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The cycle begins with the reaction between acetyl-CoA and the four-carbon oxaloacetate to form six-carbon citric acid. Through the next steps of the cycle, two of the six carbons of the citric acid leave as carbon dioxide to ultimately yield the four-carbon product, oxaloacetate, which is used again in the first step of the next cycle. During the eight reactions that take place, for every molecule of acetyl-CoA the cycle produces three NADH and one flavin adenine dinucleotide (FADH2), along with one molecule of ATP.
Figure 3.07: The Citric Acid Cycle (Krebs Cycle).
Let’s pause to take stock of what the citric acid cycle has generated from one acetyl-CoA molecule:
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The acetyl-CoA, has been oxidized to two molecules of carbon dioxide.
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Three molecules of NAD were reduced to NADH.
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One molecule of FAD was reduced to FADH\(^2\).
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One molecule of GTP (the equivalent of ATP) was produced.
Keep in mind that a reduction is really a gain of electrons. In other words, NADH and FADH\(^2\) molecules act as electron carriers and are used to generate ATP in the next stage of glucose metabolism, oxidative phosphorylation. In the next SparkNote on oxidative phosphorylation and the electron transport chain, we will learn what processes take place to ultimately derive the majority of the ATP we need to fuel our daily activity.
