264
chapter 14
Electron Transport and Oxidative Phosphorylation
TABLE 14-5
Mitochondrial Metabolite Translocators
*
Metabolites
Species Translocated (In^out)
Dicarboxylates
Malate2- ^phosphate2-
Tricarboxylates
Citrate
3
- + H+ F^malate2-
a-Ketoglutarate
a-Ketoglutarate
2
- F^malate2-
Phosphate
Phosphate- ^ OH-
Pyruvate
Pyruvate- f^OH-
Glutamate
Glutamate- ?^OH-
Glutamine
Glutamine ^glutamate- + H-
Ornithine
Omithine+^ H +
Neutral amino acids
Neutral amino acids
Acyl carnitine, carnitine
Acyl carnitine ^carnitine
Glutamate, aspartate
Glutamate- +H+r^aspartate-
ADP, ATP
a d p
3
- ^
a t p 4-
*Reproduced, with permission, from L. Ernster and G. Schatz: Mito-
chondria: A historical review.
J . C e l l B i o l .
91,227s (1981). ©1981 by
the Rockefeller University Press.
A transhydrogenase catalyzes the transfer of reduc-
ing equivalents from NADH to NADP+ to form NADPH
by utilizing energy captured by the energy conservation
mechanisms of the respiratory chain without the par-
ticipation of the phosphorylation system. Similarly, ion
translocation occurs at the expense of energy derived from
substrate oxidation in the respiratory chain. For example,
Ca2+ is transported from the cytoplasm to the mitochon-
drial matrix, through the inner mitochondrial membrane,
at the expense of proton-motive force. The Ca2+ efflux
from mitochondria is regulated so that levels of cyto-
plasmic Ca2+ that are optimal for function are achieved.
Increased cytoplasmic Ca2+ levels initiate or promote
muscle contraction (Chapter 21), glycogen breakdown
(Chapter 15), and oxidation of pyruvate (Chapter 13). De-
creased levels of Ca2+ have the opposite effect.
Translocation systems of the inner mitochondrial mem-
brane are listed in Table 14-5. Anion translocators are re-
sponsible for electroneutral movement of dicarboxylates,
tricarboxylates, a-ketoglutarate, glutamate, pyruvate, and
inorganic phosphate. Specific electrogenic translocator
systems exchange ATP for ADP, and glutamate for as-
partate, across the membrane. The metabolic function of
the translocators is to provide appropriate substrates (e.g.,
pyruvate and fatty acids) for mitochondrial oxidation that
is coupled to ATP synthesis from ADP and Pj.
The ATP/ADP translocator, also called adenine nu-
cleotide translocase, plays a vital role in the metabolism of
aerobic cells because mitochondrial ATP is primarily con-
sumed outside the mitochondria to support biosynthetic
reactions. The translocator is the most abundant protein in
the mitochondrion; two molecules per unit of respiratory
chain are present. The translocator consists of two identi-
cal hydrophobic peptides. The export of ATP4- is neces-
sarily linked to the uptake of ADP3-. This transport sys-
tem is electrogenic owing to the transport of nucleotides
of unequal charge, the driving force being the membrane
potential. Although made up of two identical subunits, the
translocator is asymmetrical in its orientation; on the C
side, it binds with ADP, and on the M side, it binds with
ATP. This asymmetry of nucleotide transport has been
demonstrated by use of the inhibitors atractyloside and
bongkrekic acid (Figure 14-20). Atractyloside is a gly-
coside found in the rhizomes of a Mediterranean thistle;
bongkrekic acid is a branched-chain unsaturated fatty acid
synthesized by a fungus found in decaying coconut meat.
The former inhibitor binds to the C side of the translocator
at the ADP site, whereas the latter binds to the M side of
the translocator at the ATP site.
Transport of Cytoplasmic NADH to Mitochondria
NADH generated in the cytoplasm during glycolysis must
be transported into the mitochondria if it is to be oxidized
in the respiratory chain. However, the inner mitochon-
drial membrane not only is impermeable to NADH and
NAD+ but contains no transport systems for these sub-
stances. Thus, the [NAD+]/[NADH] ratio is many times
higher in the cytoplasm (about
1 0 0 0
) than in mitochondria
(about
8
). The high value for this ratio in the cytoplasm
favors the glyceraldehyde-phosphate dehydrogenase reac-
tion (which is essential for glycolysis) and contributes to
a negative AG; the low value for the ratio in mitochondria
favors the oxidation of NADH in the respiratory chain. Im-
permeability to NADH is overcome by indirect transfer of
the reducing equivalents through shuttling substrates that
undergo oxidation-reduction reactions. This process con-
sists of the following steps: a reaction in which NADH re-
duces a substrate in the cytoplasm; transport of the reduced
substrate into mitochondria; and oxidation of the reduced
substrate in the respiratory chain. Two pathways that trans-
port reducing equivalents from NADH into mitochondria
have been characterized and are known as the
glycerol-
phosphate shuttle
and the
malate-aspartate shuttle.
The glycerol-phosphate shuttle is mainly associated
with wing muscle mitochondria in insects and is not
quantitatively significant in mitochondria of mammalian
muscle. However, its activity is higher in mammalian
brain and liver cells than in mammalian muscle. The
shuttle (Figure 14-21) involves two glycerol-3-phosphate
dehydrogenases, one NAD+-linked and present in the
cytoplasm, the other FAD-linked and present on the C side
of the inner mitochondrial membrane. The process begins
in the cytoplasm with the reduction of dihydroxyacetone
