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.
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