Read for comprehension. Catalyzed by isocitrate dehydrogenase, the oxidation of isocitrate by NAD+ converts a hydroxyl group to an aldehyde. This aldehyde is a β-keto acid, a species that is very susceptible to decarboxylization. The decarboxylization of this β-keto acid (oxalosuccinate) forms α-ketoglutarate. In summary, the conversion of isocitrate to α-ketoglutarate yields NADH and CO2.

We continue our discussion of the citric acid cycle with the conversion of α-ketoglutarate into succinate. Next, succinate is oxidized in a dehydrogenation reaction (catalyzed by succinate dehydrogenase), converting succinate into an alkene, fumarate, and converting FAD into FADH2. Fumarate is then hydrated to form the alcohol malate, which is then oxidized to form the aldehyde, oxaloacetate, converting NAD+ to NADH.

That's it! Don't worry if you don't have the cycle memorized to the finest detail. An example of the level that the MCAT might approach the details would be giving a list of compounds from the cycle as the correct answer for substances one may expect to find in the inner compartment of a mitochondrion. Here is a mnemonic for the cycle:

Curly Is Kicking Some Stooge Fanny, MO

This one has a limited life-span though, because nobody watches the Three Stooges anymore.

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