The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki
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The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

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150 The Mitochondrial Free Radical Theory of Aging Fig. 11.4. The Grotthuss mechanism of proton conduction in water.

A Challenge from Textbook Bioenergetics and Free Radical Chemistry Fig. 11.5. Orientation of water and hydronium layers near the membrane. density of protons at the inner membrane’s outer surface than in the bulk cytosol, and similarly a lower density at the inner surface than in the bulk matrix. The surface-to-surface difference in proton density (i.e. in pH) would be what was felt by the ATP synthase, so the idea was that a Δψ was not needed after all, because the surface-to-surface pH difference was enough to drive ATP synthesis unaided. The proposed variations of proton-motive force are shown in Figure 11.6, with notation that will be used hereafter for the forces between various locations. The proposal that proton conduction is not isotropic in three dimensions has been confirmed experimentally. Experiments in the laboratory of Teissié have shown that protons move along the surface of a membrane much faster than they move away from or towards it. 47 Other work has, conversely, demonstrated the existence of a barrier to proton movement perpendicular to the membrane. 48-50 A particular attraction of this hypothesis was that it did not simply give theoretical credence to Tedeschi’s findings: it also reconciled them with the confirmation of Mitchell’s “delocalized” model that had repeatedly been achieved using ion-distribution methods (see Section 11.3.2). This was because the ionic dyes that were used for this purpose were, necessarily, ones which were relatively soluble both in water and in lipid, so that they could diffuse across the membrane. And such molecules are exactly the sort that will act chaotropically on the water at the membrane surface: that is, disrupt its organisation, allowing protons to move in the third dimension, perpendicularly to the membrane, at a much higher rate than usual. That easier proton flow will rapidly dissipate δpc and δpm. Kell proposed that the mitochondrion responds to this collapse—which necessarily equates to a collapse of ATP synthesis—by cation transport that generates a real Δψ; that new, non-physiological Δψ then causes diffusion of the ionic dyes, whose interpretation as a real Δψ is thus correct. This potentially saves SOS from the challenge described in Section 11.3.1, that proton conduction is too rapid to allow an individual mitochondrion’s respiration rate significantly to affect the pH near its inner membrane. The faster the mitochondrion is respiring, the more of a proton-motive force it will make between the surface and the bulk (on both sides 151

150<br />

<strong>The</strong> <strong>Mitochondrial</strong> <strong>Free</strong> <strong>Radical</strong> <strong>The</strong>ory <strong>of</strong> <strong>Aging</strong><br />

Fig. 11.4. <strong>The</strong> Grotthuss mechanism <strong>of</strong> proton conduction in water.

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