The Krebs Cycle

The Krebs Cycle is the central metabolic pathway in all aerobic organisms. The cycle is a series of eight reactions that occur in the mitochondrion.  These reactions take a two carbon molecule (acetate) and completely oxidize it to carbon dioxide. The cycle is summarized in the following chemical equation:

acetyl CoA + 3 NAD + FAD + ADP + HPO4-2 ---------------> 2 CO2 + CoA + 3 NADH+ + FADH+ + ATP

Most summaries of the Krebs Cycle will usually indicate that the cycle is an aerobic process (one that requires oxygen) that produces ATP by breaking down glucose. A quick glance at the above equation will show you that no oxygen or glucose is used in the cycle and that it does not make much ATP (only one molecule for each acetyl CoA that enters the cycle). So, how did the Krebs Cycle get the reputation for aerobically producing ATP? You will have to wait to find out.

In reality, the Krebs Cycle is not only part of the pathway for the breakdown of glucose but also for the breakdown of all metabolites, including other sugars, amino acids and fatty acids. Each of these groups of molecules has a pathway that leads into the Krebs Cycle. For example, carbohydrates are converted into acetyl CoA by the process of glycolysis while fatty acids are converted into acetyl CoA by the beta oxidation pathway. In each case, the molecules are converted into products that enter the Krebs Cycle. In addition, intermediates from the Krebs Cycle can go the other direction and be used to synthesize molecules such as amino acids and fatty acids. For example, acetyl CoA can be used to synthesize fatty acids.

The following web pages are a summary of the reactions that make up the Krebs cycle. It is suggested that you have a copy of the cycle present as you page through these sites to help you better understand the 'big' picture. At the end of the tutorial, there will be a quiz to help you better understand what information you should know about the Krebs Cycle.

Note: The Krebs Cycle is usually shown as beginning with pyruvate instead of acetyl CoA. Pyruvate is a three carbon molecule that is primarily formed by glycolysis or from some amino acids. Pyruvate is converted to acetyl CoA in the mitochondrion and so serves as a direct connection to the Krebs Cycle. This reaction is not really part of the Krebs Cycle, however, since pyruvate is most often generated by glycolysis (which occurs in the cytoplasm of the cell) and the oxidation of pyruvate occurs in the mitochondrion, it is usually associated with the cycle.

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