The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

A Challenge from Textbook Bioenergetics and Free Radical Chemistry
Fig 11.3. Variation of potential near a membrane (independent of proton pumping).
have an intrinsic variation in the density of charge within themselves, due to their molecular
structure: one end of the molecule is more positively charged, the other end more negative.
Such molecules are called polar. Some polar molecules have one charged end and one
neutral end, and others have opposite charges at either end; the latter are called dipolar. Any
dipolar molecule which is placed in an electric field will have a tendency to orient itself in
such a way that its own positive charge is nearest to the negative side of the field, and conversely
its negative charge is nearest to the positive side.
The reason why all this is relevant is that water is a dipolar molecule. Its two hydrogen
atoms are attached on either side of its oxygen atom, but not in a straight line—they make
an angle of about 130˚. Thus, water molecules in an electric field will tend to “point” (if you
think of that angle as an arrow) towards the positive side of the field. There is also random
movement and reorientation going on all the time—that is what keeps water liquid—but
calculations show that the field next to the inner mitochondrial membrane will be strong
enough to inhibit that natural random orientation of water molecules, and make them mostly
line up with their oxygen atoms away from the membrane and their hydrogen atoms towards
it 28 (Fig. 11.5). Hydronium is also a dipole—the three hydrogens arrange themselves to
make a pyramid with the oxygen 46 —so it behaves the same way. Finally we come to Kell’s
insight: that this bias of orientation produces, effectively, a series of one-molecule-thick
layers of ice.* There will be a two-dimensional hydrogen-bonded network of water molecules
coating the membrane, oriented to allow ease of proton transfer, thus facilitating conduction
of protons across the face of the membrane. Not only that: the hydrogen-bond connections
between the layers will predominantly not be favorable to the relevant proton transfer, thus
impeding conduction perpendicular to the membrane. Therefore, proton conduction would
be predominantly in two dimensions. Kell proposed 41 that this semi-permeable “insulation”
of the surface water from the bulk would allow the respiratory chain to generate a greater
* Ice-like in regard to freedom of rotation, but not in regard to conduction, since the pH is around 6.5 whereas
the pH of pure ice is about 10.5. 43-45 The inferences with regard to conduction thus derive only from the
restriction on rotation caused by the electric field.
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