trans

SUBCELLULAR ORGANIZATION OF AMITOCHONDRIATE ENERGY METABOLISM 127
• evolutionary relationships of the structures
and enzymes involved in metabolism.
Although differences at the third level, i.e. in
evolutionary relationships, point to complex
events in the past, they provide little insight
into the physiology of the extant organisms.
This chapter will therefore focus on the first
two aspects.
Amitochondriate protists are essentially fermentative,
i.e. they are incapable of oxidizing
their energy substrates completely to carbon
dioxide and water. The backbone of hexose
utilization is a classical Embden–Meyerhof–
Parnas glycolytic pathway with several extensions
that lead to the known metabolic
end-products. ATP is generated by substrate
level phosphorylations without the contribution
of mitochondrial-type electron transportlinked
ATP production. In other words, ATP
production does not depend on transmembrane
proton gradients and does not require
the classical cytosol/mitochondrion compartmentation
separated by the inner mitochondrial
membrane. Such fermentative metabolism
generates only a few ATP molecules per molecule
of hexose utilized; it is ‘profligate’.
The main source of energy is carbohydrate,
primarily glucose, its oligomers and polymers. T.
foetus can also utilize fructose. Amino acids and
lipids do not support energy metabolism, with
the exception of arginine, which is metabolized
by the arginine dihydrolase pathway (Chapter
8). This is a non-oxidative ATP-generating
process, the contribution of which to the overall
energy balance of the organisms is not clarified
fully. Probably it provides only a limited part
of the ATP requirements. The absence of a mitochondrial
respiratory system does not mean
that these organisms are unable to take up and
reduce oxygen, but only that oxygen uptake is
not mediated by cytochrome oxidase and that
it is not linked to oxidative phosphorylation.
The main end-products of carbohydrate fermentation
differ in the four species, but are
primarily organic acids (acetate, succinate, and
lactate), ethanol and CO 2 . Alanine can also be a
major end-product. The relative proportions of
these depend strongly on external conditions
(pCO 2 , pO 2 , and presence or absence of exogenous
carbon sources). Usually at least two products
are formed simultaneously, indicating that
the fermentative pathways are branched and
not linear, permitting regulation of carbon flow
under various environmental conditions, as
characteristic of many prokaryotes.
The amount of energy necessary for maintenance
of the organism without growth, as determined
in chemostat cultures of T. vaginalis,
is about 50% of total energy production, a
value much higher than observed in most
microorganisms. Maintenance energy of other
amitochondriates remains to be determined.
SUBCELLULAR
ORGANIZATION OF
AMITOCHONDRIATE
ENERGY METABOLISM
While amitochondriate protists display a great
biochemical and biological diversity, they can
be assigned essentially to two types of metabolic
organization. In Type I amitochondriate
eukaryotes all processes of core energy metabolism
occur in the main cytosolic compartment
of the cell. Of the species included in this chapter
G. intestinalis and E. histolytica belong
to this metabolic type. In Type II organisms a
double membrane-bounded organelle, the
hydrogenosome, is present. Certain processes
of extended glycolysis occur in this structure,
the name of which is derived from the fact that
it produces molecular hydrogen. The two trichomonads
discussed are Type II amitochondriates
(see below). The presence of one or the
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA