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C H A P T E R
8
Amino acid and protein
metabolism
Juan José Cazzulo
Instituto de Investigaciones Biotecnológicas, Universidad Nacional de General San Martín,
Buenos Aires, Argentina
INTRODUCTION
The metabolism of proteins and amino acids
in parasites has been the subject of a number
of studies, from different research groups, over
the last two decades. As usual when studying
metabolic pathways in parasitic organisms,
most of the research has been focused on
those single enzymes, or pathways, which
differ sufficiently between the parasite and its
host. This chapter will deal mostly with such
enzymes and pathways, and therefore, since
many aspects of amino acid metabolism in
helminths are similar to those in mammals,
we will be concerned essentially with parasitic
protozoa.
Figure 8.1 summarizes the general features of
protein and amino acid metabolism in most
organisms, including parasites. Amino acids
can be obtained from one or more of three possible
sources, namely uptake from the medium
where the parasite lives, proteolysis of host
proteins, or biosynthesis. The last is, in general
restricted, and many parasites, particularly the
Protozoa, require most of the common amino
acids necessary for protein synthesis. The free
amino acids can then be used for the synthesis
of parasite proteins, or in the biosynthesis of
other compounds of physiological importance,
such as polyamines, or as fuel, for energy generation.
In the latter case, normally catabolism
starts with the transfer of the amino group to a
2-keto acid, most frequently 2-ketoglutarate, to
yield L-glutamate and the 2-keto acid corresponding
to the original amino acid, in a reaction
catalysed by an aminotransferase (also
known as transaminase). The amino group is
then liberated from L-glutamate as ammonia,
in the reaction catalyzed by L-glutamate
dehydrogenase; it can be excreted as such
(ammonotelic organisms), or converted into
urea (ureotelics) or uric acid (uricotelics). The
first type of excretion is the most common in
parasites, although a number produce urea,
Molecular Medical Parasitology
ISBN 0–12–473346–8
171
Copyright © 2003 Elsevier Science Ltd.
All rights of reproduction in any form reserved.