, an enzyme also cloned and
sequenced from E. histolytica, having sulfite as
an intermediate.
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA