Regulation of the Citric Acid Cycle
The citric acid cycle is a series of reactions that play an important part in cellular respiration, the process that extracts energy from sugar in animal and plant cells and uses that energy to power other processes in the cell. Does this Spark an idea?
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Features
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Like the other reactions in cellular respiration, the citric acid cycle is regulated largely by feedback mechanisms, where accumulation of the product of a process inhibits or slows an early step in the process. This kind of feedback mechanism helps the cell to conserve energy rather than wasting it unnecessarily.
Function
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The reduced form of nicotinamide adenine dinucleotide (NADH) and acetyl coenzyme A are products of the citric acid cycle. When these two products accumulate, the increased levels activate an enzyme called pyruvate dehydrogenase kinase, which alters and deactivates an enzyme called pyruvate dehydrogenase, and thereby reduces the rate at which fresh pyruvate (a product of glycolysis) is fed into the citric acid cycle. If levels of NADH and acetyl coenzyme A fall, another enzyme called pyruvate dehydrogenase phosphatase reactivates pyruvate dehydrogenase and thereby increases the rate at which pyruvate is fed into the cycle.
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Phosphofructokinase
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The most important way the cell regulates the citric acid cycle is through feedback inhibition of glycolysis. Adenosine triphosphate, or ATP, is a product of the citric acid cycle and of oxidative phosphorylation (which is closely linked to the citric acid cycle). As ATP accumulates, it inhibits the activity of phosophofructokinase, one of the key enzymes in glycolysis. By slowing the rate at which glycolysis takes place, the cell decreases the rate at which cellular respiration takes place until ATP levels fall again.
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References
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