trans

184 PROTEIN METABOLISM
PROLINE
Polyamines (1) (2)
(16)
Ornithine
(3)
Glutamate semialdehyde
(4)
GLUTAMATE
(5)
(12)
α-ketoglutarate
∇
-pyrroline-5-carboxylate
GLUTAMINE
(6)
NH 3
Succinate
(8)
Succinic
semialdehyde
ARGININE
(13)
Citrulline NH 3
(14)
Carbamoyl phosphate
(15)
NH 3 CO 2
TCA cycle
(7)
γ-aminobutyrate
Urea
(11)
(9)
γ-guanidinobutyrate
O 2
(10)
NH 3
CO 2 γ-guanidinobutyramide
FIGURE 8.2 Catabolic pathways of proline, glutamine, glutamate and arginine. (1) proline oxidase; (2) -pyrroline-
5-carboxylate reductase; (3) non-enzymatic reaction; (4) glutamate semialdehyde dehydrogenase; (5) glutamate
dehydrogenase; (6) glutaminase; (7) -aminobutyrate aminotransferase; (8) succinic semialdehyde dehydrogenase;
(9) arginine decarboxyoxidase; (10) -guanidinobutyramide aminohydrolase; (11) -guanidinobutyrate amidinohydrolase;
(12) arginase; (13) arginine deiminase; (14) catabolic ornithine transcarbamoylase; (15) carbamate
kinase; (16) ornithine aminotransferase.
T. vaginalis and T. foetus, ultimately leads to
ammonia and carbon dioxide formation. This
pathway is unusual in eukaryotes; it is usually
restricted to prokaryotes. In trichomonads,
ornithine transcarbamoylase and carbamate
kinase are predominantly cytosolic, whereas
the first enzyme of the pathway, arginine deiminase,
is particulate. The arginase and the
-guanidinobutyramide pathways lead to the
generation of ATP under aerobic conditions via
the TCA cycle. On the other hand, the dihydrolase
pathway is operative under anaerobic conditions,
and results in the direct formation of
ATP in the carbamate kinase reaction. Indeed,
the dihydrolase pathway appears to be responsible
for a significant proportion of the ATP
generated by these protozoa, although glucose
remains as the major fuel, at least in the case
of T. vaginalis. The enzymes of the dihydrolase
pathway are present in G. lamblia at considerably
higher levels than in T. vaginalis.
The arginase pathway is operative in many
helminths; in some trematodes, glutamate
semialdehyde enters the biosynthetic pathway
of proline, which is then the final product
of arginine catabolism.
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA