The Citric Acid Cycle
In this SparkNote on the Citric Acid Cycle, also called the Krebs Cycle, we will pick up where we left off in the last section with the aerobic product of glycolysis, pyruvate. When oxygen is present, the pyruvate moves out of the cytosol in which glycolysis took place and crosses the membrane into the matrix of the mitochondria. There, before entering the citric acid cycle proper, the pyruvate undergoes a transition stage, in which the two pyruvates are converted into two acetyl-coenzyme A (acetyl-CoA), two carbon dioxide molecules, and two NADH. Then, during the series of eight reactions that make up the citric acid cycle, the two acetyl-coA molecules are oxidized, yielding two more molecules of carbon dioxide and 2 ATP. The carbon dioxide generated in these two processes is the carbon dioxide we exhale when we breathe.
The citric acid cycle, or Krebs cycle, is central to metabolism, since at this stage a large portion of carbohydrates, lipids, and proteins are degraded by oxidation. One characteristic that marks the citric acid cycle is that it does not only have degradative functions. A number of very important coenzymes are produced in the cycle's reactions. These coenzymes go on to oxidative phosphorylation, resulting in a huge payoff of 32 ATP. Another interesting aspect of the citric acid cycle is its status as a "cycle": the final productof the cycle, oxaloacetate, is a necessary molecule for the first reaction of the cycle with acetyl-CoA.
We will begin our discussion by looking at the conversion of pyruvate to acetyl-coA, the starting material of the citric acid cycle. Next, we will follow the eight reactions of the citric acid cycle that ultimately lead to the production of oxaloacetate and numerous coenzymes that go on to be used in oxidative phosphorylation.