The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

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The Mitochondrial Free Radical Theory of Aging
11.3.1. The Spectacular “Diffusion” of Protons
When I was a schoolboy, my classmates and I were walked through an experiment that
taught the concept of pH, and in particular the fact that it is a logarithmic scale. The fact
that neutral pH is set at 7 is because of a property of water mentioned in Section 2.3.1.1: in
pure water at standard temperature and pressure, almost exactly one H2O molecule in 10 7 is
dissociated, with one proton being separated from the rest of the molecule. This means that
the molecule which remains is a hydroxide anion, OH — , and the proton attaches to a different
water molecule forming a hydronium cation, H3O + . Thus, the proportion of hydroniums
is also one in 10 7 , so the product of the proportions of hydroxide and hydronium ions to
neutral water is 10 -14 . The reason the pH scale is possible is that if alkaline chemicals are
now added to the pure water, to increase the number of hydroxides, they neutralise exactly
enough of the hydroniums so that the product of the proportion of hydroxide to neutral
water and that of hydronium remains 10 -14 . The same is true if the chemical that is added is
an acid, so increases the hydroniums instead. Thus, for example, a solution of acid that is at
pH 3 has one in 10 3 of its water molecules existing as hydronium and one in 10 11 as hydroxide.
The fact that neutral water has a pH of 7, i.e. that only one in ten million water molecules
is dissociated, means that adding a tiny amount of a strong acid (that is, an aggressive donor
of protons to water making hydronium) to a large amount of pure water can change its pH
by a detectable amount. The school experiment that I mentioned above was designed to
impress this upon us: we would first add a pH-sensitive dye, phenolphthalein, to a large
flask of pure water, and then add concentrated acid one drop at a time. The dye stayed the
same colour for a while, and then the whole flask changed colour with the addition of just
one more drop.
But that, to quote Arlo Guthrie, 27 is not what I came to tell you about. I came to talk about
a feature of that experiment which is every bit as striking as the one I just described—indeed, I
can visualise it to this day—but which was not pointed out to us and which none of us noticed
at the time. When the crucial drop is added, the whole flask changes colour at once—so quickly
that one cannot see it spreading out. Think now what happens when one adds a drop of a
strong dye to pure water. It spreads out at a highly dignified pace. The behaviour of pH change
bears no comparison.* (We will cover the mechanism behind this in some detail below.) And,
potentially, this knocks a huge hole in the logic of Section 11.2.
The flaw (or not, as I will argue here) in the logic of Section 11.2 concerns my claim
that the rate of a particular mitochondrion’s proton-pumping significantly determines the
pH—proton (strictly, hydronium) concentration—at its inner membrane’s surface. This
would be uncontroversial if it referred to anything other than protons, but protons, as
explained, diffuse in water at a phenomenal speed, orders of magnitude faster than anything
else does. So fast, in fact, that the effect of a given mitochondrion’s proton-pumping on its
own surface pH can be calculated to be infinitesimal; what matters is the average protonpumping
efficacy of all mitochondria in the cell. The pH would vary somewhat, at least
temporarily, if all the mitochondria in a cell stopped pumping protons simultaneously; but
that is not the situation which SOS proposes.
I shall present in this section my reasons for believing that the above “flaw” is itself flawed,
because the proton concentration right next to the membrane is held some way from equilibrium
with the concentration further away—and is held there by OXPHOS, such that, as SOS
requires (and as is indicated by the dismutation results noted in Section 11.2.3), 21 the rate
of OXPHOS determines the distance from equilibrium. This is a severe deviation from
* In fact, this can partly be put down to the fact that small molecules, such as hydronium, diffuse faster than
large ones such as dyes. But not totally: the observed rate of diffusion is even faster than would be predicted
on that basis. 28,29