The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

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Fig. 2.7. The respiratory chain.
The Mitochondrial Free Radical Theory of Aging
Like the earlier steps in the destruction of pyruvate, all these components are located in
mitochondria. In this case they are all embedded in the mitochondrial inner membrane,
except for cytochrome c which moves within the aqueous space between the mitochondrion’s
inner and outer membranes (the intermembrane space). Complex II is a slightly special
case: it is both a part of the TCA cycle and a part of the respiratory chain. This is because,
unlike NAD, FAD does not exist free in solution. Instead, Complex II contains a “trapped”
molecule of FAD; in its TCA cycle role it causes the transfer of electrons from succinate to
that FAD making FADH2, and in its respiratory chain role it causes the transfer of those
electrons from the FADH2 to coenzyme Q, regenerating FAD. The s,n-glycerophosphate
dehydrogenase also uses a trapped FAD, but with that exception all the other electrons that
enter the respiratory chain from the processes discussed so far—glycolysis, the pyruvate
dehydrogenase cycle and the TCA cycle—are delivered by NADH, which donates them to
Complex I. NADH molecules are not trapped but diffuse freely in the mitochondrial matrix,
so there is no similar physical association of Complex I with NADH-producing components
of these cycles.
A final note about fatty acids is appropriate here. Their breakdown to acetaldehyde also
releases hydrogen atoms; in this case, the electrons are donated in equal quantity to NAD +