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Succinate dehydrogenase is attached to the inner mitochondrial membrane. All the other enzymes of the Krebs cycle are located within the matrix of mitochondria, though.

In biological systems, there is often a relation between structural properties and function. For example, two enzymes that function sequentially in a metabolic pathway are sometimes held in close proximity to one another by a scaffold (e.g., in fatty acid synthesis). Is there a similar biochemical reason why succinate dehydrogenase is attached to the inner mitochondrial membrane, as opposed to being dissolved in the mitochondrial space?

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The reason behind this lies in the properties of FAD+. Unlike NAD+, FAD+ is not free to diffuse within the mitochondrion, it is an integral part of the inner mitochondrial membrane. Its reduced form FADH2 contributes electrons directly to ETC.

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Succinate dehydrogenase is also known as Complex II in the electron transport chain.

Although textbooks often show Complex II as taking FADH2 as a substrate, this is a little bit misleading. Succinate is the substrate, the oxidation of which facilitates the reduction of an FAD cofactor, which is already bound to Complex II. The resulting FADH2 is then oxidized in order to reduce ubiquinone, and feed electrons to Complex III.

This is a pretty good image explaining the electron transport chain.

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  • $\begingroup$ A note on the ETC image I just realized - succinate is oxidized to fumarate, not "funarate" as the image mistakenly says. $\endgroup$ – stords Feb 24 '14 at 5:07

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