trans

150 ENERGY METABOLISM – TRYPANOSOMATIDAE
Oxidative
pathway
Glucose 6-phosphate
NADP H 2 O
1
NADPH 2H
6-Phosphogluconolactone
2 H 2 O
6-Phosphogluconate
NADP
3
NADPH H
CO 2
Ribulose 5-phosphate
4 5
Xylulose
5-phosphate
6
Ribose
5-phosphate
Non-oxidative
pathway
8
Sedoheptulose
7-phosphate
Erythrose
4-phosphate
7
Glyceraldehyde
3-phosphate
Fructose
6-phosphate
Fructose
6-phosphate
Glyceraldehyde
3-phosphate
FIGURE 7.5 The hexose-monophosphate pathway. Enzymes: (1) Glucose-6-phosphate dehydrogenase;
(2) 6-phosphogluconate lactonase; (3) 6-phosphogluconate dehydrogenase; (4) ribulose 5-phosphate 3-epimerase;
(5) ribulose 5-phosphate isomerase; (6) transketolase; (7) transaldolase; (8) transketolase.
pathway may be associated with glycosomes
as well. While 50% of the activity of glucose-6-
phosphate dehydrogenase (G6PDH) in T. brucei
is found in glycosomes, its inferred protein
sequence did not reveal a peroxisome-targeting
signal. However, the T. brucei 6-phosphogluconolactonase
(6PGL), of which some 15% of
the activity is glycosomal, contains a C-terminal
motif showing similarity to the established
type of targeting signal, in agreement with its
subcellular localization. The corresponding
genes from L. mexicana have also been cloned
and sequenced and here the G6PDH contains
a PTS 1 import signal, while the 6PGL did not.
However, the sequence of the third enzyme in
the pathway, 6-phosphogluconate dehydrogenase
(6PGDH), does not contain a detectable
PTS. Interestingly, the sequences of 6PGL and
6PGDH in both organisms are of prokaryotic
origin and therefore most likely entered the
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA