The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

A Challenge from Textbook Bioenergetics and Free Radical Chemistry
standard bioenergetics, but most of my arguments are not new. The final link is due to me,
however. 30
11.3.2. Is the Bulk-to-Bulk Proton Gradient Adequate?
Mitchell’s chemiosmotic theory, described in Section 2.3.4.2, was first published 31 at a
time when the mechanism of oxidative ATP synthesis had been the object of a huge amount
of research worldwide for some years, and everyone engaged in this research had (as it turned
out) been frustrated as a result of a shared false assumption—that there existed a chemical
intermediate which transfers energy between the respiratory chain and the ATP synthase.
Because the field was so fixated on the existence of such an intermediate, it took many years
for Mitchell to win people over, even though in hindsight there can be no doubt that his
theory—even in the rather preliminary form in which he first presented it 31 —had all that a
theory ought to need in order to be compelling: elegance, simplicity, and absolute accordance
with experimental data.
This is well known. What is much less well known is that a second battle has raged over
the chemiosmotic theory, beginning in 1969, which is still unresolved. In this case it is the
other way around: a challenge to, rather than a defense of, one aspect of Mitchell’s model.
Perhaps by way of atonement for their unjustifiable reluctance to accept the original
chemiosmotic theory without a great fight, bioenergeticists have been, if anything, even
more adamant in their subsequent reluctance to entertain variations to it. This particular
variation not only has extensive experimental support: it is also very relevant to SOS. Thus
it will be explored in detail here.
The chemiosmotic theory as originally set out in 1961 31 stressed that the proton pumping
of the respiratory chain generates a proton gradient across the mitochondrial membrane,
which forces the weaker proton pump (the ATPase) into reverse, constructing rather than
hydrolysing ATP. This concise statement of the chemiosmotic hypothesis can usefully be
broken down into four testable postulates:
1. The respiratory chain transfers protons across the membrane.
2. The ATP synthase also transfers protons across the membrane.
3. Protons do not generally pass across the membrane by other means.
4. In general, ions also do not pass across it freely, only when actively transported.
In that original paper, Mitchell noted that the ATP synthase would only synthesise ATP if
there were a substantial asymmetry in the distribution of protons on either side of the enzyme;
with no asymmetry it would catalyse the reverse reaction, the hydrolysis of ATP to ADP and
phosphate. He mentioned that this asymmetry could be manifest as a difference in pH or as
an electrical potential difference, or as a combination of the two; he did not, however, speculate
on what combination was present in actual mitochondria in vivo. But in 1966 he published
a much more detailed restatement of the theory, 32 in which he laid great stress on the concept
of proton-motive force as the combination of these two components; furthermore, he
defined them more precisely than before. One, usually denoted ΔpH, is the difference in pH
between the bulk aqueous phases on either side of the membrane. The other is called Δψ,
and it derives from the fact that the membrane is generally impermeable not only to protons
but to other ions. It is the difference in electrical potential, caused by a tiny imbalance
in the concentrations of all ions added together, between the sides of the membrane. Mitchell
found it necessary to invoke Δψ in order to reconcile the chemiosmotic theory with the (by
then) known fact that the mitochondrial ΔpH in vivo is nowhere near enough to force
ATPase into reverse, i.e. to drive ATP synthesis.
It is important to note that Mitchell’s invention of the concept of the bulk-to-bulk
proton-motive force as the driving force of OXPHOS was not merely a clearer exposition of
what he had written in 1961. It incorporated a new assertion, over and above the four
145