The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

Frequently-Asked Questions
has been shown absolutely to require two things that are normally supplied by OXPHOS.
These are (a) a supply of ATP inside the mitochondrion, and (b) a proton gradient across
the inner membrane. 31,32 If OXPHOS is not happening, the mitochondrion cannot achieve
any further replication (beyond perhaps one or two more divisions using proteins it has
already imported) unless both these things are provided in some other way.
ATP is the easier one to obtain without OXPHOS. Recall from Section 9.4 that succinate
dehydrogenase is found to be upregulated in anaerobic cells, and that this implies that the
entire TCA cycle must be proceeding. But the TCA cycle occurs inside mitochondria, and
one step of it, succinyl CoA hydrolysis, generates a molecule of ATP directly.*
The proton gradient is another matter. Since the TCA cycle occurs inside mitochondria,
and generates NADH, the only way it can be maintained indefinitely is by reversal of the
usual mode of action of the malate/aspartate shuttle (see Section 2.3.2.4). This shuttle
normally imports electrons released by glycolysis, which are then fed into the respiratory
chain; now, instead, it must export electrons released by the TCA cycle, which are then fed
into the PMOR. One of the two carrier molecules that mediate the shuttle is the glutamate/
aspartate carrier, which in aerobic cells imports glutamate and exports aspartate; thus, in
anaerobic cells it exports glutamate and imports aspartate.
What has that to do with the proton gradient? Glutamate and aspartate are, indeed,
irrelevant. But there is one further feature of the glutamate/aspartate carrier which does the
trick. Figure 10.1a shows the components of a wild-type mitochondrion which are of most
relevance to the proton gradient: they include not only the respiratory chain and the ATP
synthase, but also the related metabolite carriers. Every time that the glutamate/aspartate
carrier exchanges a molecule of glutamate with one of aspartate, it also transports a proton,
in the same direction as (symport with) glutamate. Thus, in its reversed (and up-regulated)
mode of action (see Fig. 10.1b), it is exporting protons. One carrier’s protons are just as
good as another’s, for the purpose of making a gradient, so the proton gradient is maintained
and protein import is still possible. In theory, if the degree of up-regulation were sufficient,
this mode of proton export could suffice to drive ATP synthesis by Complex V (in mutant
mitochondria whose mutation was in a respiratory chain gene, so whose Complex V was
still intact); but in practice this is very unlikely, since the number of protons exported per
pyruvate molecule imported is only about 10% of what the respiratory chain achieves.
It is interesting to note that a system would exist to provide both internal ATP and a
proton gradient, even if the TCA cycle did not keep going. 33 Absence of ATP synthesis in the
mitochondrion would induce the reversal of the ATP/ADP translocase, which normally
imports ADP and exports ATP but would then import ATP generated by glycolysis (and by
other, intact mitochondria, while the cell still has some). But intramitochondrial hydrolysis
of this ATP (for protein import and other tasks) would release phosphate, so there would
also be a reversal of the phosphate carrier. (Neither the phosphate carrier nor the ATP/ADP
translocase has any mt-coded components, so this applies whatever the mtDNA mutation.)
The phosphate carrier has the same useful property as the glutamate/aspartate carrier: it
transports hydroxide ions the opposite way from phosphate, which is electrochemically the
same as transporting protons the same way as phosphate (Fig. 10.1c).
It must be acknowledged that both the intramitochondrial ATP supply and the proton
gradient are sure to be less in an anaerobic mitochondrion than in a working one, and that
its protein import will inevitably be slower as a result. This might be considered fatal to
* Not quite directly, in fact (see Section 2.3.3.2)—bacteria do it directly, but in humans what is generated
is GTP, and the extra phosphate bond is then transferred to make ATP. But the point is that OXPHOS is not
involved.
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