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Les étapes du cycle de Krebs

Le cycle de Krebs (voir image ci-dessous) commence par la combinaison du groupe acétyle de l'acétyl-CoA de la préparation du cycle de Krebs avec le composé à quatre carbones oxaloacétate pour former le composé à six carbones acide citrique. Ensuite, le citrate est oxydé le long du cycle par différentes réactions catalysées par des enzymes produisant plusieurs molécules intermédiaires. Au cours du cycle, 2 molécules supplémentaires de CO2, 1 ATP, 3 NADH, et 1 molécule de FADH2 sont formées par molécule de pyruvate. À la fin du cycle, l'oxaloacétate est régénéré pour revenir comme accepteur d'acétyle afin de relancer le 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.

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Le cycle de Krebs