The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

An Introduction to Mitochondria
Fig. 2.6. Coenzyme Q, in its three forms.
frequently in this book because in certain situations they can contribute greatly to the toxicity
of free radicals. But in the respiratory chain, these metal atoms are tightly bound with the
enzyme complexes and cytochrome c, and are not toxic.
The non-protein member of the respiratory chain, coenzyme Q, does not harbor a
transition metal atom to carry the electrons: it carries them itself, like NAD and FAD. It has
one very striking difference from NAD and FAD, though: as shown in Figure 2.6, it exists
not in two alternative states but in three. The first and third states, ubiquinone and ubiquinol,
differ in content by two electrons just like the two states of NAD and FAD. In fact, these two
are the only ones which exist free in the membrane (moving between the enzyme complexes).
The intermediate form, ubisemiquinone, exists only fleetingly while CoQ is interacting with
Complexes I or III, and is fairly tightly bound to them during this time. However, its existence
is (for the purposes of this book) the weak link in the respiratory chain, because it can
spontaneously revert to ubiquinone, and the electron it releases in so doing can, in due
course, do immense harm. We will cover the details of this toxicity in Chapter 3.
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