trans

350 ENERGY METABOLISM IN HELMINTHS
activity in the face of the elevated NADH/NAD
ratios associated with anaerobiosis. Similarly,
the regulation of E1 kinase is modified in the
adult muscle PDC. For example, E1 kinase
activity is inhibited by pyruvate and propionate,
metabolites that are elevated during
anaerobic metabolism, and is less sensitive
to stimulation by elevated NADH/NAD and
acetyl CoA/CoA ratios. The stoichiometry of
phosphorylation and inactivation of the adult
muscle PDC also differs substantially from
its aerobic mammalian counterpart, and is
designed to prevent the complete inactivation
of the complex during anaerobiosis. In mammalian
E1s, each E1 subunit of the 2 2
tetramer contains three distinct phosphorylation
sites and, given the specificity of the
mammalian E1 kinase, inactivation in vivo is
associated primarily with the phosphorylation
of site 1. However, phosphorylation at any of
the three phosphorylation sites is sufficient for
inactivation. More importantly, inactivation
is characterized by half-of-the-site reactivity,
and phosphorylation in only one of the two
E1 subunits results in the complete inactivation
of the tetramer. In contrast, the E1 in the
PDC purified from adult A. suum body wall
muscle contains only two phosphorylation
sites and inactivation is accompanied by substantially
more phosphorylation than observed
in the mammalian E1, with both E1 subunits
of the tetramer phosphorylated. These
differences effectively prevent the complete
inactivation of the complex in vivo, especially
in the presence of E1 phosphatase activity.
Interestingly, in contrast to mammalian E1
phosphatases, the A. suum enzyme is dramatically
stimulated by malate, the major mitochondrial
substrate in adult A. suum muscle.
This represents yet another regulatory modification
designed to maintain the PDC in an
active dephosphorylated state. Little is known
about the regulation of PDC activity in other
parasitic helminths, although the structural
modifications observed in the A. suum PDC do
not appear to be conserved in the F. hepatica
PDC, suggesting that while both organisms
use identical pathways to form succinate, their
enzyme systems have evolved independently.
The branched-chain fatty acids, 2-methylbutanoate
and 2-methylpentanoate are the
ultimate products of glucose degradation in
A. suum muscle and accumulate to high levels
(50 mM total) in the perienteric fluid, where
they rival Cl as the most abundant anions.
Branched-chain fatty acids appear to exit the
worm by diffusion through the cuticle, leaving
open the possibility of their further metabolism
as they pass through potentially
aerobic mitochondria in the hypodermis.
2-methylbutanoate and 2-methylpentanoate
are formed by the condensation of an acetyl
CoA and a propionyl CoA or two propionyl
CoAs, respectively, with the subsequent reduction
of the condensation products (Figure 19.2).
Enzymes in the pathway differ significantly
from the corresponding enzymes of -oxidation
found in mammalian mitochondria.
Differences might be anticipated since the
ascarid enzymes function physiologically in
the direction of acyl CoA synthesis, not oxidation.
The final reaction in the pathway, the
NADH-dependent reduction of 2-methyl
branched-chain enoyl CoAs, is rotenone-sensitive
and requires Complex I, rhodoquinone,
and electron-transport flavoprotein (ETF)
reductase and two soluble components, ETF
and 2-methyl branched-chain enoyl CoA
reductase (Figure 19.2).
Anaerobic energy generation
At least four potential sites for energy generation
have been identified in anaerobic
helminth mitochondria: substrate-level phosphorylations
coupled to the decarboxylation of
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS