Krebs cycle steps

The Krebs cycle (see image below) begins with the combination of the acetyl group of acetyl-CoA from the Krebs cycle preparation with the four-carbon compound oxaloacetate to form the six-carbon citric acid compound. Then the citrate is oxidized along the cycle by different reactions catalyzed by enzymes producing several intermediate molecules. During the cycle, 2 additional CO2 molecules, 1 ATP, 3 NADH, and 1 FADH2 molecule are formed per pyruvate molecule. In the end of the cycle, oxaloacetate is regenerated to return as an acetyl acceptor to re-start the cycle.

Diagram of labelled steps in the Krebs cycle in detail. Note that each molecule of glucose that goes through glycolysis produces two molecules of pyruvate that can enter the Krebs cycle, but these steps describe what happens to one molecule of pyruvate as it goes through the cycle. First, pyruvate, which contains 3 carbon atoms, enter the mitochondrion and undergoes a preparatory step resulting in acetyl Co A. which contains two carbon atoms and coenzyme A. During the preparatory step, one carbon atom is released from pyruvate, combining with oxygen to form carbon dioxide, while one NA Dplus is reduced to NADH. NADH will carry electrons to the electron transport chain. Next, coenzyme A. is recycled as the 2 carbon acetyl group enters the Krebs cycle. The acetyl group combines with a 4 carbon compound called oxaloacetate to form citric acid, a 6 carbon compound. Next, citric acid loses one carbon atom, which combines with oxygen to form carbon dioxide, and another molecule of NA Dplus is reduced to NADH. The result is a five carbon compound called alpha-ketoglutarate. Alpha-ketoglutarate then loses a carbon atom, which combines with oxygen to form carbon dioxide and is released. A molecule of NA Dplus is reduced to NADH and molecule of ADP is phosphirylated to produce a molecule of ATP in this step. The resulting 4 carbon compound is called succinate,and it continue through the cycle, where it is transformed into oxaloacetate. During this step, FAD is reduced to FADH2 and N A Dplus is reduced to NADH. Oxaloacetate can then join with the next acetyl group entering the Krebs cycle, and continue through the steps again as previously described. Overall, the products of one pyruvate molecule entering the Krebs cycle are 3 carbon dioxide, 4 NADH, 1 FADH2, and 1 ATP.

Figure 1: Steps in the Krebs cycle.

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