trans

126 ENERGY METABOLISM – ANAEROBIC PROTOZOA
of individual enzymes will be illustrated by a
few examples only.
THE TERMS
‘AMITOCHONDRIATE’ AND
‘ANAEROBIC’
Neither of the terms ‘amitochondriate’ and
‘anaerobic’ is entirely correct to describe the
organisms discussed here and many other
eukaryotes without typical mitochondrial functions.
Numerous objections have been raised
against both terms, and therefore they are used
in this chapter only as shorthand designations.
‘Amitochondriate’ is used in a physiological
sense and applies to organisms which lack
an electron-transport-associated energy metabolism
and are dependent exclusively on
substrate-level phosphorylations for ATP generation.
Organelles (hydrogenosomes or mitosomes)
that are assumed to share a common
ancestor with extant mitochondria are present,
however, in several or possibly all amitochondriate
groups (Chapter 12). The term
‘amitochondriate’ does not apply to protists or
multicellular organisms that dispense with
oxidative phosphorylation in certain life-cycle
stages (e.g. trypanosomatids and helminths)
or during temporary anoxia (certain marine
invertebrates).
‘Anaerobic’ is used in a biological sense and
reflects two characteristics. It signifies that the
organisms in question do not require free O 2
for survival and multiplication and are inhibited
by atmospheric O 2 concentrations. They
can tolerate lower O 2 concentrations however.
It also signifies that their ATP-generating mechanisms
do not include oxidative phosphorylation
processes that depend on a mitochondrial
type F 1 F 0 ATPase fueled by a transmembrane
proton gradient, a typical mitochondrial
process. However, O 2 , already at low levels (in
the low micromolar range), elicits metabolic
shifts toward more oxidized metabolic endproducts
and can result in an increased level
of ATP generation by the same substrate-level
phosphorylation reactions that function in the
absence of O 2 (see below, ‘Effects of Oxygen’).
These phenomena are often cited as arguments
for designating the organisms in question
as ‘microaerophiles’.
GENERAL FEATURES OF
AMITOCHONDRIATE
METABOLISM
Amitochondriate protists represent some of
the most divergent types of core metabolism
among eukaryotes. It is to be stressed, however,
that these are still quite uniform when compared
with the enormous metabolic diversity
among prokaryotes. The overall map of core
energy metabolism for amitochondriates is
largely superimposable over that of any eukaryotic
organism. Glycolysis is the main pathway
of carbohydrate utilization, and most reactions
involved in the formation of metabolic endproducts
are also known from other eukaryotes.
The map shows however a major gap, indicating
the absence of processes and constituents
of mitochondrial energy conservation. There
is no tricarboxylic acid cycle, no cytochromemediated
electron transport, no cytochrome
oxidase and no F 1 F 0 -ATPase, thus there is no
electron transport-linked ATP generation.
Mitochondriate and amitochondriate organisms
differ in a number of less conspicuous but
important aspects. Amitochondriate organisms
also show significant differences among each
other, apparent at various levels:
• subcellular organization of core metabolism
• the nature of specific reactions and the
enzymes catalyzing them
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA