trans

WHY DO TRYPANOSOMATIDAE HAVE GLYCOSOMES? 151
trypanosomatid ancestor by a mechanism of
horizontal transfer. The G6PDH, by contrast,
is of true eukaryotic origin, but this enzyme is
closely affiliated with its homologs found in
the plant cytosol.
Until recently, the role of 6PGL in metabolism
was still questionable, since 6-phosphogluconolactones
were believed to undergo
rapid spontaneous hydrolysis. Studies using
the T. brucei 6PGL together with 13 C- and
31 P-nuclear magnetic resonance spectroscopy
have now characterized the chemical scheme
and kinetic features of the oxidative branch of
this pathway. The -form of the lactone is the
only product of G6P oxidation. It leads to the
spontaneous formation of the -lactone by
intramolecular rearrangement. However, only
the -lactone undergoes spontaneous hydrolysis,
the -form being a ‘dead-end’ of this branch.
The -lactone is the only substrate for 6PGL.
Therefore, this enzyme significantly accelerates
hydrolysis of the -form, thus preventing
its conversion into the -form. Furthermore,
6PGL guards against the accumulation of -6-
phosphogluconolactone, which may be toxic
through its reaction with endogenous cellular
nucleophiles.
Suramin is a known inhibitor of T. brucei
6PGDH. A number of its analogs and several
aromatic dyes act as inhibitors of this enzyme.
The PGI transition-state inhibitor 5-phospho-
D-arabinonate also inhibits 6PGD with a K i of
50 M.
The non-oxidative branch generates from
various pentose-phosphates, the substrates for
the enzymes transketolase and transaldolase,
which transfer, respectively, two- and threecarbon
units between a variety of phosphorylated
carbohydrates. End-products of the
non-oxidative branch are the glycolytic intermediates
fructose 6-phosphate (F6P) and glyceraldehyde
3-phosphate (GA3P). The latter
branch may also function in the other direction,
where GA3P and F6P are used to generate other
phosphorylated sugars. Several of the respective
genes (e.g. transketolase) have been cloned
and sequenced, or partial gene sequences have
been identified in the Trypanosoma genome
database. None of these genes shows evidence
of horizontal gene transfer events. While in
T. brucei the enzymes of the oxidative branch
are present in both the life-cycle stages, in the
bloodstream form two enzymes of the nonoxidative
pathway, Ru5P epimerase and transketolase,
were not detected. Transaldolase was
present.
WHY DO
TRYPANOSOMATIDAE HAVE
GLYCOSOMES?
Most eukaryotes and certainly all aerobes have
peroxisomes. In general these organelles are
involved in various activities such as protection
against oxidant stress, -oxidation of fatty acids
and ether-lipid biosynthesis. Moreover, when
required, peroxisomes may adapt to specific
metabolic conditions as is the case for the specialized
peroxisomes of alkane- or methanoloxidizing
fungi. However, only in glycosomes of
the Trypanosomatidae have seven enzymes of
the glycolytic pathway been described, together
with other enzymes of carbohydrate metabolism.
Until now, no clear answer has been provided
to the question why these organisms,
contrary to other eukaryotes, have sequestered
a major part of their glycolytic pathway inside
glycosomes. With the accumulation of more
and more protein sequences in the database
and the possibility of comparing them with
their homologs from other organisms, it has
become clear that in all representatives of the
Trypanosomatidae analyzed thus far, a number
of glycolytic enzymes, but also some enzymes
involved in other pathways, such as the HMP,
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA